[c-interop] Rename @_extern to @extern

Now the feature is gated by experimental feature flag.
It's not shipped in any language release, so this rename should be fine.

Author: kateinoigakukun

docs/ReferenceGuides/UnderscoredAttributes.md

@@ -437,19 +437,19 @@ the export name.
 
 It's the equivalent of clang's `__attribute__((export_name))`.
 
-## `@_extern(<language>)`
+## `@extern(<language>)`
 
 Indicates that a particular declaration should be imported
 from the external environment.
 
-### `@_extern(wasm, module: <"moduleName">, name: <"fieldName">)`
+### `@extern(wasm, module: <"moduleName">, name: <"fieldName">)`
 
 Indicates that a particular declaration should be imported
 through WebAssembly's import interface.
 
 It's the equivalent of clang's `__attribute__((import_module("module"), import_name("field")))`.
 
-### `@_extern(c, [, <"cName">])`
+### `@extern(c, [, <"cName">])`
 
 Indicates that a particular declaration should refer to a
 C declaration with the given name. If the optional "cName"
@@ -462,7 +462,7 @@ C declarations from Swift, while `@_cdecl` is used to define
 Swift functions that can be referenced from C.
 
 Also similar to `@_silgen_name`, but a function declared with
-`@_extern(c)` is assumed to use the C ABI, while `@_silgen_name`
+`@extern(c)` is assumed to use the C ABI, while `@_silgen_name`
 assumes the Swift ABI.
 
 ## `@_fixed_layout`

include/swift/AST/Attr.def

@@ -420,7 +420,7 @@ DECL_ATTR(_section, Section,
 DECL_ATTR(_rawLayout, RawLayout,
   OnStruct | UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
   146)
-DECL_ATTR(_extern, Extern,
+DECL_ATTR(extern, Extern,
   OnFunc | AllowMultipleAttributes | ABIBreakingToAdd | ABIBreakingToRemove | APIStableToAdd | APIStableToRemove,
   147)
 SIMPLE_DECL_ATTR(_nonEscapable, NonEscapable,

include/swift/AST/Attr.h

@@ -2339,7 +2339,7 @@ class ExposeAttr : public DeclAttribute {
   }
 };
 
-/// Define the `@_extern` attribute, used to import external declarations in
+/// Define the `@extern` attribute, used to import external declarations in
 /// the specified way to interoperate with Swift.
 class ExternAttr : public DeclAttribute {
   SourceLoc LParenLoc, RParenLoc;
@@ -2364,7 +2364,7 @@ class ExternAttr : public DeclAttribute {
   const llvm::Optional<StringRef> ModuleName;
 
   /// The declaration name to import
-  /// std::nullopt if the declaration name is not specified with @_extern(c)
+  /// std::nullopt if the declaration name is not specified with @extern(c)
   const llvm::Optional<StringRef> Name;
 
   SourceLoc getLParenLoc() const { return LParenLoc; }

include/swift/AST/AttrKind.h

@@ -112,7 +112,7 @@ enum class ExposureKind: uint8_t {
 enum : unsigned { NumExposureKindBits =
   countBitsUsed(static_cast<unsigned>(ExposureKind::Last_ExposureKind)) };
 
-/// This enum represents the possible values of the @_extern attribute.
+/// This enum represents the possible values of the @extern attribute.
 enum class ExternKind: uint8_t {
   /// Reference an externally defined C function.
   /// The imported function has C function pointer representation,

include/swift/AST/DiagnosticsParse.def

@@ -1868,7 +1868,7 @@ ERROR(attr_rawlayout_expected_params,none,
       "expected %1 argument after %0 argument in @_rawLayout attribute", (StringRef, StringRef))
 
 ERROR(attr_extern_expected_label,none,
-      "expected %0 argument to @_extern attribute", (StringRef))
+      "expected %0 argument to @extern attribute", (StringRef))
 //------------------------------------------------------------------------------
 // MARK: Generics parsing diagnostics
 //------------------------------------------------------------------------------

include/swift/AST/DiagnosticsSema.def

@@ -1871,17 +1871,17 @@ ERROR(section_not_at_top_level,none,
 ERROR(section_empty_name,none,
       "@_section section name cannot be empty", ())
 
