Implement "unused pattern bindings" p2022 - parsing (#6460)

This implements proposal #2022, with changes from #3763 and leads
answers on #6448

Detection of unusedness is happening in
~~`toolchain/check/dataflow_analysis.cpp`~~ next PR. See
[here](https://github.com/burakemir/carbon-lang/tree/unused_pattern_bindings_p2022_impl_part2)
for preview.

~~All test cases with unused bindings were updated in order to avoid
polluting test output.~~

---------

Co-authored-by: Dana Jansens <danakj@orodu.net>

docs/design/pattern_matching.md

@@ -16,13 +16,14 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
         -   [Alternatives considered](#alternatives-considered)
     -   [Binding patterns](#binding-patterns)
         -   [Name binding patterns](#name-binding-patterns)
-        -   [Unused bindings](#unused-bindings)
+        -   [Anonymous bindings](#anonymous-bindings)
             -   [Alternatives considered](#alternatives-considered-1)
         -   [Compile-time bindings](#compile-time-bindings)
         -   [`auto` and type deduction](#auto-and-type-deduction)
         -   [Alternatives considered](#alternatives-considered-2)
     -   [`var`](#var)
         -   [Alternatives considered](#alternatives-considered-3)
+    -   [`unused`](#unused)
     -   [Tuple patterns](#tuple-patterns)
     -   [Struct patterns](#struct-patterns)
         -   [Alternatives considered](#alternatives-considered-4)
@@ -198,17 +199,12 @@ fn H(n: i32) {
 }
 ```
 
-#### Unused bindings
+#### Anonymous bindings
 
-A syntax like a binding but with `_` in place of an identifier, or `unused`
-before the name, can be used to ignore part of a value. Names that are qualified
-with the `unused` keyword are visible for name lookup but uses are invalid,
-including when they cause ambiguous name lookup errors. If attempted to be used,
-a compiler error will be shown to the user, instructing them to either remove
-the `unused` qualifier or remove the use.
+A syntax like a binding but with `_` in place of an identifier can be used to
+ignore part of a value.
 
 -   _binding-pattern_ ::= `_` `:` _expression_
--   _binding-pattern_ ::= `unused` _identifier_ `:` _expression_
 
 ```carbon
 fn F(n: i32) {
@@ -233,8 +229,6 @@ fn J(n: i32);
 fn G(_: i32) {}
 // ❌ Error: name of parameter does not match declaration.
 fn H(m: i32) {}
-// ✅ Does not use `n`.
-fn J(unused n: i32);
 ```
 
 ##### Alternatives considered
@@ -345,6 +339,45 @@ _pattern_ `=` _expression_ `;`.
 -   [Make `var` a binding pattern modifier](/proposals/p5164.md#make-var-a-binding-pattern-modifier)
 -   [Initialize storage once pattern matching succeeds](/proposals/p5164.md#initialize-storage-once-pattern-matching-succeeds)
 
+### `unused`
+
+When a name introduced by a binding is not used, a warning is issued. It is
+possible to avoid the warning while keeping a name, by using an `unused` marker.
+
+An `unused` marker indicates that all names in a pattern are visible for name
+lookup but uses are invalid. This includes situations when they cause ambiguous
+name lookup errors. If attempted to be used, a compiler error will be shown to
+the user, instructing them to either remove the `unused` qualifier or remove the
+use.
+
+-   _proper-pattern_ ::= `unused` _proper-pattern_
+
+An `unused` marker can be applied to any pattern and it will apply to all name
+bindings in a pattern. Nesting `unused` markers is an error. When an `unused`
+marker applies only to anonymous bindings `_` and is thus redundant, a warning
+is produced. `var` and `unused` may appear in any order in a pattern.
+
+As specified in [#3763](/proposals/p3763.md), `unused` markers may only appear
+on definitions, not on non-defining declarations. Function redeclarations that
+are also definitions may have difference due to `unused` markers, but they may
+not have different names.
+
+```carbon
+fn J(n: i32);
+
+// ✅ Does not use `n`.
+fn J(unused n: i32) { ... };
+
+fn G() {
+  match ((1, 2)) {
+    // `x` is unused
+    case (var n: i32, unused x: i32) => { F(&n); }
+    // `n` and `m` are both unused
+    case unused (n: i32, m: i32) => { J(42); }
+  }
+}
+```
+
 ### Tuple patterns
 
 A tuple of patterns can be used as a pattern.

toolchain/check/handle_binding_pattern.cpp

@@ -456,4 +456,8 @@ auto HandleParseNode(Context& context, Parse::TemplateBindingNameId node_id)
   return true;
 }
 
+auto HandleParseNode(Context& context, Parse::UnusedPatternId node_id) -> bool {
+  return context.TODO(node_id, "unused");
+}
+
 }  // namespace Carbon::Check

toolchain/check/testdata/class/import_base.carbon

@@ -21,7 +21,7 @@ base class Base {
   fn Unused[self: Self]();
 
   var x: i32;
-  var unused: i32;
+  var unused_y: i32;
 }
 
 class Child {
@@ -35,7 +35,7 @@ library "[[@TEST_NAME]]";
 import library "a";
 
 fn Run() {
-  var a: Child = {.base = {.x = 0, .unused = 1}};
+  var a: Child = {.base = {.x = 0, .unused_y = 1}};
   a.x = 2;
   a.F();
 }
@@ -54,8 +54,8 @@ fn Run() {
 // CHECK:STDOUT:   %Int.generic: %Int.type = struct_value () [concrete]
 // CHECK:STDOUT:   %i32: type = class_type @Int, @Int(%int_32) [concrete]
 // CHECK:STDOUT:   %Base.elem: type = unbound_element_type %Base, %i32 [concrete]
-// CHECK:STDOUT:   %struct_type.x.unused: type = struct_type {.x: %i32, .unused: %i32} [concrete]
-// CHECK:STDOUT:   %complete_type.20c: <witness> = complete_type_witness %struct_type.x.unused [concrete]
+// CHECK:STDOUT:   %struct_type.x.unused_y: type = struct_type {.x: %i32, .unused_y: %i32} [concrete]
+// CHECK:STDOUT:   %complete_type.cf1: <witness> = complete_type_witness %struct_type.x.unused_y [concrete]
 // CHECK:STDOUT:   %Child: type = class_type @Child [concrete]
 // CHECK:STDOUT:   %Child.elem: type = unbound_element_type %Child, %Base [concrete]
 // CHECK:STDOUT:   %struct_type.base: type = struct_type {.base: %Base} [concrete]
@@ -104,8 +104,8 @@ fn Run() {
 // CHECK:STDOUT:   %.loc8: %Base.elem = field_decl x, element0 [concrete]
 // CHECK:STDOUT:   %int_32.loc9: Core.IntLiteral = int_value 32 [concrete = constants.%int_32]
 // CHECK:STDOUT:   %i32.loc9: type = class_type @Int, @Int(constants.%int_32) [concrete = constants.%i32]
-// CHECK:STDOUT:   %.loc9: %Base.elem = field_decl unused, element1 [concrete]
-// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness constants.%struct_type.x.unused [concrete = constants.%complete_type.20c]
+// CHECK:STDOUT:   %.loc9: %Base.elem = field_decl unused_y, element1 [concrete]
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness constants.%struct_type.x.unused_y [concrete = constants.%complete_type.cf1]
 // CHECK:STDOUT:   complete_type_witness = %complete_type
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
@@ -113,7 +113,7 @@ fn Run() {
 // CHECK:STDOUT:   .F = %Base.F.decl
 // CHECK:STDOUT:   .Unused = %Base.Unused.decl
 // CHECK:STDOUT:   .x = %.loc8
-// CHECK:STDOUT:   .unused = %.loc9
+// CHECK:STDOUT:   .unused_y = %.loc9
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @Child {
@@ -143,17 +143,17 @@ fn Run() {
 // CHECK:STDOUT:   %Base: type = class_type @Base [concrete]
 // CHECK:STDOUT:   %int_32: Core.IntLiteral = int_value 32 [concrete]
 // CHECK:STDOUT:   %i32: type = class_type @Int, @Int(%int_32) [concrete]
-// CHECK:STDOUT:   %struct_type.x.unused.7d5: type = struct_type {.x: %i32, .unused: %i32} [concrete]
-// CHECK:STDOUT:   %complete_type.90f: <witness> = complete_type_witness %struct_type.x.unused.7d5 [concrete]
+// CHECK:STDOUT:   %struct_type.x.unused_y.9fc: type = struct_type {.x: %i32, .unused_y: %i32} [concrete]
+// CHECK:STDOUT:   %complete_type.2da: <witness> = complete_type_witness %struct_type.x.unused_y.9fc [concrete]
 // CHECK:STDOUT:   %struct_type.base.27a: type = struct_type {.base: %Base} [concrete]
 // CHECK:STDOUT:   %complete_type.5a1: <witness> = complete_type_witness %struct_type.base.27a [concrete]
 // CHECK:STDOUT:   %pattern_type.454: type = pattern_type %Child [concrete]
 // CHECK:STDOUT:   %int_0.5c6: Core.IntLiteral = int_value 0 [concrete]
 // CHECK:STDOUT:   %int_1.5b8: Core.IntLiteral = int_value 1 [concrete]
-// CHECK:STDOUT:   %struct_type.x.unused.c45: type = struct_type {.x: Core.IntLiteral, .unused: Core.IntLiteral} [concrete]
-// CHECK:STDOUT:   %struct.9cc: %struct_type.x.unused.c45 = struct_value (%int_0.5c6, %int_1.5b8) [concrete]
-// CHECK:STDOUT:   %struct_type.base.6c7: type = struct_type {.base: %struct_type.x.unused.c45} [concrete]
-// CHECK:STDOUT:   %struct.133: %struct_type.base.6c7 = struct_value (%struct.9cc) [concrete]
+// CHECK:STDOUT:   %struct_type.x.unused_y.76a: type = struct_type {.x: Core.IntLiteral, .unused_y: Core.IntLiteral} [concrete]
+// CHECK:STDOUT:   %struct.0bf: %struct_type.x.unused_y.76a = struct_value (%int_0.5c6, %int_1.5b8) [concrete]
+// CHECK:STDOUT:   %struct_type.base.503: type = struct_type {.base: %struct_type.x.unused_y.76a} [concrete]
+// CHECK:STDOUT:   %struct.cb5: %struct_type.base.503 = struct_value (%struct.0bf) [concrete]
 // CHECK:STDOUT:   %ImplicitAs.type.cc7: type = generic_interface_type @ImplicitAs [concrete]
 // CHECK:STDOUT:   %ImplicitAs.generic: %ImplicitAs.type.cc7 = struct_value () [concrete]
 // CHECK:STDOUT:   %ImplicitAs.type.bd9: type = facet_type <@ImplicitAs, @ImplicitAs(%i32)> [concrete]
@@ -199,12 +199,12 @@ fn Run() {
 // CHECK:STDOUT:     import Core//prelude
 // CHECK:STDOUT:     import Core//prelude/...
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %Main.import_ref.239: <witness> = import_ref Main//a, loc10_1, loaded [concrete = constants.%complete_type.90f]
+// CHECK:STDOUT:   %Main.import_ref.8f2: <witness> = import_ref Main//a, loc10_1, loaded [concrete = constants.%complete_type.2da]
 // CHECK:STDOUT:   %Main.import_ref.691 = import_ref Main//a, inst{{[0-9A-F]+}} [no loc], unloaded
 // CHECK:STDOUT:   %Main.import_ref.062: %Base.F.type = import_ref Main//a, loc5_21, loaded [concrete = constants.%Base.F]
 // CHECK:STDOUT:   %Main.import_ref.7b8 = import_ref Main//a, loc6_26, unloaded
 // CHECK:STDOUT:   %Main.import_ref.183: %Base.elem = import_ref Main//a, loc8_8, loaded [concrete = %.61a]
-// CHECK:STDOUT:   %Main.import_ref.adb = import_ref Main//a, loc9_13, unloaded
+// CHECK:STDOUT:   %Main.import_ref.7c0 = import_ref Main//a, loc9_15, unloaded
 // CHECK:STDOUT:   %Main.import_ref.44c: <witness> = import_ref Main//a, loc14_1, loaded [concrete = constants.%complete_type.5a1]
 // CHECK:STDOUT:   %Main.import_ref.d37 = import_ref Main//a, inst{{[0-9A-F]+}} [no loc], unloaded
 // CHECK:STDOUT:   %Main.import_ref.4a6 = import_ref Main//a, loc13_20, unloaded
@@ -240,14 +240,14 @@ fn Run() {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: class @Base [from "a.carbon"] {
-// CHECK:STDOUT:   complete_type_witness = imports.%Main.import_ref.239
+// CHECK:STDOUT:   complete_type_witness = imports.%Main.import_ref.8f2
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .Self = imports.%Main.import_ref.691
 // CHECK:STDOUT:   .F = imports.%Main.import_ref.062
 // CHECK:STDOUT:   .Unused = imports.%Main.import_ref.7b8
 // CHECK:STDOUT:   .x = imports.%Main.import_ref.183
-// CHECK:STDOUT:   .unused = imports.%Main.import_ref.adb
+// CHECK:STDOUT:   .unused_y = imports.%Main.import_ref.7c0
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: fn @Run() {
@@ -259,29 +259,29 @@ fn Run() {
 // CHECK:STDOUT:   %a.var: ref %Child = var %a.var_patt
 // CHECK:STDOUT:   %int_0: Core.IntLiteral = int_value 0 [concrete = constants.%int_0.5c6]
 // CHECK:STDOUT:   %int_1: Core.IntLiteral = int_value 1 [concrete = constants.%int_1.5b8]
-// CHECK:STDOUT:   %.loc7_47.1: %struct_type.x.unused.c45 = struct_literal (%int_0, %int_1) [concrete = constants.%struct.9cc]
-// CHECK:STDOUT:   %.loc7_48.1: %struct_type.base.6c7 = struct_literal (%.loc7_47.1) [concrete = constants.%struct.133]
-// CHECK:STDOUT:   %impl.elem0.loc7_47.1: %.d0f = impl_witness_access constants.%ImplicitAs.impl_witness.377, element0 [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.8ce]
-// CHECK:STDOUT:   %bound_method.loc7_47.1: <bound method> = bound_method %int_0, %impl.elem0.loc7_47.1 [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.bound.6ae]
-// CHECK:STDOUT:   %specific_fn.loc7_47.1: <specific function> = specific_function %impl.elem0.loc7_47.1, @Core.IntLiteral.as.ImplicitAs.impl.Convert(constants.%int_32) [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.specific_fn]
-// CHECK:STDOUT:   %bound_method.loc7_47.2: <bound method> = bound_method %int_0, %specific_fn.loc7_47.1 [concrete = constants.%bound_method.74d]
-// CHECK:STDOUT:   %Core.IntLiteral.as.ImplicitAs.impl.Convert.call.loc7_47.1: init %i32 = call %bound_method.loc7_47.2(%int_0) [concrete = constants.%int_0.263]
-// CHECK:STDOUT:   %.loc7_47.2: init %i32 = converted %int_0, %Core.IntLiteral.as.ImplicitAs.impl.Convert.call.loc7_47.1 [concrete = constants.%int_0.263]
-// CHECK:STDOUT:   %.loc7_48.2: ref %Base = class_element_access %a.var, element0
-// CHECK:STDOUT:   %.loc7_47.3: ref %i32 = class_element_access %.loc7_48.2, element0
-// CHECK:STDOUT:   %.loc7_47.4: init %i32 = initialize_from %.loc7_47.2 to %.loc7_47.3 [concrete = constants.%int_0.263]
-// CHECK:STDOUT:   %impl.elem0.loc7_47.2: %.d0f = impl_witness_access constants.%ImplicitAs.impl_witness.377, element0 [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.8ce]
-// CHECK:STDOUT:   %bound_method.loc7_47.3: <bound method> = bound_method %int_1, %impl.elem0.loc7_47.2 [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.bound.9ed]
-// CHECK:STDOUT:   %specific_fn.loc7_47.2: <specific function> = specific_function %impl.elem0.loc7_47.2, @Core.IntLiteral.as.ImplicitAs.impl.Convert(constants.%int_32) [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.specific_fn]
-// CHECK:STDOUT:   %bound_method.loc7_47.4: <bound method> = bound_method %int_1, %specific_fn.loc7_47.2 [concrete = constants.%bound_method.a30]
-// CHECK:STDOUT:   %Core.IntLiteral.as.ImplicitAs.impl.Convert.call.loc7_47.2: init %i32 = call %bound_method.loc7_47.4(%int_1) [concrete = constants.%int_1.47b]
-// CHECK:STDOUT:   %.loc7_47.5: init %i32 = converted %int_1, %Core.IntLiteral.as.ImplicitAs.impl.Convert.call.loc7_47.2 [concrete = constants.%int_1.47b]
-// CHECK:STDOUT:   %.loc7_47.6: ref %i32 = class_element_access %.loc7_48.2, element1
-// CHECK:STDOUT:   %.loc7_47.7: init %i32 = initialize_from %.loc7_47.5 to %.loc7_47.6 [concrete = constants.%int_1.47b]
-// CHECK:STDOUT:   %.loc7_47.8: init %Base = class_init (%.loc7_47.4, %.loc7_47.7), %.loc7_48.2 [concrete = constants.%Base.val]
-// CHECK:STDOUT:   %.loc7_48.3: init %Base = converted %.loc7_47.1, %.loc7_47.8 [concrete = constants.%Base.val]
-// CHECK:STDOUT:   %.loc7_48.4: init %Child = class_init (%.loc7_48.3), %a.var [concrete = constants.%Child.val]
-// CHECK:STDOUT:   %.loc7_3: init %Child = converted %.loc7_48.1, %.loc7_48.4 [concrete = constants.%Child.val]
+// CHECK:STDOUT:   %.loc7_49.1: %struct_type.x.unused_y.76a = struct_literal (%int_0, %int_1) [concrete = constants.%struct.0bf]
+// CHECK:STDOUT:   %.loc7_50.1: %struct_type.base.503 = struct_literal (%.loc7_49.1) [concrete = constants.%struct.cb5]
+// CHECK:STDOUT:   %impl.elem0.loc7_49.1: %.d0f = impl_witness_access constants.%ImplicitAs.impl_witness.377, element0 [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.8ce]
+// CHECK:STDOUT:   %bound_method.loc7_49.1: <bound method> = bound_method %int_0, %impl.elem0.loc7_49.1 [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.bound.6ae]
+// CHECK:STDOUT:   %specific_fn.loc7_49.1: <specific function> = specific_function %impl.elem0.loc7_49.1, @Core.IntLiteral.as.ImplicitAs.impl.Convert(constants.%int_32) [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.specific_fn]
+// CHECK:STDOUT:   %bound_method.loc7_49.2: <bound method> = bound_method %int_0, %specific_fn.loc7_49.1 [concrete = constants.%bound_method.74d]
+// CHECK:STDOUT:   %Core.IntLiteral.as.ImplicitAs.impl.Convert.call.loc7_49.1: init %i32 = call %bound_method.loc7_49.2(%int_0) [concrete = constants.%int_0.263]
+// CHECK:STDOUT:   %.loc7_49.2: init %i32 = converted %int_0, %Core.IntLiteral.as.ImplicitAs.impl.Convert.call.loc7_49.1 [concrete = constants.%int_0.263]
+// CHECK:STDOUT:   %.loc7_50.2: ref %Base = class_element_access %a.var, element0
+// CHECK:STDOUT:   %.loc7_49.3: ref %i32 = class_element_access %.loc7_50.2, element0
+// CHECK:STDOUT:   %.loc7_49.4: init %i32 = initialize_from %.loc7_49.2 to %.loc7_49.3 [concrete = constants.%int_0.263]
+// CHECK:STDOUT:   %impl.elem0.loc7_49.2: %.d0f = impl_witness_access constants.%ImplicitAs.impl_witness.377, element0 [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.8ce]
+// CHECK:STDOUT:   %bound_method.loc7_49.3: <bound method> = bound_method %int_1, %impl.elem0.loc7_49.2 [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.bound.9ed]
+// CHECK:STDOUT:   %specific_fn.loc7_49.2: <specific function> = specific_function %impl.elem0.loc7_49.2, @Core.IntLiteral.as.ImplicitAs.impl.Convert(constants.%int_32) [concrete = constants.%Core.IntLiteral.as.ImplicitAs.impl.Convert.specific_fn]
+// CHECK:STDOUT:   %bound_method.loc7_49.4: <bound method> = bound_method %int_1, %specific_fn.loc7_49.2 [concrete = constants.%bound_method.a30]
+// CHECK:STDOUT:   %Core.IntLiteral.as.ImplicitAs.impl.Convert.call.loc7_49.2: init %i32 = call %bound_method.loc7_49.4(%int_1) [concrete = constants.%int_1.47b]
+// CHECK:STDOUT:   %.loc7_49.5: init %i32 = converted %int_1, %Core.IntLiteral.as.ImplicitAs.impl.Convert.call.loc7_49.2 [concrete = constants.%int_1.47b]
+// CHECK:STDOUT:   %.loc7_49.6: ref %i32 = class_element_access %.loc7_50.2, element1
+// CHECK:STDOUT:   %.loc7_49.7: init %i32 = initialize_from %.loc7_49.5 to %.loc7_49.6 [concrete = constants.%int_1.47b]
+// CHECK:STDOUT:   %.loc7_49.8: init %Base = class_init (%.loc7_49.4, %.loc7_49.7), %.loc7_50.2 [concrete = constants.%Base.val]
+// CHECK:STDOUT:   %.loc7_50.3: init %Base = converted %.loc7_49.1, %.loc7_49.8 [concrete = constants.%Base.val]
+// CHECK:STDOUT:   %.loc7_50.4: init %Child = class_init (%.loc7_50.3), %a.var [concrete = constants.%Child.val]
+// CHECK:STDOUT:   %.loc7_3: init %Child = converted %.loc7_50.1, %.loc7_50.4 [concrete = constants.%Child.val]
 // CHECK:STDOUT:   assign %a.var, %.loc7_3
 // CHECK:STDOUT:   %Child.ref: type = name_ref Child, imports.%Main.Child [concrete = constants.%Child]
 // CHECK:STDOUT:   %a: ref %Child = ref_binding a, %a.var

toolchain/check/testdata/dataflow/unused.carbon

@@ -0,0 +1,168 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/primitives.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/dataflow/unused.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/dataflow/unused.carbon
+
+// --- todo_ensure_no_warning.carbon
+library "[[@TEST_NAME]]";
+
+fn ReturnVar() -> i32 {
+  var x: i32 = 0;
+  x = 1;
+  return x;
+   // TODO: ensure no warning.
+ }
+
+fn ReturnLet() -> i32 {
+  let y: i32 = 1;
+  return y;
+  // TODO: ensure no warning.
+}
+
+fn RefParam(ref x: i32) {
+  x = 1;
+  // TODO: ensure no warning.
+}
+
+fn ReturnedVar() -> i32 {
+  returned var x: i32 = 0;
+  return var;
+  // TODO: ensure no warning.
+}
+
+fn ReturnVarExplicit() -> i32 {
+  var x: i32 = 0;
+  return x;
+  // TODO: ensure no warning.
+}
+
+fn FieldAccess() {
+  var x: {.a: i32} = {.a = 0};
+  // Accessing a field should count as a use of x.
+  var y: i32 = x.a;
+  // TODO: ensure no warning.
+}
+
+fn ParametersUsed(x: i32) -> i32 {
+  // TODO: ensure no warning.
+  return x;
+}
+
+// --- fail_todo_unused_ref_param.carbon
+library "[[@TEST_NAME]]";
+
+// CHECK:STDERR: fail_todo_unused_ref_param.carbon:[[@LINE+4]]:19: error: semantics TODO: `unused` [SemanticsTodo]
+// CHECK:STDERR: fn RefParamUnused(unused ref y: i32) {
+// CHECK:STDERR:                   ^~~~~~~~~~~~~~~~~
+// CHECK:STDERR:
+fn RefParamUnused(unused ref y: i32) {
+  // TODO: ensure no warning
+}
+
+// --- todo_fail_local_let_named_not_used.carbon
+library "[[@TEST_NAME]]";
+
+fn F() {
+  let x: i32 = 0;
+  // TODO: warning, x is unused.
+}
+
+// --- todo_fail_local_var_named_not_used.carbon
+library "[[@TEST_NAME]]";
+
+fn F() {
+  var a: i32 = 0;
+  // TODO: warning, a is unused.
+}
+
+// --- todo_fail_local_var_shadowed_not_used.carbon
+library "[[@TEST_NAME]]";
+
+fn F() {
+  var x: i32 = 0;
+  if (true) {
+    var x: i32 = 1;
+    // TODO: warning, x is unused.
+  }
+  x = 2;
+  // TODO: warning, x is unused.
+}
+
+// --- todo_fail_param_let_named_not_used.carbon
+library "[[@TEST_NAME]]";
+
+fn F(p: i32) {
+  // TODO: warning, p is unused.
+}
+
+// --- todo_fail_param_ref_named_not_used.carbon
+library "[[@TEST_NAME]]";
+
+fn F(ref y: i32) {
+  // TODO: warning, y is unused.
+}
+
+// --- fail_todo_read_from_param_unused_ref.carbon
+library "[[@TEST_NAME]]";
+
+// CHECK:STDERR: fail_todo_read_from_param_unused_ref.carbon:[[@LINE+4]]:19: error: semantics TODO: `unused` [SemanticsTodo]
+// CHECK:STDERR: fn RefParamUnused(unused ref y: i32) {
+// CHECK:STDERR:                   ^~~~~~~~~~~~~~~~~
+// CHECK:STDERR:
+fn RefParamUnused(unused ref y: i32) {
+  var _: i32 = y;
+  // TODO: ensure error, marked "unused" but used.
+}
+
+// --- fail_todo_fail_unused_returned_var.carbon
+library "[[@TEST_NAME]]";
+
+fn F() -> i32 {
+  // CHECK:STDERR: fail_todo_fail_unused_returned_var.carbon:[[@LINE+4]]:16: error: semantics TODO: `unused` [SemanticsTodo]
+  // CHECK:STDERR:   returned var unused x: i32 = 0;
+  // CHECK:STDERR:                ^~~~~~~~~~~~~
+  // CHECK:STDERR:
+  returned var unused x: i32 = 0;
+  // TODO: ensure error, marked "unused" but used.
