Fix DWARF generation for enums

The DWARF generated for Rust enums was always somewhat unusual.
Rather than using DWARF constructs directly, it would emit magic field
names like "RUST$ENCODED$ENUM$0$Name" and "RUST$ENUM$DISR".  Since
PR #45225, though, even this has not worked -- the ad hoc scheme was
not updated to handle the wider variety of niche-filling layout
optimizations now available.

This patch changes the generated DWARF to use the standard tags meant
for this purpose; namely, DW_TAG_variant and DW_TAG_variant_part.

The patch to implement this went in to LLVM 7.  In order to work with
older versions of LLVM, and because LLVM doesn't do anything here for
PDB, the existing code is kept as a fallback mode.

Support for this DWARF is in the Rust lldb and in gdb 8.2.

Closes #32920
Closes #32924
Closes #52762
Closes #53153

1 files changed, 47 insertions, 2 deletions

diff --git a/src/rustllvm/RustWrapper.cpp b/src/rustllvm/RustWrapper.cpp
index bf7afa1b6c0..affec73e3ac 100644
--- a/src/rustllvm/RustWrapper.cpp
+++ b/src/rustllvm/RustWrapper.cpp
@@ -713,6 +713,21 @@ extern "C" LLVMMetadataRef LLVMRustDIBuilderCreateStructType(
       unwrapDI<DIType>(VTableHolder), UniqueId));
 }
 
+extern "C" LLVMMetadataRef LLVMRustDIBuilderCreateVariantPart(
+    LLVMRustDIBuilderRef Builder, LLVMMetadataRef Scope, const char *Name,
+    LLVMMetadataRef File, unsigned LineNumber, uint64_t SizeInBits,
+    uint32_t AlignInBits, LLVMRustDIFlags Flags, LLVMMetadataRef Discriminator,
+    LLVMMetadataRef Elements, const char *UniqueId) {
+#if LLVM_VERSION_GE(7, 0)
+  return wrap(Builder->createVariantPart(
+      unwrapDI<DIDescriptor>(Scope), Name, unwrapDI<DIFile>(File), LineNumber,
+      SizeInBits, AlignInBits, fromRust(Flags), unwrapDI<DIDerivedType>(Discriminator),
+      DINodeArray(unwrapDI<MDTuple>(Elements)), UniqueId));
+#else
+  abort();
+#endif
+}
+
 extern "C" LLVMMetadataRef LLVMRustDIBuilderCreateMemberType(
     LLVMRustDIBuilderRef Builder, LLVMMetadataRef Scope, const char *Name,
     LLVMMetadataRef File, unsigned LineNo, uint64_t SizeInBits,
@@ -724,6 +739,28 @@ extern "C" LLVMMetadataRef LLVMRustDIBuilderCreateMemberType(
                                         fromRust(Flags), unwrapDI<DIType>(Ty)));
 }
 
+extern "C" LLVMMetadataRef LLVMRustDIBuilderCreateVariantMemberType(
+    LLVMRustDIBuilderRef Builder, LLVMMetadataRef Scope,
+    const char *Name, LLVMMetadataRef File, unsigned LineNo, uint64_t SizeInBits,
+    uint32_t AlignInBits, uint64_t OffsetInBits, LLVMValueRef Discriminant,
+    LLVMRustDIFlags Flags, LLVMMetadataRef Ty) {
+#if LLVM_VERSION_GE(7, 0)
+  llvm::ConstantInt* D = nullptr;
+  if (Discriminant) {
+    D = unwrap<llvm::ConstantInt>(Discriminant);
+  }
+  return wrap(Builder->createVariantMemberType(unwrapDI<DIDescriptor>(Scope), Name,
+                                               unwrapDI<DIFile>(File), LineNo,
+                                               SizeInBits, AlignInBits, OffsetInBits, D,
+                                               fromRust(Flags), unwrapDI<DIType>(Ty)));
+#else
+  return wrap(Builder->createMemberType(unwrapDI<DIDescriptor>(Scope), Name,
+                                        unwrapDI<DIFile>(File), LineNo,
+                                        SizeInBits, AlignInBits, OffsetInBits,
+                                        fromRust(Flags), unwrapDI<DIType>(Ty)));
+#endif
+}
+
 extern "C" LLVMMetadataRef LLVMRustDIBuilderCreateLexicalBlock(
     LLVMRustDIBuilderRef Builder, LLVMMetadataRef Scope,
     LLVMMetadataRef File, unsigned Line, unsigned Col) {
@@ -826,11 +863,19 @@ extern "C" LLVMMetadataRef LLVMRustDIBuilderCreateEnumerationType(
     LLVMRustDIBuilderRef Builder, LLVMMetadataRef Scope, const char *Name,
     LLVMMetadataRef File, unsigned LineNumber, uint64_t SizeInBits,
     uint32_t AlignInBits, LLVMMetadataRef Elements,
-    LLVMMetadataRef ClassTy) {
+    LLVMMetadataRef ClassTy, bool IsFixed) {
+#if LLVM_VERSION_GE(7, 0)
   return wrap(Builder->createEnumerationType(
       unwrapDI<DIDescriptor>(Scope), Name, unwrapDI<DIFile>(File), LineNumber,
       SizeInBits, AlignInBits, DINodeArray(unwrapDI<MDTuple>(Elements)),
-      unwrapDI<DIType>(ClassTy)));
+      unwrapDI<DIType>(ClassTy), "", IsFixed));
+#else
+  // Ignore IsFixed on older LLVM.
+  return wrap(Builder->createEnumerationType(
+      unwrapDI<DIDescriptor>(Scope), Name, unwrapDI<DIFile>(File), LineNumber,
+      SizeInBits, AlignInBits, DINodeArray(unwrapDI<MDTuple>(Elements)),
+      unwrapDI<DIType>(ClassTy), ""));
+#endif
 }
 
 extern "C" LLVMMetadataRef LLVMRustDIBuilderCreateUnionType(