[Attributes] Support Attributes being declared as supporting an experimental late parsing mode "extension" (#88596)

This patch changes the `LateParsed` field of `Attr` in `Attr.td` to be
an instantiation of the new `LateAttrParseKind` class. The instation can be one of the following:

* `LateAttrParsingNever` - Corresponds with the false value of `LateParsed` prior to this patch (the default for an attribute).
* `LateAttrParseStandard` - Corresponds with the true value of `LateParsed` prior to this patch.
* `LateAttrParseExperimentalExt` - A new mode described below.

`LateAttrParseExperimentalExt` is an experimental extension to
`LateAttrParseStandard`. Essentially this allows
`Parser::ParseGNUAttributes(...)` to distinguish between these cases:

1. Only `LateAttrParseExperimentalExt` attributes should be late parsed.
2. Both `LateAttrParseExperimentalExt` and `LateAttrParseStandard`
  attributes should be late parsed.

Callers (and indirect callers) of `Parser::ParseGNUAttributes(...)`
indicate the desired behavior by setting a flag in the
`LateParsedAttrList` object that is passed to the function.

In addition to the above, a new driver and frontend flag
(`-fexperimental-late-parse-attributes`) with a corresponding LangOpt
(`ExperimentalLateParseAttributes`) is added that changes how
`LateAttrParseExperimentalExt` attributes are parsed.

* When the flag is disabled (default), in cases where only
  `LateAttrParsingExperimentalOnly` late parsing is requested, the
  attribute will be parsed immediately (i.e. **NOT** late parsed). This
  allows the attribute to act just like a `LateAttrParseStandard`
  attribute when the flag is disabled.

* When the flag is enabled, in cases where only
  `LateAttrParsingExperimentalOnly` late parsing is requested, the
  attribute will be late parsed.

The motivation behind this change is to allow the new `counted_by`
attribute (part of `-fbounds-safety`) to support late parsing but
**only** when `-fexperimental-late-parse-attributes` is enabled. This
attribute needs to support late parsing to allow it to refer to fields
later in a struct definition (or function parameters declared later).
However, there isn't a precedent for supporting late attribute parsing
in C so this flag allows the new behavior to exist in Clang but not be
on by default. This behavior was requested as part of the
`-fbounds-safety` RFC process
(https://discourse.llvm.org/t/rfc-enforcing-bounds-safety-in-c-fbounds-safety/70854/68).

This patch doesn't introduce any uses of `LateAttrParseExperimentalExt`.
This will be added for the `counted_by` attribute in a future patch
(https://github.com/llvm/llvm-project/pull/87596). A consequence is the
new behavior added in this patch is not yet testable. Hence, the lack of
tests covering the new behavior.

rdar://125400257

1 files changed, 34 insertions, 6 deletions

diff --git a/clang/lib/Parse/ParseDecl.cpp b/clang/lib/Parse/ParseDecl.cpp
index a7846e1..7431c25 100644
--- a/clang/lib/Parse/ParseDecl.cpp
+++ b/clang/lib/Parse/ParseDecl.cpp
@@ -91,13 +91,23 @@ static StringRef normalizeAttrName(StringRef Name) {
   return Name;
 }
 
-/// isAttributeLateParsed - Return true if the attribute has arguments that
-/// require late parsing.
-static bool isAttributeLateParsed(const IdentifierInfo &II) {
+/// returns true iff attribute is annotated with `LateAttrParseExperimentalExt`
+/// in `Attr.td`.
+static bool IsAttributeLateParsedExperimentalExt(const IdentifierInfo &II) {
+#define CLANG_ATTR_LATE_PARSED_EXPERIMENTAL_EXT_LIST
+  return llvm::StringSwitch<bool>(normalizeAttrName(II.getName()))
+#include "clang/Parse/AttrParserStringSwitches.inc"
+      .Default(false);
+#undef CLANG_ATTR_LATE_PARSED_EXPERIMENTAL_EXT_LIST
+}
+
+/// returns true iff attribute is annotated with `LateAttrParseStandard` in
+/// `Attr.td`.
+static bool IsAttributeLateParsedStandard(const IdentifierInfo &II) {
 #define CLANG_ATTR_LATE_PARSED_LIST
-    return llvm::StringSwitch<bool>(normalizeAttrName(II.getName()))
+  return llvm::StringSwitch<bool>(normalizeAttrName(II.getName()))
 #include "clang/Parse/AttrParserStringSwitches.inc"
-        .Default(false);
+      .Default(false);
 #undef CLANG_ATTR_LATE_PARSED_LIST
 }
 
@@ -222,8 +232,26 @@ void Parser::ParseGNUAttributes(ParsedAttributes &Attrs,
         continue;
       }
 
+      bool LateParse = false;
+      if (!LateAttrs)
+        LateParse = false;
+      else if (LateAttrs->lateAttrParseExperimentalExtOnly()) {
+        // The caller requested that this attribute **only** be late
+        // parsed for `LateAttrParseExperimentalExt` attributes. This will
+        // only be late parsed if the experimental language option is enabled.
+        LateParse = getLangOpts().ExperimentalLateParseAttributes &&
+                    IsAttributeLateParsedExperimentalExt(*AttrName);
+      } else {
+        // The caller did not restrict late parsing to only
+        // `LateAttrParseExperimentalExt` attributes so late parse
+        // both `LateAttrParseStandard` and `LateAttrParseExperimentalExt`
+        // attributes.
+        LateParse = IsAttributeLateParsedExperimentalExt(*AttrName) ||
+                    IsAttributeLateParsedStandard(*AttrName);
+      }
+
       // Handle "parameterized" attributes
-      if (!LateAttrs || !isAttributeLateParsed(*AttrName)) {
+      if (!LateParse) {
         ParseGNUAttributeArgs(AttrName, AttrNameLoc, Attrs, &EndLoc, nullptr,
                               SourceLocation(), ParsedAttr::Form::GNU(), D);
         continue;