+  return var;
+}
+
+// --- fail_todo_fail_read_from_unused_local_var.carbon
+library "[[@TEST_NAME]]";
+
+fn F(x: i32) {
+  var _: i32 = x;
+  // CHECK:STDERR: fail_todo_fail_read_from_unused_local_var.carbon:[[@LINE+4]]:7: error: semantics TODO: `unused` [SemanticsTodo]
+  // CHECK:STDERR:   var unused y: i32 = x;
+  // CHECK:STDERR:       ^~~~~~~~~~~~~
+  // CHECK:STDERR:
+  var unused y: i32 = x;
+  // TODO: error, marked "unused", but used
+  var unused z: i32 = y;
+}
+
+// --- fail_todo_match.carbon
+library "[[@TEST_NAME]]";
+
+fn F() -> i32 {
+  var x: i32 = 3;
+  // CHECK:STDERR: fail_todo_match.carbon:[[@LINE+4]]:3: error: semantics TODO: `HandleMatchIntroducer` [SemanticsTodo]
+  // CHECK:STDERR:   match (f(x)) {
+  // CHECK:STDERR:   ^~~~~
+  // CHECK:STDERR:
+  match (f(x)) {
+    case unused var (a: i32, b: i32) => { return 0; }
+    case (a: i32) if (a < 0) => { return 2; }
+    case var unused a: i32 if (a != x) => { return 3; }
+    default => { return 4; }
+  }
+  return 0;
+  // TODO: ensure no warning (after match is implemented).
+}

toolchain/diagnostics/diagnostic_kind.def

@@ -109,6 +109,7 @@ CARBON_DIAGNOSTIC_KIND(ExpectedVarAfterReturned)
 CARBON_DIAGNOSTIC_KIND(ExpectedChoiceDefinition)
 CARBON_DIAGNOSTIC_KIND(ExpectedChoiceAlternativeName)
 CARBON_DIAGNOSTIC_KIND(NestedVar)
+CARBON_DIAGNOSTIC_KIND(NestedUnused)
 CARBON_DIAGNOSTIC_KIND(OperatorRequiresParentheses)
 CARBON_DIAGNOSTIC_KIND(RefInsideVar)
 CARBON_DIAGNOSTIC_KIND(StatementOperatorAsSubExpr)

toolchain/lex/token_kind.def

@@ -218,6 +218,7 @@ CARBON_KEYWORD_TOKEN(Type,                "type")
 // Underscore is tokenized as a keyword because it's part of identifiers.
 CARBON_KEYWORD_TOKEN(Underscore,          "_")
 CARBON_KEYWORD_TOKEN(Unsafe,              "unsafe")
+CARBON_KEYWORD_TOKEN(Unused,              "unused")
 CARBON_KEYWORD_TOKEN(Virtual,             "virtual")
 CARBON_TOKEN_WITH_VIRTUAL_NODE(
   CARBON_KEYWORD_TOKEN(Where,             "where"))

toolchain/parse/context.h

@@ -95,6 +95,13 @@ class Context {
     // could help catch errors.
     bool in_var_pattern : 1 = false;
 
+    // Set to true to indicate that this state is handling a pattern nested
+    // inside an `unused` pattern.
+    // TODO: This is meaningful only for patterns, and the precedence fields
+    // are meaningful only for expressions, so expressing them as a union
+    // could help catch errors.
+    bool in_unused_pattern : 1 = false;
+
     // Precedence information used by expression states in order to determine
     // operator precedence. The ambient_precedence deals with how the expression
     // should interact with outside context, while the lhs_precedence is
@@ -112,7 +119,7 @@ class Context {
 
   // We expect State to fit into 12 bytes:
   //   state = 1 byte
-  //   has_error and in_var_pattern = 1 byte
+  //   has_error, in_var_pattern, and in_unused_pattern = 1 byte
   //   ambient_precedence = 1 byte
   //   lhs_precedence = 1 byte
   //   token = 4 bytes
@@ -318,11 +325,14 @@ class Context {
                .subtree_start = tree_->size()});
   }
 
-  // Pushes a new state for handling a pattern. `in_var_pattern` indicates
-  // whether that pattern is nested inside a `var` pattern.
-  auto PushStateForPattern(StateKind kind, bool in_var_pattern) -> void {
+  // Pushes a new state for handling a pattern. `in_var_pattern` and
+  // `in_unused_pattern` indicate whether that pattern is nested inside a `var`
+  // or `unused` pattern.
+  auto PushStateForPattern(StateKind kind, bool in_var_pattern,
+                           bool in_unused_pattern) -> void {
     PushState({.kind = kind,
                .in_var_pattern = in_var_pattern,
+               .in_unused_pattern = in_unused_pattern,
                .token = *position_,
                .subtree_start = tree_->size()});
   }

toolchain/parse/handle_pattern.cpp

@@ -12,15 +12,23 @@ auto HandlePattern(Context& context) -> void {
   switch (context.PositionKind()) {
     case Lex::TokenKind::OpenParen:
       context.PushStateForPattern(StateKind::PatternListAsTuple,
-                                  state.in_var_pattern);
+                                  state.in_var_pattern,
+                                  state.in_unused_pattern);
       break;
     case Lex::TokenKind::Var:
       context.PushStateForPattern(StateKind::VariablePattern,
-                                  state.in_var_pattern);
+                                  state.in_var_pattern,
+                                  state.in_unused_pattern);
+      break;
+    case Lex::TokenKind::Unused:
+      context.PushStateForPattern(StateKind::UnusedPattern,
+                                  state.in_var_pattern,
+                                  state.in_unused_pattern);
       break;
     default:
       context.PushStateForPattern(StateKind::BindingPattern,
-                                  state.in_var_pattern);
+                                  state.in_var_pattern,
+                                  state.in_unused_pattern);
       break;
   }
 }

toolchain/parse/handle_pattern_list.cpp

@@ -12,8 +12,10 @@ static auto HandlePatternListElement(Context& context, StateKind pattern_state,
                                      StateKind finish_state_kind) -> void {
   auto state = context.PopState();
 
-  context.PushStateForPattern(finish_state_kind, state.in_var_pattern);
-  context.PushStateForPattern(pattern_state, state.in_var_pattern);
+  context.PushStateForPattern(finish_state_kind, state.in_var_pattern,
+                              state.in_unused_pattern);
+  context.PushStateForPattern(pattern_state, state.in_var_pattern,
+                              state.in_unused_pattern);
 }
 
 auto HandlePatternListElementAsTuple(Context& context) -> void {
@@ -44,7 +46,8 @@ static auto HandlePatternListElementFinish(Context& context,
   if (context.ConsumeListToken(NodeKind::PatternListComma, close_token,
                                state.has_error) ==
       Context::ListTokenKind::Comma) {
-    context.PushStateForPattern(param_state_kind, state.in_var_pattern);
+    context.PushStateForPattern(param_state_kind, state.in_var_pattern,
+                                state.in_unused_pattern);
   }
 }
 
@@ -71,11 +74,13 @@ static auto HandlePatternList(Context& context, NodeKind node_kind,
     -> void {
   auto state = context.PopState();
 
-  context.PushStateForPattern(finish_state, state.in_var_pattern);
+  context.PushStateForPattern(finish_state, state.in_var_pattern,
+                              state.in_unused_pattern);
   context.AddLeafNode(node_kind, context.ConsumeChecked(open_token_kind));
 
   if (!context.PositionIs(close_token_kind)) {
-    context.PushStateForPattern(param_state, state.in_var_pattern);
+    context.PushStateForPattern(param_state, state.in_var_pattern,
+                                state.in_unused_pattern);
   }
 }
 

toolchain/parse/handle_unused.cpp

@@ -0,0 +1,36 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/parse/context.h"
+#include "toolchain/parse/handle.h"
+
+namespace Carbon::Parse {
+
+auto HandleUnusedPattern(Context& context) -> void {
+  auto state = context.PopState();
+  if (state.in_unused_pattern) {
+    CARBON_DIAGNOSTIC(NestedUnused, Error,
+                      "`unused` nested within another `unused`");
+    context.emitter().Emit(*context.position(), NestedUnused);
+    state.has_error = true;
+  }
+  context.PushState(StateKind::FinishUnusedPattern);
+  context.ConsumeChecked(Lex::TokenKind::Unused);
+
+  context.PushStateForPattern(StateKind::Pattern, state.in_var_pattern,
+                              /*in_unused_pattern=*/true);
+}
+
+auto HandleFinishUnusedPattern(Context& context) -> void {
+  auto state = context.PopState();
+  context.AddNode(NodeKind::UnusedPattern, state.token, state.has_error);
+
+  // Propagate errors to the parent state so that they can take different
+  // actions on invalid patterns.
