Revise how modules imports are done to better support import public

Historically Swift didn't have a good way to model `import public` in .proto
files. But by using `@_exported import`, we can get the majority of the way
there. This makes restructuring of proto files less likely to break Swift source
code; meaning Swift is less of a "special case" compared to other languages.

If generating with `public` or `package` visibility, then instead of importing
the whole module of for an `import public`, explicitly re-export the imports of
the types from that file. That will make them usable for the given source file
but will also let the types continue to be used by someone just importing this
module.

This also means a file with no `import public` usages becomes must simpler as it
can just rely on its direct dependencies to provide all the types that could be
used.

CompileTests/MultiModule/Sources/ImportsAPublicly/imports_a_publicly.pb.swift

@@ -10,7 +10,10 @@
 import Foundation
 import SwiftProtobuf
 
-import ModuleA
+// Use of 'import public' causes re-exports:
+@_exported import enum ModuleA.E
+@_exported import let ModuleA.Extensions_ext_str
+@_exported import struct ModuleA.A
 
 // If the compiler emits an error on this type, it is because this file
 // was generated by a version of the `protoc` Swift plug-in that is
@@ -62,6 +65,11 @@ extension ImportsAPublicly: SwiftProtobuf.Message, SwiftProtobuf._MessageImpleme
     12: .same(proto: "e"),
   ]
 
+  public var isInitialized: Bool {
+    if let v = self._a, !v.isInitialized {return false}
+    return true
+  }
+
   public mutating func decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
     while let fieldNumber = try decoder.nextFieldNumber() {
       // The use of inline closures is to circumvent an issue where the compiler

CompileTests/MultiModule/Sources/ImportsImportsAPublicly/imports_imports_a_publicly.pb.swift

@@ -10,8 +10,11 @@
 import Foundation
 import SwiftProtobuf
 
-import ImportsAPublicly
-import ModuleA
+// Use of 'import public' causes re-exports:
+@_exported import enum ModuleA.E
+@_exported import let ModuleA.Extensions_ext_str
+@_exported import struct ImportsAPublicly.ImportsAPublicly
+@_exported import struct ModuleA.A
 
 // If the compiler emits an error on this type, it is because this file
 // was generated by a version of the `protoc` Swift plug-in that is
@@ -63,6 +66,11 @@ extension ImportsImportsAPublicly: SwiftProtobuf.Message, SwiftProtobuf._Message
     22: .same(proto: "e"),
   ]
 
+  public var isInitialized: Bool {
+    if let v = self._a, !v.isInitialized {return false}
+    return true
+  }
+
   public mutating func decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
     while let fieldNumber = try decoder.nextFieldNumber() {
       // The use of inline closures is to circumvent an issue where the compiler

CompileTests/MultiModule/Sources/ModuleA/a.pb.swift

@@ -49,7 +49,7 @@ public enum E: SwiftProtobuf.Enum {
 
 }
 
-public struct A: Sendable {
+public struct A: SwiftProtobuf.ExtensibleMessage, Sendable {
   // SwiftProtobuf.Message conformance is added in an extension below. See the
   // `Message` and `Message+*Additions` files in the SwiftProtobuf library for
   // methods supported on all messages.
@@ -67,9 +67,57 @@ public struct A: Sendable {
 
   public init() {}
 
+  public var _protobuf_extensionFieldValues = SwiftProtobuf.ExtensionFieldValueSet()
   fileprivate var _e: E? = nil
 }
 
+// MARK: - Extension support defined in a.proto.
+
+// MARK: - Extension Properties
+
+// Swift Extensions on the exteneded Messages to add easy access to the declared
+// extension fields. The names are based on the extension field name from the proto
+// declaration. To avoid naming collisions, the names are prefixed with the name of
+// the scope where the extend directive occurs.
+
+extension A {
+
+  public var extStr: String {
+    get {return getExtensionValue(ext: Extensions_ext_str) ?? String()}
+    set {setExtensionValue(ext: Extensions_ext_str, value: newValue)}
+  }
+  /// Returns true if extension `Extensions_ext_str`
+  /// has been explicitly set.
+  public var hasExtStr: Bool {
+    return hasExtensionValue(ext: Extensions_ext_str)
+  }
+  /// Clears the value of extension `Extensions_ext_str`.
+  /// Subsequent reads from it will return its default value.
+  public mutating func clearExtStr() {
+    clearExtensionValue(ext: Extensions_ext_str)
+  }
+
+}
+
+// MARK: - File's ExtensionMap: A_Extensions
+
+/// A `SwiftProtobuf.SimpleExtensionMap` that includes all of the extensions defined by
+/// this .proto file. It can be used any place an `SwiftProtobuf.ExtensionMap` is needed
+/// in parsing, or it can be combined with other `SwiftProtobuf.SimpleExtensionMap`s to create
+/// a larger `SwiftProtobuf.SimpleExtensionMap`.
+public let A_Extensions: SwiftProtobuf.SimpleExtensionMap = [
+  Extensions_ext_str
+]
+
+// Extension Objects - The only reason these might be needed is when manually
+// constructing a `SimpleExtensionMap`, otherwise, use the above _Extension Properties_
+// accessors for the extension fields on the messages directly.
+
+public let Extensions_ext_str = SwiftProtobuf.MessageExtension<SwiftProtobuf.OptionalExtensionField<SwiftProtobuf.ProtobufString>, A>(
+  _protobuf_fieldNumber: 100,
+  fieldName: "ext_str"
+)
+
 // MARK: - Code below here is support for the SwiftProtobuf runtime.
 
 extension E: SwiftProtobuf._ProtoNameProviding {
@@ -86,13 +134,20 @@ extension A: SwiftProtobuf.Message, SwiftProtobuf._MessageImplementationBase, Sw
     1: .same(proto: "e"),
   ]
 
+  public var isInitialized: Bool {
+    if !_protobuf_extensionFieldValues.isInitialized {return false}
+    return true
+  }
+
   public mutating func decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
     while let fieldNumber = try decoder.nextFieldNumber() {
       // The use of inline closures is to circumvent an issue where the compiler
       // allocates stack space for every case branch when no optimizations are
       // enabled. https://github.com/apple/swift-protobuf/issues/1034
       switch fieldNumber {
       case 1: try { try decoder.decodeSingularEnumField(value: &self._e) }()
+      case 100..<1001:
+        try { try decoder.decodeExtensionField(values: &_protobuf_extensionFieldValues, messageType: A.self, fieldNumber: fieldNumber) }()
       default: break
       }
     }
@@ -106,12 +161,14 @@ extension A: SwiftProtobuf.Message, SwiftProtobuf._MessageImplementationBase, Sw
     try { if let v = self._e {
       try visitor.visitSingularEnumField(value: v, fieldNumber: 1)
     } }()
+    try visitor.visitExtensionFields(fields: _protobuf_extensionFieldValues, start: 100, end: 1001)
     try unknownFields.traverse(visitor: &visitor)
   }
 
   public static func ==(lhs: A, rhs: A) -> Bool {
     if lhs._e != rhs._e {return false}
     if lhs.unknownFields != rhs.unknownFields {return false}
+    if lhs._protobuf_extensionFieldValues != rhs._protobuf_extensionFieldValues {return false}
     return true
   }
 }

CompileTests/MultiModule/Tests/Test1/test1.swift

@@ -1,5 +1,5 @@
 import ImportsAPublicly
-import ModuleA  // Needed because `import public` doesn't help Swift
+// Don't need to import ModuleA because of the file being a `import public`
 
 import XCTest
 

CompileTests/MultiModule/Tests/Test1/uses_a_transitively.pb.swift

@@ -11,7 +11,6 @@ import Foundation
 import SwiftProtobuf
 
 import ImportsAPublicly
-import ModuleA
 
 // If the compiler emits an error on this type, it is because this file
 // was generated by a version of the `protoc` Swift plug-in that is
@@ -63,6 +62,11 @@ extension UsesATransitively: SwiftProtobuf.Message, SwiftProtobuf._MessageImplem
     102: .same(proto: "e"),
   ]
 
+  public var isInitialized: Bool {
+    if let v = self._a, !v.isInitialized {return false}
+    return true
+  }
+
   public mutating func decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
     while let fieldNumber = try decoder.nextFieldNumber() {
       // The use of inline closures is to circumvent an issue where the compiler

