Clarify what was missing in binding pattern errors (#4314)

Co-authored-by: Josh L <josh11b@users.noreply.github.com>

toolchain/check/testdata/function/declaration/no_prelude/fail_pattern_in_signature.carbon

@@ -8,7 +8,7 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/function/declaration/no_prelude/fail_pattern_in_signature.carbon
 
-// CHECK:STDERR: fail_pattern_in_signature.carbon:[[@LINE+7]]:6: ERROR: Expected binding pattern.
+// CHECK:STDERR: fail_pattern_in_signature.carbon:[[@LINE+7]]:6: ERROR: Expected name in binding pattern.
 // CHECK:STDERR: fn F((a: {}, b: {}), c: {});
 // CHECK:STDERR:      ^
 // CHECK:STDERR:

toolchain/parse/handle_binding_pattern.cpp

@@ -25,11 +25,13 @@ auto HandleBindingPattern(Context& context) -> void {
   }
 
   // Handle an invalid pattern introducer for parameters and variables.
-  auto on_error = [&]() {
+  auto on_error = [&](llvm::StringLiteral expected) {
     if (!state.has_error) {
       CARBON_DIAGNOSTIC(ExpectedBindingPattern, Error,
-                        "Expected binding pattern.");
-      context.emitter().Emit(*context.position(), ExpectedBindingPattern);
+                        "Expected {0} in binding pattern.",
+                        llvm::StringLiteral);
+      context.emitter().Emit(*context.position(), ExpectedBindingPattern,
+                             expected);
       state.has_error = true;
     }
   };
@@ -50,7 +52,7 @@ auto HandleBindingPattern(Context& context) -> void {
     // Add a placeholder for the name.
     context.AddLeafNode(NodeKind::IdentifierName, *context.position(),
                         /*has_error=*/true);
-    on_error();
+    on_error("name");
   }
 
   if (auto kind = context.PositionKind();
@@ -63,7 +65,7 @@ auto HandleBindingPattern(Context& context) -> void {
     context.PushState(state);
     context.PushStateForExpr(PrecedenceGroup::ForType());
   } else {
-    on_error();
+    on_error("`:` or `:!`");
     // Add a placeholder for the type.
     context.AddLeafNode(NodeKind::InvalidParse, *context.position(),
                         /*has_error=*/true);

toolchain/parse/testdata/basics/fail_paren_match_regression.carbon

@@ -8,7 +8,7 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/basics/fail_paren_match_regression.carbon
 
-// CHECK:STDERR: fail_paren_match_regression.carbon:[[@LINE+8]]:5: ERROR: Expected binding pattern.
+// CHECK:STDERR: fail_paren_match_regression.carbon:[[@LINE+8]]:5: ERROR: Expected name in binding pattern.
 // CHECK:STDERR: var = (foo {})
 // CHECK:STDERR:     ^
 // CHECK:STDERR:

toolchain/parse/testdata/function/declaration/fail_with_identifier_as_param.carbon

@@ -8,7 +8,7 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/function/declaration/fail_with_identifier_as_param.carbon
 
-// CHECK:STDERR: fail_with_identifier_as_param.carbon:[[@LINE+3]]:11: ERROR: Expected binding pattern.
+// CHECK:STDERR: fail_with_identifier_as_param.carbon:[[@LINE+3]]:11: ERROR: Expected `:` or `:!` in binding pattern.
 // CHECK:STDERR: fn foo(bar);
 // CHECK:STDERR:           ^
 fn foo(bar);

toolchain/parse/testdata/generics/impl/fail_impl.carbon

@@ -54,7 +54,7 @@ impl forall f32;
 // CHECK:STDERR:
 impl forall [] u32;
 
-// CHECK:STDERR: fail_impl.carbon:[[@LINE+8]]:21: ERROR: Expected binding pattern.
+// CHECK:STDERR: fail_impl.carbon:[[@LINE+8]]:21: ERROR: Expected `:` or `:!` in binding pattern.
 // CHECK:STDERR: impl forall [invalid] i8;
 // CHECK:STDERR:                     ^
 // CHECK:STDERR:

toolchain/parse/testdata/generics/interface/fail_self_param_syntax.carbon

@@ -9,13 +9,13 @@
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/generics/interface/fail_self_param_syntax.carbon
 
 interface Foo {
-  // CHECK:STDERR: fail_self_param_syntax.carbon:[[@LINE+4]]:13: ERROR: Expected binding pattern.
+  // CHECK:STDERR: fail_self_param_syntax.carbon:[[@LINE+4]]:13: ERROR: Expected `:` or `:!` in binding pattern.
   // CHECK:STDERR:   fn Sub[me Self](b: Self) -> Self;
   // CHECK:STDERR:             ^~~~
   // CHECK:STDERR:
   fn Sub[me Self](b: Self) -> Self;
 
-  // CHECK:STDERR: fail_self_param_syntax.carbon:[[@LINE+3]]:10: ERROR: Expected binding pattern.
+  // CHECK:STDERR: fail_self_param_syntax.carbon:[[@LINE+3]]:10: ERROR: Expected name in binding pattern.
   // CHECK:STDERR:   fn Mul[Self](b: Self) -> Self;
   // CHECK:STDERR:          ^~~~
   fn Mul[Self](b: Self) -> Self;

toolchain/parse/testdata/let/fail_bad_name.carbon

@@ -8,7 +8,7 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/let/fail_bad_name.carbon
 