+  if (state.has_error) {
+    context.ReturnErrorOnState();
+  }
+}
+
+}  // namespace Carbon::Parse

toolchain/parse/handle_var.cpp

@@ -25,7 +25,8 @@ static auto HandleVar(Context& context, StateKind finish_state_kind,
     context.AddLeafNode(NodeKind::ReturnedModifier, returned_token);
   }
 
-  context.PushStateForPattern(StateKind::Pattern, /*in_var_pattern=*/true);
+  context.PushStateForPattern(StateKind::Pattern, /*in_var_pattern=*/true,
+                              /*in_unused_pattern=*/false);
 }
 
 auto HandleVarAsRegular(Context& context) -> void {
@@ -142,7 +143,8 @@ auto HandleVariablePattern(Context& context) -> void {
   context.PushState(StateKind::FinishVariablePattern);
   context.ConsumeChecked(Lex::TokenKind::Var);
 
-  context.PushStateForPattern(StateKind::Pattern, /*in_var_pattern=*/true);
+  context.PushStateForPattern(StateKind::Pattern, /*in_var_pattern=*/true,
+                              state.in_unused_pattern);
 }
 
 auto HandleFinishVariablePattern(Context& context) -> void {

toolchain/parse/node_kind.def

@@ -170,6 +170,7 @@ CARBON_PARSE_NODE_KIND(ArrayExprComma)
 CARBON_PARSE_NODE_KIND(ArrayExpr)
 
 CARBON_PARSE_NODE_KIND(RefBindingName)
+CARBON_PARSE_NODE_KIND(UnusedPattern)
 CARBON_PARSE_NODE_KIND(LetBindingPattern)
 CARBON_PARSE_NODE_KIND(AssociatedConstantNameAndType)
 CARBON_PARSE_NODE_KIND(VarBindingPattern)

toolchain/parse/state.def

@@ -1035,6 +1035,10 @@ CARBON_PARSE_STATE(ParenExprFinish)
 //  ...
 // ^
 //   1. BindingPattern
+//
+//  ...
+// ^
+//   1. UnusedPattern
 CARBON_PARSE_STATE(Pattern)
 
 // Handles the initial part of a binding pattern, enqueuing type expression
@@ -1085,6 +1089,21 @@ CARBON_PARSE_STATE(VariablePattern)
 //   (state done)
 CARBON_PARSE_STATE(FinishVariablePattern)
 
+// Handles `unused` in a pattern context.
+//
+// unused ...
+// ^~~~~~
+//   1. Pattern
+//   2. FinishUnusedPattern
+CARBON_PARSE_STATE(UnusedPattern)
+
+// Finishes `unused` in a pattern context.
+//
+// unused ...
+//           ^
+//   (state done)
+CARBON_PARSE_STATE(FinishUnusedPattern)
+
 // Handles a single statement. While typically within a statement block, this
 // can also be used for error recovery where we expect a statement block and
 // are missing braces.

toolchain/parse/testdata/var/unused.carbon

@@ -0,0 +1,470 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/parse/testdata/var/unused.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/var/unused.carbon
+
+// --- fail_unused_does_not_start_declaration.carbon
+
+// CHECK:STDERR: fail_unused_does_not_start_declaration.carbon:[[@LINE+4]]:1: error: unrecognized declaration introducer [UnrecognizedDecl]
+// CHECK:STDERR: unused y: i32;
+// CHECK:STDERR: ^~~~~~
+// CHECK:STDERR:
+unused y: i32;
+
+// --- let_var_unused.carbon
+var unused y: i32;
+let unused y: i32;
+let unused ref w: i32 = 0;
+
+fn F(unused x: i32, var unused b: i32, unused ref c: i32) {
+  var unused y: i32;
+  let unused z: i32;
+  let unused ref w: i32 = 0;
+  let unused ref t: str = "hello";
+}
+
+// --- unused_tuple.carbon
+let (x: (), var unused y: ()) = ((), ());
+let (x: (), unused var y: ()) = ((), ());
+
+let unused (x: (), y: ()) = ((), ());
+let var (x: (), unused y: ()) = ((), ());
+let var unused (x: (), y: ()) = ((), ());
+
+// --- var_unused_function.carbon
+fn F(x: (), var unused y: ());
+
+// --- var_unused_match.carbon
+fn F() -> i32 {
+  var x: i32 = 3;
+  match (f(x)) {
+    case unused var (a: i32, b: i32) => { return 0; }
+    case (a: i32) if (a < 0) => { return 2; }
+    case var unused a: i32 if (a != x) => { return 3; }
+    default => { return 4; }
+  }
+  return 0;
+}
+
+// --- unused_var_function.carbon
+fn F(x: (), unused var y: ());
+
+// --- fail_ref_unused.carbon
+
+// CHECK:STDERR: fail_ref_unused.carbon:[[@LINE+4]]:10: error: expected name in binding pattern [ExpectedBindingPattern]
+// CHECK:STDERR: fn F(ref unused y: i32);
+// CHECK:STDERR:          ^~~~~~
+// CHECK:STDERR:
+fn F(ref unused y: i32);
+
+// --- fail_nested.carbon
+
+// CHECK:STDERR: fail_nested.carbon:[[@LINE+4]]:12: error: `unused` nested within another `unused` [NestedUnused]
+// CHECK:STDERR: var unused unused x: i32 = 0;
+// CHECK:STDERR:            ^~~~~~
+// CHECK:STDERR:
+var unused unused x: i32 = 0;
+
+// --- fail_nested_parens.carbon
+
+// CHECK:STDERR: fail_nested_parens.carbon:[[@LINE+4]]:13: error: `unused` nested within another `unused` [NestedUnused]
+// CHECK:STDERR: var unused (unused y: i32) = (0,);
+// CHECK:STDERR:             ^~~~~~
+// CHECK:STDERR:
+var unused (unused y: i32) = (0,);
+
+// CHECK:STDOUT: - filename: fail_unused_does_not_start_declaration.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'InvalidParseStart', text: 'unused', has_error: yes},
+// CHECK:STDOUT:     {kind: 'InvalidParseSubtree', text: ';', has_error: yes, subtree_size: 2},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
+// CHECK:STDOUT: - filename: let_var_unused.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'VariableIntroducer', text: 'var'},
+// CHECK:STDOUT:             {kind: 'IdentifierNameNotBeforeParams', text: 'y'},
+// CHECK:STDOUT:             {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:           {kind: 'VarBindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:         {kind: 'UnusedPattern', text: 'unused', subtree_size: 4},
+// CHECK:STDOUT:       {kind: 'VariablePattern', text: 'var', subtree_size: 5},
+// CHECK:STDOUT:     {kind: 'VariableDecl', text: ';', subtree_size: 7},
+// CHECK:STDOUT:       {kind: 'LetIntroducer', text: 'let'},
+// CHECK:STDOUT:           {kind: 'IdentifierNameNotBeforeParams', text: 'y'},
+// CHECK:STDOUT:           {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:         {kind: 'LetBindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'UnusedPattern', text: 'unused', subtree_size: 4},
+// CHECK:STDOUT:     {kind: 'LetDecl', text: ';', subtree_size: 6},
+// CHECK:STDOUT:       {kind: 'LetIntroducer', text: 'let'},
+// CHECK:STDOUT:             {kind: 'IdentifierNameNotBeforeParams', text: 'w'},
+// CHECK:STDOUT:           {kind: 'RefBindingName', text: 'ref', subtree_size: 2},
+// CHECK:STDOUT:           {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:         {kind: 'LetBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:       {kind: 'UnusedPattern', text: 'unused', subtree_size: 5},
+// CHECK:STDOUT:       {kind: 'LetInitializer', text: '='},
+// CHECK:STDOUT:       {kind: 'IntLiteral', text: '0'},
+// CHECK:STDOUT:     {kind: 'LetDecl', text: ';', subtree_size: 9},
+// CHECK:STDOUT:         {kind: 'FunctionIntroducer', text: 'fn'},
+// CHECK:STDOUT:         {kind: 'IdentifierNameBeforeParams', text: 'F'},
+// CHECK:STDOUT:           {kind: 'ExplicitParamListStart', text: '('},
+// CHECK:STDOUT:               {kind: 'IdentifierNameNotBeforeParams', text: 'x'},
+// CHECK:STDOUT:               {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:             {kind: 'LetBindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:           {kind: 'UnusedPattern', text: 'unused', subtree_size: 4},
+// CHECK:STDOUT:           {kind: 'PatternListComma', text: ','},
+// CHECK:STDOUT:                 {kind: 'IdentifierNameNotBeforeParams', text: 'b'},
+// CHECK:STDOUT:                 {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:               {kind: 'VarBindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:             {kind: 'UnusedPattern', text: 'unused', subtree_size: 4},
+// CHECK:STDOUT:           {kind: 'VariablePattern', text: 'var', subtree_size: 5},
+// CHECK:STDOUT:           {kind: 'PatternListComma', text: ','},
+// CHECK:STDOUT:                 {kind: 'IdentifierNameNotBeforeParams', text: 'c'},
+// CHECK:STDOUT:               {kind: 'RefBindingName', text: 'ref', subtree_size: 2},
+// CHECK:STDOUT:               {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:             {kind: 'LetBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:           {kind: 'UnusedPattern', text: 'unused', subtree_size: 5},
+// CHECK:STDOUT:         {kind: 'ExplicitParamList', text: ')', subtree_size: 18},
+// CHECK:STDOUT:       {kind: 'FunctionDefinitionStart', text: '{', subtree_size: 21},
+// CHECK:STDOUT:         {kind: 'VariableIntroducer', text: 'var'},
+// CHECK:STDOUT:               {kind: 'IdentifierNameNotBeforeParams', text: 'y'},
+// CHECK:STDOUT:               {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:             {kind: 'VarBindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:           {kind: 'UnusedPattern', text: 'unused', subtree_size: 4},