CompileTests/MultiModule/Tests/Test2/test2.swift

@@ -1,5 +1,5 @@
 import ImportsImportsAPublicly
-import ModuleA  // Needed because `import public` doesn't help Swift
+// Don't need to import ModuleA because of the file being a `import public`
 
 import XCTest
 

CompileTests/MultiModule/Tests/Test2/uses_a_transitively2.pb.swift

@@ -10,9 +10,7 @@
 import Foundation
 import SwiftProtobuf
 
-import ImportsAPublicly
 import ImportsImportsAPublicly
-import ModuleA
 
 // If the compiler emits an error on this type, it is because this file
 // was generated by a version of the `protoc` Swift plug-in that is
@@ -64,6 +62,11 @@ extension UsesATransitively2: SwiftProtobuf.Message, SwiftProtobuf._MessageImple
     122: .same(proto: "e"),
   ]
 
+  public var isInitialized: Bool {
+    if let v = self._a, !v.isInitialized {return false}
+    return true
+  }
+
   public mutating func decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
     while let fieldNumber = try decoder.nextFieldNumber() {
       // The use of inline closures is to circumvent an issue where the compiler

Protos/CompileTests/MultiModule/Sources/ModuleA/a.proto

@@ -6,4 +6,9 @@ enum E {
 
 message A {
   optional E e = 1;
+  extensions 100 to 1000;
+}
+
+extend A {
+  optional string ext_str = 100;
 }

Reference/CompileTests/MultiModule/Sources/ImportsAPublicly/imports_a_publicly.pb.swift

@@ -10,7 +10,10 @@
 import Foundation
 import SwiftProtobuf
 
-import ModuleA
+// Use of 'import public' causes re-exports:
+@_exported import enum ModuleA.E
+@_exported import let ModuleA.Extensions_ext_str
+@_exported import struct ModuleA.A
 
 // If the compiler emits an error on this type, it is because this file
 // was generated by a version of the `protoc` Swift plug-in that is
@@ -62,6 +65,11 @@ extension ImportsAPublicly: SwiftProtobuf.Message, SwiftProtobuf._MessageImpleme
     12: .same(proto: "e"),
   ]
 
+  public var isInitialized: Bool {
+    if let v = self._a, !v.isInitialized {return false}
+    return true
+  }
+
   public mutating func decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
     while let fieldNumber = try decoder.nextFieldNumber() {
       // The use of inline closures is to circumvent an issue where the compiler

Reference/CompileTests/MultiModule/Sources/ImportsImportsAPublicly/imports_imports_a_publicly.pb.swift

@@ -10,8 +10,11 @@
 import Foundation
 import SwiftProtobuf
 
-import ImportsAPublicly
-import ModuleA
+// Use of 'import public' causes re-exports:
+@_exported import enum ModuleA.E
+@_exported import let ModuleA.Extensions_ext_str
+@_exported import struct ImportsAPublicly.ImportsAPublicly
+@_exported import struct ModuleA.A
 
 // If the compiler emits an error on this type, it is because this file
 // was generated by a version of the `protoc` Swift plug-in that is
@@ -63,6 +66,11 @@ extension ImportsImportsAPublicly: SwiftProtobuf.Message, SwiftProtobuf._Message
     22: .same(proto: "e"),
   ]
 
+  public var isInitialized: Bool {
+    if let v = self._a, !v.isInitialized {return false}
+    return true
+  }
+
   public mutating func decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
     while let fieldNumber = try decoder.nextFieldNumber() {
       // The use of inline closures is to circumvent an issue where the compiler

Reference/CompileTests/MultiModule/Sources/ModuleA/a.pb.swift

@@ -49,7 +49,7 @@ public enum E: SwiftProtobuf.Enum {
 
 }
 
-public struct A: Sendable {
+public struct A: SwiftProtobuf.ExtensibleMessage, Sendable {
   // SwiftProtobuf.Message conformance is added in an extension below. See the
   // `Message` and `Message+*Additions` files in the SwiftProtobuf library for
   // methods supported on all messages.
@@ -67,9 +67,57 @@ public struct A: Sendable {
 
   public init() {}
 
+  public var _protobuf_extensionFieldValues = SwiftProtobuf.ExtensionFieldValueSet()
   fileprivate var _e: E? = nil
 }
 
