Add documentation for entity declaration design work (#4230)

Trying to pull in key elements of #3762, #3763, and #3980 (decl matching
and `extern`, essentially). These aren't specific to any particular
declaration type, but are common to entities, so suggesting a new doc
oriented on that.

There's probably more that could be said here, I'm just focused on
getting the recent formal discussion mirrored into the design.

---------

Co-authored-by: josh11b <15258583+josh11b@users.noreply.github.com>

docs/design/declaring_entities.md

@@ -0,0 +1,240 @@
+# Declaring entities
+
+<!--
+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
+-->
+
+<!-- toc -->
+
+## Table of contents
+
+-   [Overview](#overview)
+-   [Matching redeclarations of an entity](#matching-redeclarations-of-an-entity)
+    -   [Details](#details)
+-   [`extern` and `extern library`](#extern-and-extern-library)
+    -   [Valid scopes for `extern`](#valid-scopes-for-extern)
+    -   [Effect on indirect imports](#effect-on-indirect-imports)
+-   [Alternatives considered](#alternatives-considered)
+-   [References](#references)
+
+<!-- tocstop -->
+
+## Overview
+
+Entities may have up to three declarations:
+
+-   An optional, owning forward declaration.
+    -   For example, `class MyClass;`.
+    -   This must come before the definition. The API file is considered to be
+        before the implementation file.
+-   A required, owning definition.
+    -   For example, `class MyClass { ... }`.
+    -   The definition might be the _only_ declaration.
+-   An optional, non-owning `extern library "<owning_library>"` declaration.
+    -   For example, `extern library "OtherLibrary" class MyClass;`.
+    -   It must be in a separate library from the definition.
+    -   The owning library's API file must import the `extern` declaration, and
+        must also contain a declaration.
+    -   The owning library's declarations must have the `extern` modifier
+        (without `library`).
+        -   For example, `extern class MyClass;`.
+
+For example, a library can have a forward declaration of an entity in the API
+file, and use the implementation file for the entity's definition. Putting the
+definition in an implementation file this way can reduce the dependencies for
+API file evaluation, improving compile time. This is commonly done with
+functions. For example:
+
+```
+library "MyLibrary";
+
+fn DoSomething();
+```
+
+```
+impl library "MyLibrary";
+
+fn DoSomething() {
+  ...
+}
+```
+
+## Matching redeclarations of an entity
+
+In order to determine whether two redeclarations refer to the same entity, we
+apply the rules:
+
+-   Two named declarations _declare the same entity_ if they have the same scope
+    and the same name. This includes imported declarations.
+-   When two named declarations declare the same entity, the second is said to
+    be a _redeclaration_.
+-   Two owned declarations _differ_ if they don't syntactically match.
+    -   Otherwise, if one is a non-owned `extern library` declaration,
+        declarations differ if they don't match semantically.
+-   The program is invalid if it contains two declarations of the same entity
+    that differ.
+
+```carbon
+class A {
+  // This function will be redeclared in order to provide a definition.
+  fn F(n: i32);
+}
+
+// ✅ Valid: The declaration matches syntactically.
+fn A.F(n: i32) {}
+
+// ❌ Invalid: The parameter name differs.
+fn A.F(m: i32) {}
+
+// ❌ Invalid: The parameter type differs syntactically.
+fn A.F(n: (i32)) {}
+```
+
+### Details
+
+TODO: Figure out what details to pull from
+[#3762](https://github.com/carbon-language/carbon-lang/pull/3762) and
+[#3763](https://github.com/carbon-language/carbon-lang/pull/3763).
+
+## `extern` and `extern library`
+
+There are two forms of the `extern` modifier:
+
+-   On an owning declaration, `extern` limits access to the definition.
+    -   The entity must be directly imported in order to use of the definition.
+    -   An `extern library` declaration is optional.
+-   On a non-owning declaration, `extern library` allows references to an entity
+    without depending on the owning library.
+    -   The library name indicates where the entity is defined.
+    -   This can be used to improve build performance, such as by splitting out
+        a declaration in order to reduce a library's dependencies.
+
+For example, a use of both might look like:
+
+```
+library "owner";
+
+// This `import` is required due to the `extern library`, but we also make use
+// of `MyClassFactory` below. This is a circular use of `MyClass` that we
+// couldn't split between libraries without `extern`.