+// CHECK:STDOUT:         {kind: 'VariablePattern', text: 'var', subtree_size: 5},
+// CHECK:STDOUT:       {kind: 'VariableDecl', text: ';', subtree_size: 7},
+// CHECK:STDOUT:         {kind: 'LetIntroducer', text: 'let'},
+// CHECK:STDOUT:             {kind: 'IdentifierNameNotBeforeParams', text: 'z'},
+// CHECK:STDOUT:             {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:           {kind: 'LetBindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:         {kind: 'UnusedPattern', text: 'unused', subtree_size: 4},
+// CHECK:STDOUT:       {kind: 'LetDecl', text: ';', subtree_size: 6},
+// CHECK:STDOUT:         {kind: 'LetIntroducer', text: 'let'},
+// CHECK:STDOUT:               {kind: 'IdentifierNameNotBeforeParams', text: 'w'},
+// CHECK:STDOUT:             {kind: 'RefBindingName', text: 'ref', subtree_size: 2},
+// CHECK:STDOUT:             {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:           {kind: 'LetBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:         {kind: 'UnusedPattern', text: 'unused', subtree_size: 5},
+// CHECK:STDOUT:         {kind: 'LetInitializer', text: '='},
+// CHECK:STDOUT:         {kind: 'IntLiteral', text: '0'},
+// CHECK:STDOUT:       {kind: 'LetDecl', text: ';', subtree_size: 9},
+// CHECK:STDOUT:         {kind: 'LetIntroducer', text: 'let'},
+// CHECK:STDOUT:               {kind: 'IdentifierNameNotBeforeParams', text: 't'},
+// CHECK:STDOUT:             {kind: 'RefBindingName', text: 'ref', subtree_size: 2},
+// CHECK:STDOUT:             {kind: 'StringTypeLiteral', text: 'str'},
+// CHECK:STDOUT:           {kind: 'LetBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:         {kind: 'UnusedPattern', text: 'unused', subtree_size: 5},
+// CHECK:STDOUT:         {kind: 'LetInitializer', text: '='},
+// CHECK:STDOUT:         {kind: 'StringLiteral', text: '"hello"'},
+// CHECK:STDOUT:       {kind: 'LetDecl', text: ';', subtree_size: 9},
+// CHECK:STDOUT:     {kind: 'FunctionDefinition', text: '}', subtree_size: 53},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
+// CHECK:STDOUT: - filename: unused_tuple.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'LetIntroducer', text: 'let'},
+// CHECK:STDOUT:         {kind: 'TuplePatternStart', text: '('},
+// CHECK:STDOUT:           {kind: 'IdentifierNameNotBeforeParams', text: 'x'},
+// CHECK:STDOUT:             {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:           {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:         {kind: 'LetBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:         {kind: 'PatternListComma', text: ','},
+// CHECK:STDOUT:               {kind: 'IdentifierNameNotBeforeParams', text: 'y'},
+// CHECK:STDOUT:                 {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:               {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:             {kind: 'VarBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:           {kind: 'UnusedPattern', text: 'unused', subtree_size: 5},
+// CHECK:STDOUT:         {kind: 'VariablePattern', text: 'var', subtree_size: 6},
+// CHECK:STDOUT:       {kind: 'TuplePattern', text: ')', subtree_size: 13},
+// CHECK:STDOUT:       {kind: 'LetInitializer', text: '='},
+// CHECK:STDOUT:         {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:           {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:         {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:         {kind: 'TupleLiteralComma', text: ','},
+// CHECK:STDOUT:           {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:         {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:       {kind: 'TupleLiteral', text: ')', subtree_size: 7},
+// CHECK:STDOUT:     {kind: 'LetDecl', text: ';', subtree_size: 23},
+// CHECK:STDOUT:       {kind: 'LetIntroducer', text: 'let'},
+// CHECK:STDOUT:         {kind: 'TuplePatternStart', text: '('},
+// CHECK:STDOUT:           {kind: 'IdentifierNameNotBeforeParams', text: 'x'},
+// CHECK:STDOUT:             {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:           {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:         {kind: 'LetBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:         {kind: 'PatternListComma', text: ','},
+// CHECK:STDOUT:               {kind: 'IdentifierNameNotBeforeParams', text: 'y'},
+// CHECK:STDOUT:                 {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:               {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:             {kind: 'VarBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:           {kind: 'VariablePattern', text: 'var', subtree_size: 5},
+// CHECK:STDOUT:         {kind: 'UnusedPattern', text: 'unused', subtree_size: 6},
+// CHECK:STDOUT:       {kind: 'TuplePattern', text: ')', subtree_size: 13},
+// CHECK:STDOUT:       {kind: 'LetInitializer', text: '='},
+// CHECK:STDOUT:         {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:           {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:         {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:         {kind: 'TupleLiteralComma', text: ','},
+// CHECK:STDOUT:           {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:         {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:       {kind: 'TupleLiteral', text: ')', subtree_size: 7},
+// CHECK:STDOUT:     {kind: 'LetDecl', text: ';', subtree_size: 23},
+// CHECK:STDOUT:       {kind: 'LetIntroducer', text: 'let'},
+// CHECK:STDOUT:           {kind: 'TuplePatternStart', text: '('},
+// CHECK:STDOUT:             {kind: 'IdentifierNameNotBeforeParams', text: 'x'},
+// CHECK:STDOUT:               {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:             {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:           {kind: 'LetBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:           {kind: 'PatternListComma', text: ','},
+// CHECK:STDOUT:             {kind: 'IdentifierNameNotBeforeParams', text: 'y'},
+// CHECK:STDOUT:               {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:             {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:           {kind: 'LetBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:         {kind: 'TuplePattern', text: ')', subtree_size: 11},
+// CHECK:STDOUT:       {kind: 'UnusedPattern', text: 'unused', subtree_size: 12},
+// CHECK:STDOUT:       {kind: 'LetInitializer', text: '='},
+// CHECK:STDOUT:         {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:           {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:         {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:         {kind: 'TupleLiteralComma', text: ','},
+// CHECK:STDOUT:           {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:         {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:       {kind: 'TupleLiteral', text: ')', subtree_size: 7},
+// CHECK:STDOUT:     {kind: 'LetDecl', text: ';', subtree_size: 22},
+// CHECK:STDOUT:       {kind: 'LetIntroducer', text: 'let'},
+// CHECK:STDOUT:           {kind: 'TuplePatternStart', text: '('},
+// CHECK:STDOUT:             {kind: 'IdentifierNameNotBeforeParams', text: 'x'},
+// CHECK:STDOUT:               {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:             {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:           {kind: 'VarBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:           {kind: 'PatternListComma', text: ','},
+// CHECK:STDOUT:               {kind: 'IdentifierNameNotBeforeParams', text: 'y'},
+// CHECK:STDOUT:                 {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:               {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:             {kind: 'VarBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:           {kind: 'UnusedPattern', text: 'unused', subtree_size: 5},
+// CHECK:STDOUT:         {kind: 'TuplePattern', text: ')', subtree_size: 12},
+// CHECK:STDOUT:       {kind: 'VariablePattern', text: 'var', subtree_size: 13},
+// CHECK:STDOUT:       {kind: 'LetInitializer', text: '='},
+// CHECK:STDOUT:         {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:           {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:         {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:         {kind: 'TupleLiteralComma', text: ','},
+// CHECK:STDOUT:           {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:         {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:       {kind: 'TupleLiteral', text: ')', subtree_size: 7},
+// CHECK:STDOUT:     {kind: 'LetDecl', text: ';', subtree_size: 23},
+// CHECK:STDOUT:       {kind: 'LetIntroducer', text: 'let'},
