[Compile Time Values] Add syntactic verification of valid expressions in '@const' contexts

Syntactically verify that initializer expressions of '@const' variables and argument expressions to '@const' parameters consist strictly of syntactically-verifiable set of basic values and operations

Author: artemcm

include/swift/AST/DiagnosticsSema.def

@@ -816,9 +816,34 @@ ERROR(cannot_return_value_from_void_func,none,
 NOTE(add_return_type_note,none,
      "did you mean to add a return type?", ())
 
-ERROR(expect_compile_time_const,none,
+ERROR(expect_compile_time_literal,none,
       "expect a compile-time constant literal", ())
 
+ERROR(const_unsupported_enum_associated_value,none,
+      "enums with associated values not supported in a '@const' expression", ())
+ERROR(const_unsupported_operator,none,
+      "unsupported operator in a '@const' expression", ())
+ERROR(const_unsupported_type,none,
+      "unsupported type in a '@const' expression", ())
+ERROR(const_unsupported_type_expr,none,
+      "type expressions not supported in a '@const' expression", ())
+ERROR(const_unsupported_closure,none,
+      "closures not supported in a '@const' expression", ())
+ERROR(const_unsupported_keypath,none,
+      "keypaths not supported in a '@const' expression", ())
+ERROR(const_opaque_decl_ref,none,
+      "unable to resolve variable reference in a '@const' expression", ())
+ERROR(const_opaque_func_decl_ref,none,
+      "unable to resolve function reference in a '@const' expression", ())
+ERROR(const_non_convention_c_conversion,none,
+      "only 'convention(c)' function values are supported in a '@const' expression", ())
+ERROR(const_opaque_callee,none,
+      "unable to resolve callee in a '@const' expression", ())
+ERROR(const_non_const_param,none,
+      "reference to a non-'@const' parameter in a '@const' expression", ())
+ERROR(const_unknown_default,none,
+      "not supported in a '@const' expression", ())
+
 //------------------------------------------------------------------------------
 // MARK: Import Resolution
 //------------------------------------------------------------------------------

include/swift/Basic/Features.def

@@ -497,6 +497,9 @@ EXPERIMENTAL_FEATURE(InlineArrayTypeSugar, false)
 /// Allow declaration of compile-time values
 EXPERIMENTAL_FEATURE(CompileTimeValues, true)
 
+/// Allow declaration of compile-time values
+EXPERIMENTAL_FEATURE(CompileTimeValuesPreview, false)
+
 /// Allow function body macros applied to closures.
 EXPERIMENTAL_FEATURE(ClosureBodyMacro, true)
 

lib/AST/FeatureSet.cpp

@@ -408,6 +408,10 @@ static bool usesFeatureCompileTimeValues(Decl *decl) {
          decl->getAttrs().hasAttribute<ConstInitializedAttr>();
 }
 
+static bool usesFeatureCompileTimeValuesPreview(Decl *decl) {
+  return false;
+}
+
 static bool usesFeatureClosureBodyMacro(Decl *decl) {
   return false;
 }

lib/Sema/CMakeLists.txt

@@ -38,6 +38,7 @@ add_swift_host_library(swiftSema STATIC
   DerivedConformance/DerivedConformanceRawRepresentable.cpp
   ImportResolution.cpp
   InstrumenterSupport.cpp
+  LegalConstExprVerifier.cpp
   LookupVisibleDecls.cpp
   MiscDiagnostics.cpp
   OpenedExistentials.cpp

lib/Sema/CSDiagnostics.cpp

@@ -6521,7 +6521,7 @@ bool ExtraneousReturnFailure::diagnoseAsError() {
 }
 
 bool NotCompileTimeLiteralFailure::diagnoseAsError() {
-  emitDiagnostic(diag::expect_compile_time_const);
+  emitDiagnostic(diag::expect_compile_time_literal);
   return true;
 }