+import library "factory";
+
+extern class MyClass {
+  fn Make() -> MyClass* {
+    return MyClassFactory();
+  }
+
+  var val: i32 = 0;
+}
+```
+
+```
+library "factory";
+
+// Declares `MyClass` so that `MyClassFactory` can return it.
+extern library "owner" class MyClass;
+
+fn MyClassFactory(val: i32) -> MyClass*;
+```
+
+```
+impl library "factory";
+
+// Imports the definition of `MyClass`.
+import library "owner";
+
+extern fn MyClassFactory(val: i32) -> MyClass* {
+  var c: MyClass* = new MyClass();
+  c->val = val;
+  return c;
+}
+```
+
+### Valid scopes for `extern`
+
+The `extern` modifier is only valid on namespace-scoped entities, including in
+the file scope. In other words, `class C { extern fn F(); }` is invalid.
+
+### Effect on indirect imports
+
+Indirect imports won't see the definition of an `extern` entity. We expect this
+to primarily affect return types of functions. If an incomplete type is
+encountered this way, it can be resolved by directly importing the definition.
+For example:
+
+```
+library "type";
+
+// Because this is `extern`, the definition must be directly imported.
+extern class MyType { var x: i32 }
+```
+
+```
+library "make_type";
+
+import library "type";
+
+// Here we have a function which returns the type.
+fn MakeMyType() -> MyType*;
+```
+
+```
+library "invalid_use";
+
+import library "make_type";
+
+fn InvalidUse() -> i32 {
+  // ❌ Invalid: `MyType` is incomplete because it's `extern` and not directly
+  // imported. `x` cannot be accessed.
+  return MakeMyType()->x;
+}
+```
+
+```
+library "valid_use";
+
+import library "make_type";
+
+// ✅ Valid: By directly importing the definition, we can now access `x`.
+import library "type";
+
+fn ValidUse() -> i32 {
+  return MakeMyType()->x;
+}
+```
+
+## Alternatives considered
+
+-   [Other modifier keyword merging approaches](/proposals/p3762.md#other-modifier-keyword-merging-approaches)
+-   [No `extern` keyword](/proposals/p3762.md#no-extern-keyword)
+-   [Looser restrictions on declarations](/proposals/p3762.md#looser-restrictions-on-declarations)
+-   [`extern` naming](/proposals/p3762.md#extern-naming)
+-   [Default `extern` to private](/proposals/p3762.md#default-extern-to-private)
+-   [Opaque types](/proposals/p3762.md#opaque-types)
+-   [Require a library provide its own `extern` declarations](/proposals/p3762.md#require-a-library-provide-its-own-extern-declarations)
+-   [Allow cross-package `extern` declarations](/proposals/p3762.md#allow-cross-package-extern-declarations)
+-   [Use a partially or fully semantic rule](/proposals/p3763.md#use-a-partially-or-fully-semantic-rule)
+-   [Use package-wide name poisoning](/proposals/p3763.md#use-package-wide-name-poisoning)
+-   [Allow shadowing in implementation file after use in API file](/proposals/p3763.md#allow-shadowing-in-implementation-file-after-use-in-api-file)
+-   [Allow multiple non-owning declarations, remove the import requirement, or both](/proposals/p3980.md#allow-multiple-non-owning-declarations-remove-the-import-requirement-or-both)
+-   [Total number of allowed declarations (owning and non-owning)](/proposals/p3980.md#total-number-of-allowed-declarations-owning-and-non-owning)
+    -   [Do not restrict the number of forward declarations](/proposals/p3980.md#do-not-restrict-the-number-of-forward-declarations)
+    -   [Allow up to two declarations total](/proposals/p3980.md#allow-up-to-two-declarations-total)
+    -   [Allow up to four declarations total](/proposals/p3980.md#allow-up-to-four-declarations-total)
+-   [Don't require a modifier on the owning declarations](/proposals/p3980.md#dont-require-a-modifier-on-the-owning-declarations)
+-   [Only require `extern` on the first owning declaration](/proposals/p3980.md#only-require-extern-on-the-first-owning-declaration)
+-   [Separate require-direct-import from non-owning declarations](/proposals/p3980.md#separate-require-direct-import-from-non-owning-declarations)
+-   [Other `extern` syntaxes](/proposals/p3980.md#other-extern-syntaxes)
+-   [Have types with `extern` members re-export them](/proposals/p3980.md#have-types-with-extern-members-re-export-them)
+-   [Require syntactic matching for `extern library` declarations](/proposals/p3980.md#require-syntactic-matching-for-extern-library-declarations)
+
+## References
+
+-   Proposal
+    [#3762: Merging forward declarations](https://github.com/carbon-language/carbon-lang/pull/3762)
+-   Proposal
+    [#3763: Matching redeclarations](https://github.com/carbon-language/carbon-lang/pull/3763)
+-   Proposal
+    [#3980: Singular `extern` declarations](https://github.com/carbon-language/carbon-lang/pull/3980)