+// MARK: - Extension support defined in a.proto.
+
+// MARK: - Extension Properties
+
+// Swift Extensions on the exteneded Messages to add easy access to the declared
+// extension fields. The names are based on the extension field name from the proto
+// declaration. To avoid naming collisions, the names are prefixed with the name of
+// the scope where the extend directive occurs.
+
+extension A {
+
+  public var extStr: String {
+    get {return getExtensionValue(ext: Extensions_ext_str) ?? String()}
+    set {setExtensionValue(ext: Extensions_ext_str, value: newValue)}
+  }
+  /// Returns true if extension `Extensions_ext_str`
+  /// has been explicitly set.
+  public var hasExtStr: Bool {
+    return hasExtensionValue(ext: Extensions_ext_str)
+  }
+  /// Clears the value of extension `Extensions_ext_str`.
+  /// Subsequent reads from it will return its default value.
+  public mutating func clearExtStr() {
+    clearExtensionValue(ext: Extensions_ext_str)
+  }
+
+}
+
+// MARK: - File's ExtensionMap: A_Extensions
+
+/// A `SwiftProtobuf.SimpleExtensionMap` that includes all of the extensions defined by
+/// this .proto file. It can be used any place an `SwiftProtobuf.ExtensionMap` is needed
+/// in parsing, or it can be combined with other `SwiftProtobuf.SimpleExtensionMap`s to create
+/// a larger `SwiftProtobuf.SimpleExtensionMap`.
+public let A_Extensions: SwiftProtobuf.SimpleExtensionMap = [
+  Extensions_ext_str
+]
+
+// Extension Objects - The only reason these might be needed is when manually
+// constructing a `SimpleExtensionMap`, otherwise, use the above _Extension Properties_
+// accessors for the extension fields on the messages directly.
+
+public let Extensions_ext_str = SwiftProtobuf.MessageExtension<SwiftProtobuf.OptionalExtensionField<SwiftProtobuf.ProtobufString>, A>(
+  _protobuf_fieldNumber: 100,
+  fieldName: "ext_str"
+)
+
 // MARK: - Code below here is support for the SwiftProtobuf runtime.
 
 extension E: SwiftProtobuf._ProtoNameProviding {
@@ -86,13 +134,20 @@ extension A: SwiftProtobuf.Message, SwiftProtobuf._MessageImplementationBase, Sw
     1: .same(proto: "e"),
   ]
 
+  public var isInitialized: Bool {
+    if !_protobuf_extensionFieldValues.isInitialized {return false}
+    return true
+  }
+
   public mutating func decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
     while let fieldNumber = try decoder.nextFieldNumber() {
       // The use of inline closures is to circumvent an issue where the compiler
       // allocates stack space for every case branch when no optimizations are
       // enabled. https://github.com/apple/swift-protobuf/issues/1034
       switch fieldNumber {
       case 1: try { try decoder.decodeSingularEnumField(value: &self._e) }()
+      case 100..<1001:
+        try { try decoder.decodeExtensionField(values: &_protobuf_extensionFieldValues, messageType: A.self, fieldNumber: fieldNumber) }()
       default: break
       }
     }
@@ -106,12 +161,14 @@ extension A: SwiftProtobuf.Message, SwiftProtobuf._MessageImplementationBase, Sw
     try { if let v = self._e {
       try visitor.visitSingularEnumField(value: v, fieldNumber: 1)
     } }()
+    try visitor.visitExtensionFields(fields: _protobuf_extensionFieldValues, start: 100, end: 1001)
     try unknownFields.traverse(visitor: &visitor)
   }
 
   public static func ==(lhs: A, rhs: A) -> Bool {
     if lhs._e != rhs._e {return false}
     if lhs.unknownFields != rhs.unknownFields {return false}
+    if lhs._protobuf_extensionFieldValues != rhs._protobuf_extensionFieldValues {return false}
     return true
   }
 }

Reference/CompileTests/MultiModule/Tests/Test1/uses_a_transitively.pb.swift

@@ -11,7 +11,6 @@ import Foundation
 import SwiftProtobuf
 
 import ImportsAPublicly
-import ModuleA
 
 // If the compiler emits an error on this type, it is because this file
 // was generated by a version of the `protoc` Swift plug-in that is
@@ -63,6 +62,11 @@ extension UsesATransitively: SwiftProtobuf.Message, SwiftProtobuf._MessageImplem
     102: .same(proto: "e"),
   ]
 
+  public var isInitialized: Bool {
+    if let v = self._a, !v.isInitialized {return false}
+    return true
+  }
+
   public mutating func decodeMessage<D: SwiftProtobuf.Decoder>(decoder: inout D) throws {
     while let fieldNumber = try decoder.nextFieldNumber() {
       // The use of inline closures is to circumvent an issue where the compiler