Name the captured upvars for closures/generators in debuginfo

Previously, debuggers print closures as something like
```
y::main::closure-0 (0x7fffffffdd34)
```
The pointer actually references to an upvar. It is not
very obvious, especially for beginners.

It's because upvars don't have names before, as they
are packed into a tuple. This commit names the upvars,
so we can expect to see something like
```
y::main::closure-0 {_captured_ref__b: 0x[...]}
```

Author: lrh2000

compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs

@@ -1289,14 +1289,36 @@ struct TupleMemberDescriptionFactory<'tcx> {
 
 impl<'tcx> TupleMemberDescriptionFactory<'tcx> {
     fn create_member_descriptions(&self, cx: &CodegenCx<'ll, 'tcx>) -> Vec<MemberDescription<'ll>> {
+        // For closures and generators, name the captured upvars
+        // with the help of `CapturedPlace::to_mangled_name`.
+        let closure_def_id = match *self.ty.kind() {
+            ty::Generator(def_id, ..) => def_id.as_local(),
+            ty::Closure(def_id, ..) => def_id.as_local(),
+            _ => None,
+        };
+        let captures = match closure_def_id {
+            Some(local_def_id) => {
+                let typeck_results = cx.tcx.typeck(local_def_id);
+                let captures = typeck_results
+                    .closure_min_captures_flattened(local_def_id.to_def_id())
+                    .collect::<Vec<_>>();
+                Some(captures)
+            }
+            _ => None,
+        };
+
         let layout = cx.layout_of(self.ty);
         self.component_types
             .iter()
             .enumerate()
             .map(|(i, &component_type)| {
                 let (size, align) = cx.size_and_align_of(component_type);
+                let name = captures
+                    .as_ref()
+                    .map(|c| c[i].to_mangled_name(cx.tcx))
+                    .unwrap_or_else(|| format!("__{}", i));
                 MemberDescription {
-                    name: format!("__{}", i),
+                    name,
                     type_metadata: type_metadata(cx, component_type, self.span),
                     offset: layout.fields.offset(i),
                     size,

compiler/rustc_middle/src/ty/closure.rs

@@ -159,6 +159,54 @@ impl CapturedPlace<'tcx> {
         place_to_string_for_capture(tcx, &self.place)
     }
 
+    /// Returns mangled names of captured upvars. Here are some examples:
+    ///  - `_captured_val__name__field`
+    ///  - `_captured_ref__name__field`
+    ///
+    /// The purpose is to use those names in debuginfo. They should be human-understandable.
+    /// Without the names, the end users may get confused when the debuggers just print some
+    /// pointers in closures or generators.
+    pub fn to_mangled_name(&self, tcx: TyCtxt<'tcx>) -> String {
+        let prefix = match self.info.capture_kind {
+            ty::UpvarCapture::ByValue(_) => "_captured_val__",
+            ty::UpvarCapture::ByRef(_) => "_captured_ref__",
+        };
+
+        let hir_id = match self.place.base {
+            HirPlaceBase::Upvar(upvar_id) => upvar_id.var_path.hir_id,
+            base => bug!("Expected an upvar, found {:?}", base),
+        };
+        let name = tcx.hir().name(hir_id);
+
+        let mut ty = self.place.base_ty;
+        let mut fields = String::new();
+        for proj in self.place.projections.iter() {
+            match proj.kind {
+                HirProjectionKind::Field(idx, variant) => match ty.kind() {
+                    ty::Tuple(_) => fields = format!("{}__{}", fields, idx),
+                    ty::Adt(def, ..) => {
+                        fields = format!(
+                            "{}__{}",
+                            fields,
+                            def.variants[variant].fields[idx as usize].ident.name.as_str(),
+                        );
+                    }
+                    ty => {
+                        bug!("Unexpected type {:?} for `Field` projection", ty)
+                    }
+                },
+
+                // Ignore derefs for now, as they are likely caused by
+                // autoderefs that don't appear in the original code.
+                HirProjectionKind::Deref => {}
+                proj => bug!("Unexpected projection {:?} in captured place", proj),
+            }
+            ty = proj.ty;
+        }
+
+        prefix.to_owned() + &name.to_string() + &fields
+    }
+
     /// Returns the hir-id of the root variable for the captured place.
     /// e.g., if `a.b.c` was captured, would return the hir-id for `a`.
     pub fn get_root_variable(&self) -> hir::HirId {