+// CHECK:STDOUT:             {kind: 'TuplePatternStart', text: '('},
+// CHECK:STDOUT:               {kind: 'IdentifierNameNotBeforeParams', text: 'x'},
+// CHECK:STDOUT:                 {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:               {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:             {kind: 'VarBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:             {kind: 'PatternListComma', text: ','},
+// CHECK:STDOUT:               {kind: 'IdentifierNameNotBeforeParams', text: 'y'},
+// CHECK:STDOUT:                 {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:               {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:             {kind: 'VarBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:           {kind: 'TuplePattern', text: ')', subtree_size: 11},
+// CHECK:STDOUT:         {kind: 'UnusedPattern', text: 'unused', subtree_size: 12},
+// CHECK:STDOUT:       {kind: 'VariablePattern', text: 'var', subtree_size: 13},
+// CHECK:STDOUT:       {kind: 'LetInitializer', text: '='},
+// CHECK:STDOUT:         {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:           {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:         {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:         {kind: 'TupleLiteralComma', text: ','},
+// CHECK:STDOUT:           {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:         {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:       {kind: 'TupleLiteral', text: ')', subtree_size: 7},
+// CHECK:STDOUT:     {kind: 'LetDecl', text: ';', subtree_size: 23},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
+// CHECK:STDOUT: - filename: var_unused_function.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'FunctionIntroducer', text: 'fn'},
+// CHECK:STDOUT:       {kind: 'IdentifierNameBeforeParams', text: 'F'},
+// CHECK:STDOUT:         {kind: 'ExplicitParamListStart', text: '('},
+// CHECK:STDOUT:           {kind: 'IdentifierNameNotBeforeParams', text: 'x'},
+// CHECK:STDOUT:             {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:           {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:         {kind: 'LetBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:         {kind: 'PatternListComma', text: ','},
+// CHECK:STDOUT:               {kind: 'IdentifierNameNotBeforeParams', text: 'y'},
+// CHECK:STDOUT:                 {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:               {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:             {kind: 'VarBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:           {kind: 'UnusedPattern', text: 'unused', subtree_size: 5},
+// CHECK:STDOUT:         {kind: 'VariablePattern', text: 'var', subtree_size: 6},
+// CHECK:STDOUT:       {kind: 'ExplicitParamList', text: ')', subtree_size: 13},
+// CHECK:STDOUT:     {kind: 'FunctionDecl', text: ';', subtree_size: 16},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
+// CHECK:STDOUT: - filename: var_unused_match.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:         {kind: 'FunctionIntroducer', text: 'fn'},
+// CHECK:STDOUT:         {kind: 'IdentifierNameBeforeParams', text: 'F'},
+// CHECK:STDOUT:           {kind: 'ExplicitParamListStart', text: '('},
+// CHECK:STDOUT:         {kind: 'ExplicitParamList', text: ')', subtree_size: 2},
+// CHECK:STDOUT:           {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:         {kind: 'ReturnType', text: '->', subtree_size: 2},
+// CHECK:STDOUT:       {kind: 'FunctionDefinitionStart', text: '{', subtree_size: 7},
+// CHECK:STDOUT:         {kind: 'VariableIntroducer', text: 'var'},
+// CHECK:STDOUT:             {kind: 'IdentifierNameNotBeforeParams', text: 'x'},
+// CHECK:STDOUT:             {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:           {kind: 'VarBindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:         {kind: 'VariablePattern', text: 'var', subtree_size: 4},
+// CHECK:STDOUT:         {kind: 'VariableInitializer', text: '='},
+// CHECK:STDOUT:         {kind: 'IntLiteral', text: '3'},
+// CHECK:STDOUT:       {kind: 'VariableDecl', text: ';', subtree_size: 8},
+// CHECK:STDOUT:           {kind: 'MatchIntroducer', text: 'match'},
+// CHECK:STDOUT:             {kind: 'MatchConditionStart', text: '('},
+// CHECK:STDOUT:                 {kind: 'IdentifierNameExpr', text: 'f'},
+// CHECK:STDOUT:               {kind: 'CallExprStart', text: '(', subtree_size: 2},
+// CHECK:STDOUT:               {kind: 'IdentifierNameExpr', text: 'x'},
+// CHECK:STDOUT:             {kind: 'CallExpr', text: ')', subtree_size: 4},
+// CHECK:STDOUT:           {kind: 'MatchCondition', text: ')', subtree_size: 6},
+// CHECK:STDOUT:         {kind: 'MatchStatementStart', text: '{', subtree_size: 8},
+// CHECK:STDOUT:             {kind: 'MatchCaseIntroducer', text: 'case'},
+// CHECK:STDOUT:                   {kind: 'TuplePatternStart', text: '('},
+// CHECK:STDOUT:                     {kind: 'IdentifierNameNotBeforeParams', text: 'a'},
+// CHECK:STDOUT:                     {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:                   {kind: 'VarBindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:                   {kind: 'PatternListComma', text: ','},
+// CHECK:STDOUT:                     {kind: 'IdentifierNameNotBeforeParams', text: 'b'},
+// CHECK:STDOUT:                     {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:                   {kind: 'VarBindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:                 {kind: 'TuplePattern', text: ')', subtree_size: 9},
+// CHECK:STDOUT:               {kind: 'VariablePattern', text: 'var', subtree_size: 10},
+// CHECK:STDOUT:             {kind: 'UnusedPattern', text: 'unused', subtree_size: 11},
+// CHECK:STDOUT:             {kind: 'MatchCaseEqualGreater', text: '=>'},
+// CHECK:STDOUT:           {kind: 'MatchCaseStart', text: '{', subtree_size: 14},
+// CHECK:STDOUT:             {kind: 'ReturnStatementStart', text: 'return'},
+// CHECK:STDOUT:             {kind: 'IntLiteral', text: '0'},
+// CHECK:STDOUT:           {kind: 'ReturnStatement', text: ';', subtree_size: 3},
+// CHECK:STDOUT:         {kind: 'MatchCase', text: '}', subtree_size: 18},
+// CHECK:STDOUT:             {kind: 'MatchCaseIntroducer', text: 'case'},
+// CHECK:STDOUT:               {kind: 'TuplePatternStart', text: '('},
+// CHECK:STDOUT:                 {kind: 'IdentifierNameNotBeforeParams', text: 'a'},
+// CHECK:STDOUT:                 {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:               {kind: 'LetBindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:             {kind: 'TuplePattern', text: ')', subtree_size: 5},
+// CHECK:STDOUT:               {kind: 'MatchCaseGuardIntroducer', text: 'if'},
+// CHECK:STDOUT:               {kind: 'MatchCaseGuardStart', text: '('},
+// CHECK:STDOUT:                 {kind: 'IdentifierNameExpr', text: 'a'},
+// CHECK:STDOUT:                 {kind: 'IntLiteral', text: '0'},
+// CHECK:STDOUT:               {kind: 'InfixOperatorLess', text: '<', subtree_size: 3},
+// CHECK:STDOUT:             {kind: 'MatchCaseGuard', text: ')', subtree_size: 6},
+// CHECK:STDOUT:             {kind: 'MatchCaseEqualGreater', text: '=>'},
+// CHECK:STDOUT:           {kind: 'MatchCaseStart', text: '{', subtree_size: 14},
+// CHECK:STDOUT:             {kind: 'ReturnStatementStart', text: 'return'},
+// CHECK:STDOUT:             {kind: 'IntLiteral', text: '2'},
+// CHECK:STDOUT:           {kind: 'ReturnStatement', text: ';', subtree_size: 3},
+// CHECK:STDOUT:         {kind: 'MatchCase', text: '}', subtree_size: 18},
+// CHECK:STDOUT:             {kind: 'MatchCaseIntroducer', text: 'case'},
+// CHECK:STDOUT:                   {kind: 'IdentifierNameNotBeforeParams', text: 'a'},
+// CHECK:STDOUT:                   {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:                 {kind: 'VarBindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:               {kind: 'UnusedPattern', text: 'unused', subtree_size: 4},
+// CHECK:STDOUT:             {kind: 'VariablePattern', text: 'var', subtree_size: 5},
+// CHECK:STDOUT:               {kind: 'MatchCaseGuardIntroducer', text: 'if'},
+// CHECK:STDOUT:               {kind: 'MatchCaseGuardStart', text: '('},
+// CHECK:STDOUT:                 {kind: 'IdentifierNameExpr', text: 'a'},
+// CHECK:STDOUT:                 {kind: 'IdentifierNameExpr', text: 'x'},
+// CHECK:STDOUT:               {kind: 'InfixOperatorExclaimEqual', text: '!=', subtree_size: 3},
+// CHECK:STDOUT:             {kind: 'MatchCaseGuard', text: ')', subtree_size: 6},