-// @_extern
+// @extern
 ERROR(attr_extern_experimental,none,
-      "@_extern requires '-enable-experimental-feature Extern'", ())
+      "@extern requires '-enable-experimental-feature Extern'", ())
 ERROR(extern_not_at_top_level_func,none,
-      "@_extern attribute can only be applied to global functions", ())
+      "@extern attribute can only be applied to global functions", ())
 ERROR(extern_empty_c_name,none,
-      "expected non-empty C name in @_extern attribute", ())
+      "expected non-empty C name in @extern attribute", ())
 ERROR(extern_only_non_other_attr,none,
-      "@_extern attribute cannot be applied to an '@%0' declaration", (StringRef))
+      "@extern attribute cannot be applied to an '@%0' declaration", (StringRef))
 WARNING(extern_c_maybe_invalid_name, none,
-        "C name '%0' may be invalid; explicitly specify the name in @_extern(c) to suppress this warning",
+        "C name '%0' may be invalid; explicitly specify the name in @extern(c) to suppress this warning",
         (StringRef))
 
 ERROR(c_func_variadic, none,

include/swift/Basic/Features.def

@@ -256,7 +256,7 @@ EXPERIMENTAL_FEATURE(StructLetDestructuring, true)
 /// lifetime-dependent results.
 EXPERIMENTAL_FEATURE(NonEscapableTypes, false)
 
-/// Enable the `@_extern` attribute.
+/// Enable the `@extern` attribute.
 EXPERIMENTAL_FEATURE(Extern, true)
 
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE

include/swift/Parse/Parser.h

@@ -1095,7 +1095,7 @@ class Parser {
   ParserResult<DifferentiableAttr> parseDifferentiableAttribute(SourceLoc AtLoc,
                                                                 SourceLoc Loc);
 
-  /// Parse the @_extern attribute.
+  /// Parse the @extern attribute.
   bool parseExternAttribute(DeclAttributes &Attributes, bool &DiscardAttribute,
                             StringRef AttrName, SourceLoc AtLoc, SourceLoc Loc);
 

include/swift/SIL/SILFunction.h

@@ -303,7 +303,7 @@ class SILFunction
   /// empty.
   StringRef WasmExportName;
 
-  /// Name of a Wasm import module and field if @_extern(wasm) attribute
+  /// Name of a Wasm import module and field if @extern(wasm) attribute
   llvm::Optional<std::pair<StringRef, StringRef>> WasmImportModuleAndField;
 
   /// Has value if there's a profile for this function
@@ -1293,14 +1293,14 @@ class SILFunction
   StringRef wasmExportName() const { return WasmExportName; }
   void setWasmExportName(StringRef value) { WasmExportName = value; }
 
-  /// Return Wasm import module name if @_extern(wasm) was used otherwise empty
+  /// Return Wasm import module name if @extern(wasm) was used otherwise empty
   StringRef wasmImportModuleName() const {
     if (WasmImportModuleAndField)
       return WasmImportModuleAndField->first;
     return StringRef();
   }
 
-  /// Return Wasm import field name if @_extern(wasm) was used otherwise empty
+  /// Return Wasm import field name if @extern(wasm) was used otherwise empty
   StringRef wasmImportFieldName() const {
     if (WasmImportModuleAndField)
       return WasmImportModuleAndField->second;

lib/AST/Attr.cpp

@@ -1141,7 +1141,7 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options,
 
   case DAK_Extern: {
     auto *Attr = cast<ExternAttr>(this);
-    Printer.printAttrName("@_extern");
+    Printer.printAttrName("@extern");
     Printer << "(";
     switch (Attr->getExternKind()) {
     case ExternKind::C:
@@ -1152,7 +1152,7 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options,
       break;
     case ExternKind::Wasm:
       Printer << "wasm";
-      // @_extern(wasm) always has names.
+      // @extern(wasm) always has names.
       Printer << ", module: \"" << *Attr->ModuleName << "\"";
       Printer << ", name: \"" << *Attr->Name << "\")";
       break;
@@ -1731,7 +1731,7 @@ StringRef DeclAttribute::getAttrName() const {
   case DAK_RawLayout:
     return "_rawLayout";
   case DAK_Extern:
-    return "_extern";
+    return "extern";
   }
   llvm_unreachable("bad DeclAttrKind");
 }