Consolidate checking for msvc when generating debuginfo

If the target we're generating code for is msvc, then we do two main
things differently: we generate type names in a C++ style instead of a
Rust style and we generate debuginfo for enums differently.

I've refactored the code so that there is one function
(`cpp_like_debuginfo`) which determines if we should use the C++ style
of naming types and other debuginfo generation or the regular Rust one.

compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs

@@ -18,6 +18,7 @@ use crate::llvm::debuginfo::{
 use crate::value::Value;
 
 use cstr::cstr;
+use rustc_codegen_ssa::debuginfo::type_names::cpp_like_debuginfo;
 use rustc_codegen_ssa::traits::*;
 use rustc_data_structures::fingerprint::Fingerprint;
 use rustc_data_structures::fx::FxHashMap;
@@ -933,16 +934,16 @@ fn basic_type_metadata<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>, t: Ty<'tcx>) -> &'l
 
     // When targeting MSVC, emit MSVC style type names for compatibility with
     // .natvis visualizers (and perhaps other existing native debuggers?)
-    let msvc_like_names = cx.tcx.sess.target.is_like_msvc;
+    let cpp_like_debuginfo = cpp_like_debuginfo(cx.tcx);
 
     let (name, encoding) = match t.kind() {
         ty::Never => ("!", DW_ATE_unsigned),
         ty::Tuple(elements) if elements.is_empty() => ("()", DW_ATE_unsigned),
         ty::Bool => ("bool", DW_ATE_boolean),
         ty::Char => ("char", DW_ATE_unsigned_char),
-        ty::Int(int_ty) if msvc_like_names => (int_ty.msvc_basic_name(), DW_ATE_signed),
-        ty::Uint(uint_ty) if msvc_like_names => (uint_ty.msvc_basic_name(), DW_ATE_unsigned),
-        ty::Float(float_ty) if msvc_like_names => (float_ty.msvc_basic_name(), DW_ATE_float),
+        ty::Int(int_ty) if cpp_like_debuginfo => (int_ty.msvc_basic_name(), DW_ATE_signed),
+        ty::Uint(uint_ty) if cpp_like_debuginfo => (uint_ty.msvc_basic_name(), DW_ATE_unsigned),
+        ty::Float(float_ty) if cpp_like_debuginfo => (float_ty.msvc_basic_name(), DW_ATE_float),
         ty::Int(int_ty) => (int_ty.name_str(), DW_ATE_signed),
         ty::Uint(uint_ty) => (uint_ty.name_str(), DW_ATE_unsigned),
         ty::Float(float_ty) => (float_ty.name_str(), DW_ATE_float),
@@ -959,7 +960,7 @@ fn basic_type_metadata<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>, t: Ty<'tcx>) -> &'l
         )
     };
 
-    if !msvc_like_names {
+    if !cpp_like_debuginfo {
         return ty_metadata;
     }
 
@@ -1521,13 +1522,6 @@ fn prepare_union_metadata<'ll, 'tcx>(
 // Enums
 //=-----------------------------------------------------------------------------
 
-/// DWARF variant support is only available starting in LLVM 8, but
-/// on MSVC we have to use the fallback mode, because LLVM doesn't
-/// lower variant parts to PDB.
-fn use_enum_fallback(cx: &CodegenCx<'_, '_>) -> bool {
-    cx.sess().target.is_like_msvc
-}
-
 // FIXME(eddyb) maybe precompute this? Right now it's computed once
 // per generator monomorphization, but it doesn't depend on substs.
 fn generator_layout_and_saved_local_names<'tcx>(
@@ -1602,7 +1596,10 @@ impl<'ll, 'tcx> EnumMemberDescriptionFactory<'ll, 'tcx> {
             _ => bug!(),
         };
 
-        let fallback = use_enum_fallback(cx);
+        // While LLVM supports generating debuginfo for variant types (enums), it doesn't support
+        // lowering that debuginfo to CodeView records for msvc targets. So if we are targeting
+        // msvc, then we need to use a different, fallback encoding of the debuginfo.
+        let fallback = cpp_like_debuginfo(cx.tcx);
         // This will always find the metadata in the type map.
         let self_metadata = type_metadata(cx, self.enum_type, self.span);
 
@@ -2155,7 +2152,10 @@ fn prepare_enum_metadata<'ll, 'tcx>(
         return FinalMetadata(discriminant_type_metadata(tag.value));
     }
 
-    if use_enum_fallback(cx) {
+    // While LLVM supports generating debuginfo for variant types (enums), it doesn't support
+    // lowering that debuginfo to CodeView records for msvc targets. So if we are targeting
+    // msvc, then we need to use a different encoding of the debuginfo.
+    if cpp_like_debuginfo(tcx) {
         let discriminant_type_metadata = match layout.variants {
             Variants::Single { .. } => None,
             Variants::Multiple { tag_encoding: TagEncoding::Niche { .. }, tag, .. }