Generate more precise generator names

Currently all generators are named with a `generator$N` suffix,
regardless of where they come from. This means an `async fn` shows up as
a generator in stack traces, which can be surprising to async
programmers since they should not need to know that async functions are
implementated using generators.

This change generators a different name depending on the generator kind,
allowing us to tell whether the generator is the result of an async
block, an async closure, an async fn, or a plain generator.

Author: eholk

compiler/rustc_codegen_ssa/src/debuginfo/type_names.rs

@@ -519,12 +519,18 @@ fn push_unqualified_item_name(
             output.push_str(tcx.crate_name(def_id.krate).as_str());
         }
         DefPathData::ClosureExpr if tcx.generator_kind(def_id).is_some() => {
+            let key = match tcx.generator_kind(def_id).unwrap() {
+                hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Block) => "async_block",
+                hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure) => "async_closure",
+                hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn) => "async_fn",
+                hir::GeneratorKind::Gen => "generator",
+            };
             // Generators look like closures, but we want to treat them differently
             // in the debug info.
             if cpp_like_debuginfo(tcx) {
-                write!(output, "generator${}", disambiguated_data.disambiguator).unwrap();
+                write!(output, "{}${}", key, disambiguated_data.disambiguator).unwrap();
             } else {
-                write!(output, "{{generator#{}}}", disambiguated_data.disambiguator).unwrap();
+                write!(output, "{{{}#{}}}", key, disambiguated_data.disambiguator).unwrap();
             }
         }
         _ => match disambiguated_data.data.name() {

src/test/codegen/async-fn-debug-msvc.rs

@@ -17,7 +17,7 @@ async fn async_fn_test() {
 // FIXME: No way to reliably check the filename.
 
 // CHECK-DAG:  [[ASYNC_FN:!.*]] = !DINamespace(name: "async_fn_test"
-// CHECK-DAG:  [[GEN:!.*]] = !DICompositeType(tag: DW_TAG_union_type, name: "generator$0"
+// CHECK-DAG:  [[GEN:!.*]] = !DICompositeType(tag: DW_TAG_union_type, name: "async_fn$0"
 // CHECK:      {{!.*}} = !DIDerivedType(tag: DW_TAG_member, name: "variant0", scope: [[GEN]],
 // For brevity, we only check the struct name and members of the last variant.
 // CHECK-SAME: file: [[FILE:![0-9]*]], line: 11,