[Clang] Support free positioning of labels inside compound statements in C

[Izaron]

Summary

Implements part of paper P2324R2
https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2508.pdf
Fixes https://github.com/llvm/llvm-project/issues/61507

[Izaron]

A big TODO: rename ext_c_label_end_of_compound_statement to something like ext_c_label_free_positioning. Since we found out it is more complete than just the end of statements.

clang/lib/Parse/ParseStmt.cpp

695–697

Place in code where it matters: https://github.com/llvm/llvm-project/blob/84870c4abbacfb97ca025c9219b724f381611380/clang/lib/Parse/ParseStmt.cpp#L237

clang/test/OpenMP/threadprivate_messages.cpp

141

TODO: find out if this is okay? (I've got no idea what it does)

clang/test/Parser/c2x-label.c

5–6

TODO: make sure a warning is printed. I couldn't find correct conditions for the warning, I tried and it produced false positive warnings in some other tests =(

P.S. Its C++'s counterpart: https://github.com/llvm/llvm-project/blob/main/clang/test/Parser/cxx2b-label.cpp

[aaron.ballman]

clang/lib/Parse/ParseStmt.cpp

745

At this point, you know whether you've parsed a declaration statement or not, so you could potentially add the extension diagnostic logic here.

clang/test/OpenMP/threadprivate_messages.cpp

141

CC @mikerice @jyu2 @jdoerfert for opinions on OpenMP impact. There are FIXMEs in the code related to this: https://github.com/llvm/llvm-project/blob/main/clang/lib/Parse/ParseOpenMP.cpp#L2671

clang/test/Parser/c2x-label.c

5–6

Yeah, this would be specific to C because C++ has always allowed a declaration there. I left a comment above with a potential idea.

clang/lib/Parse/ParseStmt.cpp

 ///       labeled-statement:
 ///         label statement
 ///
 StmtResult Parser::ParseLabeledStatement(ParsedAttributes &Attrs,
                                          ParsedStmtContext StmtCtx) {
   assert(Tok.is(tok::identifier) && Tok.getIdentifierInfo() &&
          "Not an identifier!");
 
-  // The substatement is always a 'statement', not a 'declaration', but is
-  // otherwise in the same context as the labeled-statement.
-  StmtCtx &= ~ParsedStmtContext::AllowDeclarationsInC;
-
   Token IdentTok = Tok;  // Save the whole token.
   ConsumeToken();  // eat the identifier.
 
   assert(Tok.is(tok::colon) && "Not a label!");
 
   // identifier ':' statement
   SourceLocation ColonLoc = ConsumeToken();
 
StmtResult Parser::ParseLabeledStatement(ParsedAttributes &Attrs,
 
   // If we've not parsed a statement yet, parse one now.
   if (!SubStmt.isInvalid() && !SubStmt.isUsable())
     SubStmt = ParseStatement(nullptr, StmtCtx);
 
   // Broken substmt shouldn't prevent the label from being added to the AST.
   if (SubStmt.isInvalid())
     SubStmt = Actions.ActOnNullStmt(ColonLoc);
 
   LabelDecl *LD = Actions.LookupOrCreateLabel(IdentTok.getIdentifierInfo(),
                                               IdentTok.getLocation());
   Actions.ProcessDeclAttributeList(Actions.CurScope, LD, Attrs);
   Attrs.clear();
 
   return Actions.ActOnLabelStmt(IdentTok.getLocation(), LD, ColonLoc,
                                 SubStmt.get());
 }

clang/test/OpenMP/allocate_messages.cpp

 #pragma omp allocate(d1)
   {
     ++a;
     d2 = 0;
 #pragma omp allocate(d3) // expected-error {{'#pragma omp allocate' must appear in the scope of the 'd3' variable declaration}}
   }
 #pragma omp allocate(d3)
 label:
-#pragma omp allocate(d4) // expected-error {{'#pragma omp allocate' cannot be an immediate substatement}}
+#pragma omp allocate(d4)
 
 #pragma omp allocate(a) // expected-error {{'#pragma omp allocate' must appear in the scope of the 'a' variable declaration}}
   return (y);
 #pragma omp allocate(d) // expected-error {{'#pragma omp allocate' must appear in the scope of the 'd' variable declaration}}
 #pragma omp parallel allocate(d) // expected-error {{the referenced item is not found in any private clause on the same directive}}
   ;
 }

clang/test/OpenMP/threadprivate_messages.cpp

 #pragma omp threadprivate(d2) // expected-error {{'#pragma omp threadprivate' must precede all references to variable 'd2'}}
 #pragma omp threadprivate(d1)
   {
   ++a;d2=0;
 #pragma omp threadprivate(d3) // expected-error {{'#pragma omp threadprivate' must appear in the scope of the 'd3' variable declaration}}
   }
 #pragma omp threadprivate(d3)
 label:
-#pragma omp threadprivate(d4) // expected-error {{'#pragma omp threadprivate' cannot be an immediate substatement}}
+#pragma omp threadprivate(d4)
 
 #pragma omp threadprivate(a) // expected-error {{'#pragma omp threadprivate' must appear in the scope of the 'a' variable declaration}}
   return (y);
 #pragma omp threadprivate(d) // expected-error {{'#pragma omp threadprivate' must appear in the scope of the 'd' variable declaration}}
 }

clang/test/Parser/c2x-label.c

 // RUN: %clang_cc1 -fsyntax-only -std=c17 -Wc2x-compat -verify=c17 %s
 // RUN: %clang_cc1 -fsyntax-only -std=c2x -Wpre-c2x-compat -verify=c2x %s
 
 void test_label_in_func() {
-    int x;
 label1:
+    int x;
+label2:
     x = 1;
-label2: label3: label4:
+label3: label4: label5:
 } // c17-warning {{label at end of compound statement is a C2x extension}} \
      c2x-warning {{label at end of compound statement is incompatible with C standards before C2x}}
 
 int test_label_in_switch(int v) {
     switch (v) {
     case 1:
         return 1;
     case 2: