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, 30 insertions, 3 deletions

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index 1bdeccc..0c1f88b 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -2891,7 +2891,8 @@ void Parser::ParseAlignmentSpecifier(ParsedAttributes &Attrs,
 }
 
 ExprResult Parser::ParseExtIntegerArgument() {
-  assert(Tok.is(tok::kw__ExtInt) && "Not an extended int type");
+  assert(Tok.isOneOf(tok::kw__ExtInt, tok::kw__BitInt) &&
+         "Not an extended int type");
   ConsumeToken();
 
   BalancedDelimiterTracker T(*this, tok::l_paren);
@@ -3882,11 +3883,13 @@ void Parser::ParseDeclarationSpecifiers(DeclSpec &DS,
       isInvalid = DS.SetTypeSpecType(DeclSpec::TST_int, Loc, PrevSpec,
                                      DiagID, Policy);
       break;
-    case tok::kw__ExtInt: {
+    case tok::kw__ExtInt:
+    case tok::kw__BitInt: {
+      DiagnoseBitIntUse(Tok);
       ExprResult ER = ParseExtIntegerArgument();
       if (ER.isInvalid())
         continue;
-      isInvalid = DS.SetExtIntType(Loc, ER.get(), PrevSpec, DiagID, Policy);
+      isInvalid = DS.SetBitIntType(Loc, ER.get(), PrevSpec, DiagID, Policy);
       ConsumedEnd = PrevTokLocation;
       break;
     }
@@ -5015,6 +5018,7 @@ bool Parser::isKnownToBeTypeSpecifier(const Token &Tok) const {
   case tok::kw_char32_t:
   case tok::kw_int:
   case tok::kw__ExtInt:
+  case tok::kw__BitInt:
   case tok::kw___bf16:
   case tok::kw_half:
   case tok::kw_float:
@@ -5097,6 +5101,7 @@ bool Parser::isTypeSpecifierQualifier() {
   case tok::kw_char32_t:
   case tok::kw_int:
   case tok::kw__ExtInt:
+  case tok::kw__BitInt:
   case tok::kw_half:
   case tok::kw___bf16:
   case tok::kw_float:
@@ -5268,6 +5273,7 @@ bool Parser::isDeclarationSpecifier(bool DisambiguatingWithExpression) {
 
   case tok::kw_int:
   case tok::kw__ExtInt:
+  case tok::kw__BitInt:
   case tok::kw_half:
   case tok::kw___bf16:
   case tok::kw_float:
@@ -7476,3 +7482,24 @@ bool Parser::TryAltiVecTokenOutOfLine(DeclSpec &DS, SourceLocation Loc,
   }
   return false;
 }
+
+void Parser::DiagnoseBitIntUse(const Token &Tok) {
+  // If the token is for _ExtInt, diagnose it as being deprecated. Otherwise,
+  // the token is about _BitInt and gets (potentially) diagnosed as use of an
+  // extension.
+  assert(Tok.isOneOf(tok::kw__ExtInt, tok::kw__BitInt) &&
+         "expected either an _ExtInt or _BitInt token!");
+
+  SourceLocation Loc = Tok.getLocation();
+  if (Tok.is(tok::kw__ExtInt)) {
+    Diag(Loc, diag::warn_ext_int_deprecated)
+        << FixItHint::CreateReplacement(Loc, "_BitInt");
+  } else {
+    // In C2x mode, diagnose that the use is not compatible with pre-C2x modes.
+    // Otherwise, diagnose that the use is a Clang extension.
+    if (getLangOpts().C2x)
+      Diag(Loc, diag::warn_c17_compat_bit_int);
+    else
+      Diag(Loc, diag::ext_bit_int) << getLangOpts().CPlusPlus;
+  }
+}