-// CHECK:STDERR: fail_bad_name.carbon:[[@LINE+3]]:5: ERROR: Expected binding pattern.
+// CHECK:STDERR: fail_bad_name.carbon:[[@LINE+3]]:5: ERROR: Expected name in binding pattern.
 // CHECK:STDERR: let ? = 4;
 // CHECK:STDERR:     ^
 let ? = 4;

toolchain/parse/testdata/let/fail_empty.carbon

@@ -8,7 +8,7 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/let/fail_empty.carbon
 
-// CHECK:STDERR: fail_empty.carbon:[[@LINE+3]]:4: ERROR: Expected binding pattern.
+// CHECK:STDERR: fail_empty.carbon:[[@LINE+3]]:4: ERROR: Expected name in binding pattern.
 // CHECK:STDERR: let;
 // CHECK:STDERR:    ^
 let;

toolchain/parse/testdata/let/fail_missing_name.carbon

@@ -10,7 +10,7 @@
 
 // --- fail_runtime_binding.carbon
 
-// CHECK:STDERR: fail_runtime_binding.carbon:[[@LINE+4]]:5: ERROR: Expected binding pattern.
+// CHECK:STDERR: fail_runtime_binding.carbon:[[@LINE+4]]:5: ERROR: Expected name in binding pattern.
 // CHECK:STDERR: let : i32 = 4;
 // CHECK:STDERR:     ^
 // CHECK:STDERR:
@@ -18,7 +18,7 @@ let : i32 = 4;
 
 // --- fail_complietime_binding.carbon
 
-// CHECK:STDERR: fail_complietime_binding.carbon:[[@LINE+3]]:5: ERROR: Expected binding pattern.
+// CHECK:STDERR: fail_complietime_binding.carbon:[[@LINE+3]]:5: ERROR: Expected name in binding pattern.
 // CHECK:STDERR: let :! bool = true;
 // CHECK:STDERR:     ^~
 let :! bool = true;

toolchain/parse/testdata/let/fail_missing_type.carbon

@@ -8,7 +8,7 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/let/fail_missing_type.carbon
 
-// CHECK:STDERR: fail_missing_type.carbon:[[@LINE+3]]:7: ERROR: Expected binding pattern.
+// CHECK:STDERR: fail_missing_type.carbon:[[@LINE+3]]:7: ERROR: Expected `:` or `:!` in binding pattern.
 // CHECK:STDERR: let a = 4;
 // CHECK:STDERR:       ^
 let a = 4;

toolchain/parse/testdata/let/fail_no_semi.carbon

@@ -10,7 +10,7 @@
 
 let
 
-// CHECK:STDERR: fail_no_semi.carbon:[[@LINE+16]]:21: ERROR: Expected binding pattern.
+// CHECK:STDERR: fail_no_semi.carbon:[[@LINE+16]]:21: ERROR: Expected name in binding pattern.
 // CHECK:STDERR: // CHECK:STDOUT:   ]
 // CHECK:STDERR:                     ^
 // CHECK:STDERR:

toolchain/parse/testdata/match/fail_missing_case_pattern.carbon

@@ -10,7 +10,7 @@
 
 fn f() -> i32 {
   match (3) {
-    // CHECK:STDERR: fail_missing_case_pattern.carbon:[[@LINE+3]]:10: ERROR: Expected binding pattern.
+    // CHECK:STDERR: fail_missing_case_pattern.carbon:[[@LINE+3]]:10: ERROR: Expected name in binding pattern.
     // CHECK:STDERR:     case => { return 2; }
     // CHECK:STDERR:          ^~
     case => { return 2; }

toolchain/parse/testdata/package_expr/fail_in_name.carbon

@@ -8,7 +8,7 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/package_expr/fail_in_name.carbon
 
-// CHECK:STDERR: fail_in_name.carbon:[[@LINE+4]]:5: ERROR: Expected binding pattern.
+// CHECK:STDERR: fail_in_name.carbon:[[@LINE+4]]:5: ERROR: Expected name in binding pattern.
 // CHECK:STDERR: var package.val: i32;
 // CHECK:STDERR:     ^~~~~~~
 // CHECK:STDERR:

toolchain/parse/testdata/var/fail_bad_name.carbon

@@ -8,7 +8,7 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/var/fail_bad_name.carbon
 
-// CHECK:STDERR: fail_bad_name.carbon:[[@LINE+3]]:5: ERROR: Expected binding pattern.
+// CHECK:STDERR: fail_bad_name.carbon:[[@LINE+3]]:5: ERROR: Expected name in binding pattern.
 // CHECK:STDERR: var *;
 // CHECK:STDERR:     ^
 var *;

toolchain/parse/testdata/var/fail_empty.carbon

@@ -8,7 +8,7 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/parse/testdata/var/fail_empty.carbon
 
-// CHECK:STDERR: fail_empty.carbon:[[@LINE+3]]:4: ERROR: Expected binding pattern.
+// CHECK:STDERR: fail_empty.carbon:[[@LINE+3]]:4: ERROR: Expected name in binding pattern.
 // CHECK:STDERR: var;
 // CHECK:STDERR:    ^
 var;

toolchain/parse/testdata/var/fail_no_semi.carbon

@@ -10,7 +10,7 @@
 
 var
 
-// CHECK:STDERR: fail_no_semi.carbon:[[@LINE+16]]:21: ERROR: Expected binding pattern.
+// CHECK:STDERR: fail_no_semi.carbon:[[@LINE+16]]:21: ERROR: Expected name in binding pattern.
 // CHECK:STDERR: // CHECK:STDOUT:   ]
 // CHECK:STDERR:                     ^
 // CHECK:STDERR: