Auto merge of #96316 - michaelwoerister:debuginfo-fix-unit-msvc, r=wesleywiser

debuginfo: Emit ZST struct debuginfo for unit type when CPP-like debuginfo is enabled

As already discovered in 24a728a8eb, PDB does not play well with custom basic types. This PR extends to the fix to `()`: Instead of a custom basic type, we treat it like an empty tuple (i.e. it is described as a struct which happens to have no fields).

Before this change anything with a `()` in it would cause trouble, which is especially bad for `*const ()` and `*mut ()` which are often used for opaque pointers. E.g. the test case added in this PR would look like:
```
0:000>  dx _ref
Error: Unable to bind name '_ref'
0:000>  dx _ptr
Error: Unable to bind name '_ptr'
0:000>  dx _local
Error: Unable to bind name '_local'
0:000>  dx _field,d
_field,d         [Type: unit_type::_TypeContainingUnitField]
    [+0x008] _a               : 123 [Type: unsigned int]
    [+0x000] _unit            : Unexpected failure to dereference object
    [+0x000] _b               : 456 [Type: unsigned __int64]
0:000>  dx ((__int64 *)_ptr),x
Error: Unable to bind name '_ptr'
```

With the PR it produces the expected output:
```
0:000>  dx _ref
_ref             : 0x7ff6f2012230 : () [Type: tuple$<> *]
0:000>  dx _ptr
_ptr             : 0x7e8ddffc20 : () [Type: tuple$<> *]
0:000>  dx _local
_local           : () [Type: tuple$<>]
0:000>  dx _field,d
_field,d         [Type: unit_type::_TypeContainingUnitField]
    [+0x008] _a               : 123 [Type: unsigned int]
    [+0x000] _unit            : () [Type: tuple$<>]
    [+0x000] _b               : 456 [Type: unsigned __int64]
0:000>  dx ((__int64 *)_ptr),x
((__int64 *)_ptr),x : 0x7e8ddffc20 : 0x1122334455667788 [Type: __int64 *]
```

r? `@wesleywiser`

compiler/rustc_codegen_llvm/src/debuginfo/metadata.rs

@@ -437,11 +437,9 @@ pub fn type_di_node<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>, t: Ty<'tcx>) -> &'ll D
 
     let DINodeCreationResult { di_node, already_stored_in_typemap } = match *t.kind() {
         ty::Never | ty::Bool | ty::Char | ty::Int(_) | ty::Uint(_) | ty::Float(_) => {
-            DINodeCreationResult::new(build_basic_type_di_node(cx, t), false)
-        }
-        ty::Tuple(elements) if elements.is_empty() => {
-            DINodeCreationResult::new(build_basic_type_di_node(cx, t), false)
+            build_basic_type_di_node(cx, t)
         }
+        ty::Tuple(elements) if elements.is_empty() => build_basic_type_di_node(cx, t),
         ty::Array(..) => build_fixed_size_array_di_node(cx, unique_type_id, t),
         ty::Slice(_) | ty::Str => build_slice_type_di_node(cx, t, unique_type_id),
         ty::Dynamic(..) => build_dyn_type_di_node(cx, t, unique_type_id),
@@ -640,7 +638,10 @@ impl MsvcBasicName for ty::FloatTy {
     }
 }
 
-fn build_basic_type_di_node<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>, t: Ty<'tcx>) -> &'ll DIType {
+fn build_basic_type_di_node<'ll, 'tcx>(
+    cx: &CodegenCx<'ll, 'tcx>,
+    t: Ty<'tcx>,
+) -> DINodeCreationResult<'ll> {
     debug!("build_basic_type_di_node: {:?}", t);
 
     // When targeting MSVC, emit MSVC style type names for compatibility with
@@ -649,7 +650,13 @@ fn build_basic_type_di_node<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>, t: Ty<'tcx>) -
 
     let (name, encoding) = match t.kind() {
         ty::Never => ("!", DW_ATE_unsigned),
-        ty::Tuple(elements) if elements.is_empty() => ("()", DW_ATE_unsigned),
+        ty::Tuple(elements) if elements.is_empty() => {
+            if cpp_like_debuginfo {
+                return build_tuple_type_di_node(cx, UniqueTypeId::for_ty(cx.tcx, t));
+            } else {
+                ("()", DW_ATE_unsigned)
+            }
+        }
         ty::Bool => ("bool", DW_ATE_boolean),
         ty::Char => ("char", DW_ATE_UTF),
         ty::Int(int_ty) if cpp_like_debuginfo => (int_ty.msvc_basic_name(), DW_ATE_signed),
@@ -672,14 +679,14 @@ fn build_basic_type_di_node<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>, t: Ty<'tcx>) -
     };
 
     if !cpp_like_debuginfo {
-        return ty_di_node;
+        return DINodeCreationResult::new(ty_di_node, false);
     }
 
     let typedef_name = match t.kind() {
         ty::Int(int_ty) => int_ty.name_str(),
         ty::Uint(uint_ty) => uint_ty.name_str(),
         ty::Float(float_ty) => float_ty.name_str(),
-        _ => return ty_di_node,
+        _ => return DINodeCreationResult::new(ty_di_node, false),
     };
 
     let typedef_di_node = unsafe {
@@ -694,7 +701,7 @@ fn build_basic_type_di_node<'ll, 'tcx>(cx: &CodegenCx<'ll, 'tcx>, t: Ty<'tcx>) -
         )
     };
 
-    typedef_di_node
+    DINodeCreationResult::new(typedef_di_node, false)
 }
 
 fn build_foreign_type_di_node<'ll, 'tcx>(

src/test/debuginfo/unit-type.rs

@@ -0,0 +1,71 @@
+// compile-flags:-g
+
+// We only test Rust-aware versions of GDB:
+// min-gdb-version: 8.2
+
+// === GDB TESTS ===================================================================================
+
+// gdb-command: run
+
+// gdb-command: print _ref
+// gdb-check: $1 = (*mut ()) 0x[...]
+
+// gdb-command: print _ptr
+// gdb-check: $2 = (*mut ()) 0x[...]
+
+// gdb-command: print _local
+// gdb-check: $3 = ()
+
+// gdb-command: print _field
+// gdb-check: $4 = unit_type::_TypeContainingUnitField {_a: 123, _unit: (), _b: 456}
+
+// Check that we can cast "void pointers" to their actual type in the debugger
+// gdb-command: print /x *(_ptr as *const u64)
+// gdb-check: $5 = 0x1122334455667788
+
+// === CDB TESTS ===================================================================================
+
+// cdb-command: g
+// cdb-check: Breakpoint 0 hit
+
+// cdb-command: dx _ref
+// cdb-check: _ref             : 0x[...] : () [Type: tuple$<> *]
+
+// cdb-command: dx _ptr
+// cdb-check: _ptr             : 0x[...] : () [Type: tuple$<> *]
+
+// cdb-command: dx _local
+// cdb-check: _local           : () [Type: tuple$<>]
+
+// cdb-command: dx _field,d
+// cdb-check: _field,d         [Type: unit_type::_TypeContainingUnitField]
+// cdb-check:     [+0x[...]] _a               : 123 [Type: unsigned int]
+// cdb-check:     [+0x[...]] _unit            : () [Type: tuple$<>]
+// cdb-check:     [+0x[...]] _b               : 456 [Type: unsigned __int64]
+
+// Check that we can cast "void pointers" to their actual type in the debugger
+// cdb-command: dx ((__int64 *)_ptr),x
+// cdb-check: ((__int64 *)_ptr),x : 0x[...] : 0x1122334455667788 [Type: __int64 *]
+// cdb-check:     0x1122334455667788 [Type: __int64]
+
+struct _TypeContainingUnitField {
+    _a: u32,
+    _unit: (),
+    _b: u64,
+}
+
+fn foo(_ref: &(), _ptr: *const ()) {
+    let _local = ();
+    let _field = _TypeContainingUnitField { _a: 123, _unit: (), _b: 456 };
+
+    zzz(); // #break
+}
+
+fn main() {
+    let pointee = 0x1122_3344_5566_7788i64;
+
+    foo(&(), &pointee as *const i64 as *const ());
+}
+
+#[inline(never)]
+fn zzz() {}