+// CHECK:STDOUT:             {kind: 'MatchCaseEqualGreater', text: '=>'},
+// CHECK:STDOUT:           {kind: 'MatchCaseStart', text: '{', subtree_size: 14},
+// CHECK:STDOUT:             {kind: 'ReturnStatementStart', text: 'return'},
+// CHECK:STDOUT:             {kind: 'IntLiteral', text: '3'},
+// CHECK:STDOUT:           {kind: 'ReturnStatement', text: ';', subtree_size: 3},
+// CHECK:STDOUT:         {kind: 'MatchCase', text: '}', subtree_size: 18},
+// CHECK:STDOUT:             {kind: 'MatchDefaultIntroducer', text: 'default'},
+// CHECK:STDOUT:             {kind: 'MatchDefaultEqualGreater', text: '=>'},
+// CHECK:STDOUT:           {kind: 'MatchDefaultStart', text: '{', subtree_size: 3},
+// CHECK:STDOUT:             {kind: 'ReturnStatementStart', text: 'return'},
+// CHECK:STDOUT:             {kind: 'IntLiteral', text: '4'},
+// CHECK:STDOUT:           {kind: 'ReturnStatement', text: ';', subtree_size: 3},
+// CHECK:STDOUT:         {kind: 'MatchDefault', text: '}', subtree_size: 7},
+// CHECK:STDOUT:       {kind: 'MatchStatement', text: '}', subtree_size: 70},
+// CHECK:STDOUT:         {kind: 'ReturnStatementStart', text: 'return'},
+// CHECK:STDOUT:         {kind: 'IntLiteral', text: '0'},
+// CHECK:STDOUT:       {kind: 'ReturnStatement', text: ';', subtree_size: 3},
+// CHECK:STDOUT:     {kind: 'FunctionDefinition', text: '}', subtree_size: 89},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
+// CHECK:STDOUT: - filename: unused_var_function.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'FunctionIntroducer', text: 'fn'},
+// CHECK:STDOUT:       {kind: 'IdentifierNameBeforeParams', text: 'F'},
+// CHECK:STDOUT:         {kind: 'ExplicitParamListStart', text: '('},
+// CHECK:STDOUT:           {kind: 'IdentifierNameNotBeforeParams', text: 'x'},
+// CHECK:STDOUT:             {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:           {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:         {kind: 'LetBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:         {kind: 'PatternListComma', text: ','},
+// CHECK:STDOUT:               {kind: 'IdentifierNameNotBeforeParams', text: 'y'},
+// CHECK:STDOUT:                 {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:               {kind: 'TupleLiteral', text: ')', subtree_size: 2},
+// CHECK:STDOUT:             {kind: 'VarBindingPattern', text: ':', subtree_size: 4},
+// CHECK:STDOUT:           {kind: 'VariablePattern', text: 'var', subtree_size: 5},
+// CHECK:STDOUT:         {kind: 'UnusedPattern', text: 'unused', subtree_size: 6},
+// CHECK:STDOUT:       {kind: 'ExplicitParamList', text: ')', subtree_size: 13},
+// CHECK:STDOUT:     {kind: 'FunctionDecl', text: ';', subtree_size: 16},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
+// CHECK:STDOUT: - filename: fail_ref_unused.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'FunctionIntroducer', text: 'fn'},
+// CHECK:STDOUT:       {kind: 'IdentifierNameBeforeParams', text: 'F'},
+// CHECK:STDOUT:         {kind: 'ExplicitParamListStart', text: '('},
+// CHECK:STDOUT:           {kind: 'IdentifierNameNotBeforeParams', text: 'unused', has_error: yes},
+// CHECK:STDOUT:           {kind: 'InvalidParse', text: 'unused', has_error: yes},
+// CHECK:STDOUT:         {kind: 'LetBindingPattern', text: 'ref', has_error: yes, subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'ExplicitParamList', text: ')', has_error: yes, subtree_size: 5},
+// CHECK:STDOUT:     {kind: 'FunctionDecl', text: ';', subtree_size: 8},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
+// CHECK:STDOUT: - filename: fail_nested.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'VariableIntroducer', text: 'var'},
+// CHECK:STDOUT:               {kind: 'IdentifierNameNotBeforeParams', text: 'x'},
+// CHECK:STDOUT:               {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:             {kind: 'VarBindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:           {kind: 'UnusedPattern', text: 'unused', subtree_size: 4},
+// CHECK:STDOUT:         {kind: 'UnusedPattern', text: 'unused', subtree_size: 5},
+// CHECK:STDOUT:       {kind: 'VariablePattern', text: 'var', subtree_size: 6},
+// CHECK:STDOUT:       {kind: 'VariableInitializer', text: '='},
+// CHECK:STDOUT:       {kind: 'IntLiteral', text: '0'},
+// CHECK:STDOUT:     {kind: 'VariableDecl', text: ';', subtree_size: 10},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]
+// CHECK:STDOUT: - filename: fail_nested_parens.carbon
+// CHECK:STDOUT:   parse_tree: [
+// CHECK:STDOUT:     {kind: 'FileStart', text: ''},
+// CHECK:STDOUT:       {kind: 'VariableIntroducer', text: 'var'},
+// CHECK:STDOUT:             {kind: 'TuplePatternStart', text: '('},
+// CHECK:STDOUT:                 {kind: 'IdentifierNameNotBeforeParams', text: 'y'},
+// CHECK:STDOUT:                 {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:               {kind: 'VarBindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:             {kind: 'UnusedPattern', text: 'unused', subtree_size: 4},
+// CHECK:STDOUT:           {kind: 'TuplePattern', text: ')', subtree_size: 6},
+// CHECK:STDOUT:         {kind: 'UnusedPattern', text: 'unused', subtree_size: 7},
+// CHECK:STDOUT:       {kind: 'VariablePattern', text: 'var', subtree_size: 8},
+// CHECK:STDOUT:       {kind: 'VariableInitializer', text: '='},
+// CHECK:STDOUT:         {kind: 'TupleLiteralStart', text: '('},
+// CHECK:STDOUT:         {kind: 'IntLiteral', text: '0'},
+// CHECK:STDOUT:         {kind: 'TupleLiteralComma', text: ','},
+// CHECK:STDOUT:       {kind: 'TupleLiteral', text: ')', subtree_size: 4},
+// CHECK:STDOUT:     {kind: 'VariableDecl', text: ';', subtree_size: 15},
+// CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
+// CHECK:STDOUT:   ]

toolchain/parse/testdata/var/var.carbon

@@ -9,9 +9,11 @@
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/var/var.carbon
 
 var v: i32 = 0;
+var _: i32 = 0;
 var w: i32;
 fn F() {
   var s: str = "hello";
+  var _: str = "anonymous";
 }
 
 // CHECK:STDOUT: - filename: var.carbon
@@ -26,6 +28,14 @@ fn F() {
 // CHECK:STDOUT:       {kind: 'IntLiteral', text: '0'},
 // CHECK:STDOUT:     {kind: 'VariableDecl', text: ';', subtree_size: 8},
 // CHECK:STDOUT:       {kind: 'VariableIntroducer', text: 'var'},
+// CHECK:STDOUT:           {kind: 'UnderscoreName', text: '_'},
+// CHECK:STDOUT:           {kind: 'IntTypeLiteral', text: 'i32'},
+// CHECK:STDOUT:         {kind: 'VarBindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:       {kind: 'VariablePattern', text: 'var', subtree_size: 4},
+// CHECK:STDOUT:       {kind: 'VariableInitializer', text: '='},
+// CHECK:STDOUT:       {kind: 'IntLiteral', text: '0'},
+// CHECK:STDOUT:     {kind: 'VariableDecl', text: ';', subtree_size: 8},
+// CHECK:STDOUT:       {kind: 'VariableIntroducer', text: 'var'},
 // CHECK:STDOUT:           {kind: 'IdentifierNameNotBeforeParams', text: 'w'},
 // CHECK:STDOUT:           {kind: 'IntTypeLiteral', text: 'i32'},
 // CHECK:STDOUT:         {kind: 'VarBindingPattern', text: ':', subtree_size: 3},
@@ -44,6 +54,14 @@ fn F() {
 // CHECK:STDOUT:         {kind: 'VariableInitializer', text: '='},
 // CHECK:STDOUT:         {kind: 'StringLiteral', text: '"hello"'},
 // CHECK:STDOUT:       {kind: 'VariableDecl', text: ';', subtree_size: 8},
-// CHECK:STDOUT:     {kind: 'FunctionDefinition', text: '}', subtree_size: 14},
+// CHECK:STDOUT:         {kind: 'VariableIntroducer', text: 'var'},
+// CHECK:STDOUT:             {kind: 'UnderscoreName', text: '_'},
+// CHECK:STDOUT:             {kind: 'StringTypeLiteral', text: 'str'},
+// CHECK:STDOUT:           {kind: 'VarBindingPattern', text: ':', subtree_size: 3},
+// CHECK:STDOUT:         {kind: 'VariablePattern', text: 'var', subtree_size: 4},
+// CHECK:STDOUT:         {kind: 'VariableInitializer', text: '='},
+// CHECK:STDOUT:         {kind: 'StringLiteral', text: '"anonymous"'},
+// CHECK:STDOUT:       {kind: 'VariableDecl', text: ';', subtree_size: 8},
+// CHECK:STDOUT:     {kind: 'FunctionDefinition', text: '}', subtree_size: 22},
 // CHECK:STDOUT:     {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT:   ]

toolchain/parse/typed_nodes.h

@@ -324,6 +324,15 @@ struct Namespace {
 // Pattern nodes
 // -------------
 
+// An unused pattern: `unused pattern`.
+struct UnusedPattern {
+  static constexpr auto Kind = NodeKind::UnusedPattern.Define(
+      {.category = NodeCategory::Pattern, .child_count = 1});
+
+  Lex::UnusedTokenIndex token;
+  AnyPatternId inner;
+};
+
 // A ref binding name: `ref name`.
 struct RefBindingName {
   static constexpr auto Kind =