Introduce _BitInt, deprecate _ExtInt

WG14 adopted the _ExtInt feature from Clang for C23, but renamed the
type to be _BitInt. This patch does the vast majority of the work to
rename _ExtInt to _BitInt, which accounts for most of its size. The new
type is exposed in older C modes and all C++ modes as a conforming
extension. However, there are functional changes worth calling out:

* Deprecates _ExtInt with a fix-it to help users migrate to _BitInt.
* Updates the mangling for the type.
* Updates the documentation and adds a release note to warn users what
is going on.
* Adds new diagnostics for use of _BitInt to call out when it's used as
a Clang extension or as a pre-C23 compatibility concern.
* Adds new tests for the new diagnostic behaviors.

I want to call out the ABI break specifically. We do not believe that
this break will cause a significant imposition for early adopters of
the feature, and so this is being done as a full break. If it turns out
there are critical uses where recompilation is not an option for some
reason, we can consider using ABI tags to ease the transition.

1 files changed, 6 insertions, 6 deletions

diff --git a/clang/lib/Sema/DeclSpec.cpp b/clang/lib/Sema/DeclSpec.cpp
index 4405f29..d4dc790 100644
--- a/clang/lib/Sema/DeclSpec.cpp
+++ b/clang/lib/Sema/DeclSpec.cpp
@@ -365,7 +365,7 @@ bool Declarator::isDeclarationOfFunction() const {
     case TST_half:
     case TST_int:
     case TST_int128:
-    case TST_extint:
+    case TST_bitint:
     case TST_struct:
     case TST_interface:
     case TST_union:
@@ -551,7 +551,7 @@ const char *DeclSpec::getSpecifierName(DeclSpec::TST T,
   case DeclSpec::TST_char32:      return "char32_t";
   case DeclSpec::TST_int:         return "int";
   case DeclSpec::TST_int128:      return "__int128";
-  case DeclSpec::TST_extint:      return "_ExtInt";
+  case DeclSpec::TST_bitint:      return "_BitInt";
   case DeclSpec::TST_half:        return "half";
   case DeclSpec::TST_float:       return "float";
   case DeclSpec::TST_double:      return "double";
@@ -932,7 +932,7 @@ bool DeclSpec::SetTypeSpecError() {
   return false;
 }
 
-bool DeclSpec::SetExtIntType(SourceLocation KWLoc, Expr *BitsExpr,
+bool DeclSpec::SetBitIntType(SourceLocation KWLoc, Expr *BitsExpr,
                              const char *&PrevSpec, unsigned &DiagID,
                              const PrintingPolicy &Policy) {
   assert(BitsExpr && "no expression provided!");
@@ -945,7 +945,7 @@ bool DeclSpec::SetExtIntType(SourceLocation KWLoc, Expr *BitsExpr,
     return true;
   }
 
-  TypeSpecType = TST_extint;
+  TypeSpecType = TST_bitint;
   ExprRep = BitsExpr;
   TSTLoc = KWLoc;
   TSTNameLoc = KWLoc;
@@ -1252,7 +1252,7 @@ void DeclSpec::Finish(Sema &S, const PrintingPolicy &Policy) {
       TypeSpecType = TST_int; // unsigned -> unsigned int, signed -> signed int.
     else if (TypeSpecType != TST_int && TypeSpecType != TST_int128 &&
              TypeSpecType != TST_char && TypeSpecType != TST_wchar &&
-             !IsFixedPointType && TypeSpecType != TST_extint) {
+             !IsFixedPointType && TypeSpecType != TST_bitint) {
       S.Diag(TSSLoc, diag::err_invalid_sign_spec)
         << getSpecifierName((TST)TypeSpecType, Policy);
       // signed double -> double.
@@ -1302,7 +1302,7 @@ void DeclSpec::Finish(Sema &S, const PrintingPolicy &Policy) {
                                                  " double");
       TypeSpecType = TST_double;   // _Complex -> _Complex double.
     } else if (TypeSpecType == TST_int || TypeSpecType == TST_char ||
-               TypeSpecType == TST_extint) {
+               TypeSpecType == TST_bitint) {
       // Note that this intentionally doesn't include _Complex _Bool.
       if (!S.getLangOpts().CPlusPlus)
         S.Diag(TSTLoc, diag::ext_integer_complex);