SE-0492: Stabilize @_section/@_used into @section/@used

Removes the underscored prefixes from the @_section and @_used attributes, making them public as @section and @used respectively. The SymbolLinkageMarkers experimental feature has been removed as these attributes are now part of the standard language. Implemented expression syntactic checking rules per SE-0492.

Major parts:
- Renamed @_section to @section and @_used to @used
- Removed the SymbolLinkageMarkers experimental feature
- Added parsing support for the old underscored names with deprecation warnings
- Updated all tests and examples to use the new attribute names
- Added syntactic validation for @section to align with SE-0492 (reusing the legality checker by @artemcm)
- Changed @DebugDescription macro to explicitly use a tuple type instead of type inferring it, to comply with the expression syntax rules
- Added a testcase for the various allowed and disallowed syntactic forms, `test/ConstValues/SectionSyntactic.swift`.

Author: kubamracek

docs/ReferenceGuides/UnderscoredAttributes.md

@@ -1117,12 +1117,6 @@ if it is never explicitly bound using a typed pointer method like
 bound, it must only be used with compatible typed memory accesses for as long
 as the binding is active.
 
-## `@_section("section_name")`
-
-Places a global variable or a top-level function into a section of the object
-file with the given name. It's the equivalent of clang's
-`__attribute__((section))`.
-
 ## `@_semantics("uniquely.recognized.id")`
 
 Allows the optimizer to make use of some key invariants in performance critical
@@ -1320,12 +1314,6 @@ for more details.
 
 This `async` function uses the pre-SE-0338 semantics of unsafely inheriting the caller's executor.  This is an underscored feature because the right way of inheriting an executor is to pass in the required executor and switch to it.  Unfortunately, there are functions in the standard library which need to inherit their caller's executor but cannot change their ABI because they were not defined as `@_alwaysEmitIntoClient` in the initial release.
 
-## `@_used`
-
-Marks a global variable or a top-level function as "used externally" even if it
-does not have visible users in the compilation unit. It's the equivalent of
-clang's `__attribute__((used))`.
-
 ## `@_weakLinked`
 
 Allows a declaration to be weakly-referenced, i.e., any references emitted by

include/swift/AST/Attr.h

@@ -714,7 +714,7 @@ class SILGenNameAttr : public DeclAttribute {
   }
 };
 
-/// Defines the @_section attribute.
+/// Defines the @section attribute.
 class SectionAttr : public DeclAttribute {
 public:
   SectionAttr(StringRef Name, SourceLoc AtLoc, SourceRange Range, bool Implicit)

include/swift/AST/DeclAttr.def

@@ -753,12 +753,12 @@ DECL_ATTR(attached, MacroRole,
   142)
 DECL_ATTR_ALIAS(freestanding, MacroRole)
 
-SIMPLE_DECL_ATTR(_used, Used,
+SIMPLE_DECL_ATTR(used, Used,
   OnAbstractFunction | OnVar,
   UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | ForbiddenInABIAttr,
   143)
 
-DECL_ATTR(_section, Section,
+DECL_ATTR(section, Section,
   OnAbstractFunction | OnVar,
   UserInaccessible | ABIBreakingToAdd | ABIBreakingToRemove | APIBreakingToAdd | APIBreakingToRemove | ForbiddenInABIAttr,
   144)

include/swift/AST/DiagnosticsSema.def

@@ -2097,11 +2097,9 @@ ERROR(cdecl_feature_required,none,
       "'@c' requires '-enable-experimental-feature CDecl'",
       ())
 
-// @_used and @_section
-ERROR(section_linkage_markers_disabled,none,
-      "attribute requires '-enable-experimental-feature SymbolLinkageMarkers'", ())
+// @used and @section
 ERROR(section_empty_name,none,
-      "'@_section' section name cannot be empty", ())
+      "'@section' section name cannot be empty", ())
 
 // @_silgen_name and friends
 WARNING(reserved_runtime_symbol_name,none,

include/swift/Basic/Features.def

@@ -323,9 +323,6 @@ EXPERIMENTAL_FEATURE(FullTypedThrows, false)
 EXPERIMENTAL_FEATURE(SameElementRequirements, false)
 EXPERIMENTAL_FEATURE(KeyPathWithMethodMembers, false)
 
-// Whether to enable @_used and @_section attributes
-EXPERIMENTAL_FEATURE(SymbolLinkageMarkers, true)
-
 // Whether to emit an Embedded Swift module with "deferred" code generation,
 // meaning that the only code that will be emitted into the object file is
 // code that was marked as "never emit into client". For everything else,

include/swift/IRGen/Linking.h

@@ -1850,7 +1850,7 @@ class LinkEntity {
   ///
   /// This applies only in the Embedded Swift linkage model, and is used for
   /// any symbols that have not been explicitly requested to have unique
-  /// definitions (e.g., with @_used).
+  /// definitions (e.g., with @used).
   bool hasNonUniqueDefinition() const;
 
 #undef LINKENTITY_GET_FIELD

include/swift/SIL/SILFunction.h

@@ -325,7 +325,7 @@ class SILFunction
   /// If empty, use the SIL function's name directly.
   StringRef AsmName;
 
-  /// Name of a section if @_section attribute was used, otherwise empty.
+  /// Name of a section if @section attribute was used, otherwise empty.
   StringRef Section;
 
   /// Name of a Wasm export if @_expose(wasm) attribute was used, otherwise
@@ -396,7 +396,7 @@ class SILFunction
   /// would indicate.
   unsigned HasCReferences : 1;
 
-  /// Whether attribute @_used was present
+  /// Whether attribute @used was present
   unsigned MarkedAsUsed : 1;
 
   /// Whether cross-module references to this function should always use weak
@@ -1424,15 +1424,15 @@ class SILFunction
     return V && V->getAttrs().hasAttribute<AlwaysEmitIntoClientAttr>();
   }
 
-  /// Return whether this function has attribute @_used on it
+  /// Return whether this function has attribute @used on it
   bool markedAsUsed() const { return MarkedAsUsed; }
   void setMarkedAsUsed(bool value) { MarkedAsUsed = value; }
 
   /// Return custom assembler name, otherwise empty.
   StringRef asmName() const { return AsmName; }
   void setAsmName(StringRef value) { AsmName = value; }
 
-  /// Return custom section name if @_section was used, otherwise empty
+  /// Return custom section name if @section was used, otherwise empty
   StringRef section() const { return Section; }
   void setSection(StringRef value) { Section = value; }
 

include/swift/SIL/SILGlobalVariable.h

@@ -64,7 +64,7 @@ class SILGlobalVariable
   /// the mangled name of the variable will be used instead.
   StringRef AsmName;
 
-  /// Name of a section if @_section attribute was used, otherwise empty.
+  /// Name of a section if @section attribute was used, otherwise empty.
   StringRef Section;
 
   /// The lowered type of the variable.
@@ -86,7 +86,7 @@ class SILGlobalVariable
   /// once (either in its declaration, or once later), making it immutable.
   unsigned IsLet : 1;
 
-  /// Whether this declaration was marked `@_used`, meaning that it should be
+  /// Whether this declaration was marked `@used`, meaning that it should be
   /// added to the llvm.used list.
   unsigned IsUsed : 1;
 
@@ -162,7 +162,7 @@ class SILGlobalVariable
   StringRef asmName() const { return AsmName; }
   void setAsmName(StringRef value) { AsmName = value; }
 
-  /// Return custom section name if @_section was used, otherwise empty
+  /// Return custom section name if @section was used, otherwise empty
   StringRef section() const { return Section; }
   void setSection(StringRef value) { Section = value; }
 
@@ -242,7 +242,7 @@ class SILGlobalVariable
     StaticInitializerBlock.eraseAllInstructions(Module);
   }
 
-  /// Returns true if this global variable has `@_used` attribute.
+  /// Returns true if this global variable has `@used` attribute.
   bool markedAsUsed() const { return IsUsed; }
 
   void setMarkedAsUsed(bool used) { IsUsed = used; }

lib/AST/Attr.cpp

@@ -1323,7 +1323,7 @@ bool DeclAttribute::printImpl(ASTPrinter &Printer, const PrintOptions &Options,
   }
 
   case DeclAttrKind::Section:
-    Printer.printAttrName("@_section");
+    Printer.printAttrName("@section");
     Printer << "(\"" << cast<SectionAttr>(this)->Name << "\")";
     break;
 

lib/AST/Decl.cpp

@@ -1342,7 +1342,7 @@ bool AbstractStorageDecl::isCompileTimeLiteral() const {
 }
 
 bool AbstractStorageDecl::isConstValue() const {
-  return getAttrs().hasAttribute<ConstValAttr>();
+  return getAttrs().hasAttribute<ConstValAttr>() || getAttrs().hasAttribute<SectionAttr>();
 }
 
 bool AbstractStorageDecl::isTransparent() const {