…ters in structs in C (llvm#93121) Fixes llvm#92687. Previously the attribute was only allowed on flexible array members. This patch patch changes this to also allow the attribute on pointer fields in structs and also allows late parsing of the attribute in some contexts. For example this previously wasn't allowed: ``` struct BufferTypeDeclAttributePosition { size_t count; char* buffer __counted_by(count); // Now allowed } ``` Note the attribute is prevented on pointee types where the size isn't known at compile time. In particular pointee types that are: * Incomplete (e.g. `void`) and sizeless types * Function types (e.g. the pointee of a function pointer) * Struct types with a flexible array member This patch also introduces late parsing of the attribute when used in the declaration attribute position. For example ``` struct BufferTypeDeclAttributePosition { char* buffer __counted_by(count); // Now allowed size_t count; } ``` is now allowed but **only** when passing `-fexperimental-late-parse-attributes`. The motivation for using late parsing here is to avoid breaking the data layout of structs in existing code that want to use the `counted_by` attribute. This patch is the first use of `LateAttrParseExperimentalExt` in `Attr.td` that was introduced in a previous patch. Note by allowing the attribute on struct member pointers this now allows the possiblity of writing the attribute in the type attribute position. For example: ``` struct BufferTypeAttributePosition { size_t count; char *__counted_by(count) buffer; // Now allowed } ``` However, the attribute in this position is still currently parsed immediately rather than late parsed. So this will not parse currently: ``` struct BufferTypeAttributePosition { char *__counted_by(count) buffer; // Fails to parse size_t count; } ``` The intention is to lift this restriction in future patches. It has not been done in this patch to keep this size of this commit small. There are also several other follow up changes that will need to be addressed in future patches: * Make late parsing working with anonymous structs (see `on_pointer_anon_buf` in `attr-counted-by-late-parsed-struct-ptrs.c`). * Allow `counted_by` on more subjects (e.g. parameters, returns types) when `-fbounds-safety` is enabled. * Make use of the attribute on pointer types in code gen (e.g. for `_builtin_dynamic_object_size` and UBSan's array-bounds checks). This work is heavily based on a patch originally written by Yeoul Na. ** Differences between llvm#93121 and this patch ** * The memory leak that caused llvm#93121 to be reverted (see llvm#92687) should now be fixed. See "The Memory Leak". * The fix to `pragma-attribute-supported-attributes-list.test` (originally in cef6387) has been incorporated into this patch. * A relaxation of counted_by semantics (originally in 112eadd) has been incorporated into this patch. * The assert in `Parser::DistributeCLateParsedAttrs` has been removed because that broke downstream code. * The switch statement in `Parser::ParseLexedCAttribute` has been removed in favor of using `Parser::ParseGNUAttributeArgs` which does the same thing but is more feature complete. * The `EnterScope` parameter has been plumbed through `Parser::ParseLexedCAttribute` and `Parser::ParseLexedCAttributeList`. It currently doesn't do anything but it will be needed in future commits. ** The Memory Leak ** The problem was that these lines parsed the attributes but then did nothing to free the memory ``` assert(!getLangOpts().CPlusPlus); for (auto *LateAttr : LateFieldAttrs) ParseLexedCAttribute(*LateAttr); ``` To fix this this a new `Parser::ParseLexedCAttributeList` method has been added (based on `Parser::ParseLexedAttributeList`) which does the necessary memory management. The intention is to merge these two methods together so there is just one implementation in a future patch. A more principled fixed here would be to fix the ownership of the `LateParsedAttribute` objects. In principle `LateParsedAttrList` should own its pointers exclusively and be responsible for deallocating them. Unfortunately this is complicated by `LateParsedAttribute` objects also being stored in another data structure (`LateParsedDeclarations`) as can be seen below (`LA` gets stored in two places). ``` // Handle attributes with arguments that require late parsing. LateParsedAttribute *LA = new LateParsedAttribute(this, *AttrName, AttrNameLoc); LateAttrs->push_back(LA); // Attributes in a class are parsed at the end of the class, along // with other late-parsed declarations. if (!ClassStack.empty() && !LateAttrs->parseSoon()) getCurrentClass().LateParsedDeclarations.push_back(LA); ``` this means the ownership of LateParsedAttribute objects isn't very clear. rdar://125400257

Reverted 0ec3b972e and cef6387e5 #92687

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