src/test/debuginfo/captured-fields.rs

@@ -0,0 +1,87 @@
+// compile-flags:-g
+
+// === GDB TESTS ===================================================================================
+
+// gdb-command:run
+// gdb-command:print test
+// gdbr-check:$1 = captured_fields::main::{closure#0} {_captured_ref__my_ref__my_field1: 0x[...]}
+// gdb-command:continue
+// gdb-command:print test
+// gdbr-check:$2 = captured_fields::main::{closure#1} {_captured_ref__my_ref__my_field2: 0x[...]}
+// gdb-command:continue
+// gdb-command:print test
+// gdbr-check:$3 = captured_fields::main::{closure#2} {_captured_ref__my_ref: 0x[...]}
+// gdb-command:continue
+// gdb-command:print test
+// gdbr-check:$4 = captured_fields::main::{closure#3} {_captured_val__my_ref: 0x[...]}
+// gdb-command:continue
+// gdb-command:print test
+// gdbr-check:$5 = captured_fields::main::{closure#4} {_captured_val__my_var: captured_fields::MyStruct {my_field1: 11, my_field2: 22}}
+// gdb-command:continue
+
+// === LLDB TESTS ==================================================================================
+
+// lldb-command:run
+// lldb-command:print test
+// lldbg-check:(captured_fields::main::{closure#0}) $0 = { _captured_ref__my_ref__my_field1 = 0x[...] }
+// lldb-command:continue
+// lldb-command:print test
+// lldbg-check:(captured_fields::main::{closure#1}) $1 = { _captured_ref__my_ref__my_field2 = 0x[...] }
+// lldb-command:continue
+// lldb-command:print test
+// lldbg-check:(captured_fields::main::{closure#2}) $2 = { _captured_ref__my_ref = 0x[...] }
+// lldb-command:continue
+// lldb-command:print test
+// lldbg-check:(captured_fields::main::{closure#3}) $3 = { _captured_val__my_ref = 0x[...] }
+// lldb-command:continue
+// lldb-command:print test
+// lldbg-check:(captured_fields::main::{closure#4}) $4 = { _captured_val__my_var = { my_field1 = 11 my_field2 = 22 } }
+// lldb-command:continue
+
+#![feature(capture_disjoint_fields)]
+#![allow(unused)]
+
+struct MyStruct {
+    my_field1: u32,
+    my_field2: u32,
+}
+
+fn main() {
+    let mut my_var = MyStruct {
+        my_field1: 11,
+        my_field2: 22,
+    };
+    let my_ref = &mut my_var;
+
+    let test = || {
+        let a = &mut my_ref.my_field1;
+    };
+
+    _zzz(); // #break
+
+    let test = || {
+        let a = &my_ref.my_field2;
+    };
+
+    _zzz(); // #break
+
+    let test = || {
+        let a = &my_ref;
+    };
+
+    _zzz(); // #break
+
+    let test = || {
+        let a = my_ref;
+    };
+
+    _zzz(); // #break
+
+    let test = || {
+        let a = my_var;
+    };
+
+    _zzz(); // #break
+}
+
+fn _zzz() {}

src/test/debuginfo/generator-objects.rs

@@ -11,16 +11,16 @@
 
 // gdb-command:run
 // gdb-command:print b
-// gdb-check:$1 = generator_objects::main::{generator#0}::Unresumed(0x[...])
+// gdb-check:$1 = generator_objects::main::{generator#0}::Unresumed{_captured_ref__a: 0x[...]}
 // gdb-command:continue
 // gdb-command:print b
-// gdb-check:$2 = generator_objects::main::{generator#0}::Suspend0{c: 6, d: 7, __0: 0x[...]}
+// gdb-check:$2 = generator_objects::main::{generator#0}::Suspend0{c: 6, d: 7, _captured_ref__a: 0x[...]}
 // gdb-command:continue
 // gdb-command:print b
-// gdb-check:$3 = generator_objects::main::{generator#0}::Suspend1{c: 7, d: 8, __0: 0x[...]}
+// gdb-check:$3 = generator_objects::main::{generator#0}::Suspend1{c: 7, d: 8, _captured_ref__a: 0x[...]}
 // gdb-command:continue
 // gdb-command:print b
-// gdb-check:$4 = generator_objects::main::{generator#0}::Returned(0x[...])
+// gdb-check:$4 = generator_objects::main::{generator#0}::Returned{_captured_ref__a: 0x[...]}
 
 // === LLDB TESTS ==================================================================================
 

src/test/debuginfo/issue-57822.rs

@@ -11,17 +11,17 @@
 // gdb-command:run
 
 // gdb-command:print g
-// gdb-check:$1 = issue_57822::main::{closure#1} (issue_57822::main::{closure#0} (1))
+// gdb-check:$1 = issue_57822::main::{closure#1} {_captured_val__f: issue_57822::main::{closure#0} {_captured_val__x: 1}}
 
 // gdb-command:print b
-// gdb-check:$2 = issue_57822::main::{generator#3}::Unresumed(issue_57822::main::{generator#2}::Unresumed(2))
+// gdb-check:$2 = issue_57822::main::{generator#3}::Unresumed{_captured_val__a: issue_57822::main::{generator#2}::Unresumed{_captured_val__y: 2}}
 
 // === LLDB TESTS ==================================================================================
 
 // lldb-command:run
 
 // lldb-command:print g
-// lldbg-check:(issue_57822::main::{closure#1}) $0 = { 0 = { 0 = 1 } }
+// lldbg-check:(issue_57822::main::{closure#1}) $0 = { _captured_val__f = { _captured_val__x = 1 } }
 
 // lldb-command:print b
 // lldbg-check:(issue_57822::main::{generator#3}) $1 =