Fix closed over variables not available in debuginfo for Windows MSVC

The issue was that the resulting debuginfo was too complex for LLVM to
translate into CodeView records correctly. As a result, it simply
ignored the debuginfo which meant Windows debuggers could not display
any closed over variables when stepping inside a closure.

This fixes that by spilling additional variables to the stack so that
the resulting debuginfo is simple (just `*my_variable.dbg.spill`) and
LLVM can generate the correct CV records.

compiler/rustc_codegen_ssa/src/mir/debuginfo.rs

@@ -6,7 +6,7 @@ use rustc_middle::ty;
 use rustc_session::config::DebugInfo;
 use rustc_span::symbol::{kw, Symbol};
 use rustc_span::{BytePos, Span};
-use rustc_target::abi::{LayoutOf, Size};
+use rustc_target::abi::Size;
 
 use super::operand::{OperandRef, OperandValue};
 use super::place::PlaceRef;
@@ -265,33 +265,25 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
  None => continue,
  };
 
- let mut layout = base.layout;
  let mut direct_offset = Size::ZERO;
  // FIXME(eddyb) use smallvec here.
  let mut indirect_offsets = vec![];
+ let mut place = base;
 
  for elem in &var.projection[..] {
  match *elem {
  mir::ProjectionElem::Deref => {
  indirect_offsets.push(Size::ZERO);
- layout = bx.cx().layout_of(
- layout
- .ty
- .builtin_deref(true)
- .unwrap_or_else(|| {
- span_bug!(var.source_info.span, "cannot deref `{}`", layout.ty)
- })
- .ty,
- );
+ place = place.project_deref(bx);
  }
  mir::ProjectionElem::Field(field, _) => {
  let i = field.index();
  let offset = indirect_offsets.last_mut().unwrap_or(&mut direct_offset);
- *offset += layout.fields.offset(i);
- layout = layout.field(bx.cx(), i);
+ *offset += place.layout.fields.offset(i);
+ place = place.project_field(bx, i);
  }
  mir::ProjectionElem::Downcast(_, variant) => {
- layout = layout.for_variant(bx.cx(), variant);
+ place = place.project_downcast(bx, variant);
  }
  _ => span_bug!(
  var.source_info.span,
@@ -301,7 +293,39 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
  }
  }
 
- bx.dbg_var_addr(dbg_var, dbg_loc, base.llval, direct_offset, &indirect_offsets);
+ // When targeting MSVC, create extra allocas for arguments instead of pointing multiple
+ // dbg_var_addr() calls into the same alloca with offsets. MSVC uses CodeView records
+ // not DWARF and LLVM doesn't support translating the resulting
+ // [DW_OP_deref, DW_OP_plus_uconst, offset, DW_OP_deref] debug info to CodeView.
+ // Creating extra allocas on the stack makes the resulting debug info simple enough
+ // that LLVM can generate correct CodeView records and thus the values appear in the
+ // debugger. (#83709)
+ let should_create_individual_allocas = bx.cx().sess().target.is_like_msvc
+ && self.mir.local_kind(local) == mir::LocalKind::Arg
+ // LLVM can handle simple things but anything more complex than just a direct
+ // offset or one indirect offset of 0 is too complex for it to generate CV records
+ // correctly.
+ && (direct_offset != Size::ZERO
+ || !matches!(&indirect_offsets[..], [Size::ZERO] | []));
+
+ if should_create_individual_allocas {
+ // Create a variable which will be a pointer to the actual value
+ let ptr_ty = bx.tcx().mk_ty(ty::RawPtr(ty::TypeAndMut {
+ mutbl: mir::Mutability::Mut,
+ ty: place.layout.ty,
+ }));
+ let ptr_layout = bx.layout_of(ptr_ty);
+ let alloca = PlaceRef::alloca(bx, ptr_layout);
+ bx.set_var_name(alloca.llval, &(var.name.to_string() + ".dbg.spill"));
+
+ // Write the pointer to the variable
+ bx.store(place.llval, alloca.llval, alloca.align);
+
+ // Point the debug info to `*alloca` for the current variable
+ bx.dbg_var_addr(dbg_var, dbg_loc, alloca.llval, Size::ZERO, &[Size::ZERO]);
+ } else {
+ bx.dbg_var_addr(dbg_var, dbg_loc, base.llval, direct_offset, &indirect_offsets);
+ }
  }
  }
 

compiler/rustc_codegen_ssa/src/mir/place.rs

@@ -402,6 +402,18 @@ impl<'a, 'tcx, V: CodegenObject> PlaceRef<'tcx, V> {
  downcast
  }
 
+ pub fn project_deref<Bx: BuilderMethods<'a, 'tcx, Value = V>>(&self, bx: &mut Bx) -> Self {
+ let target_ty = self.layout.ty.builtin_deref(true).expect("failed to deref");
+ let layout = bx.layout_of(target_ty.ty);
+
+ PlaceRef {
+ llval: bx.load(self.llval, self.align),
+ llextra: None,
+ layout,
+ align: layout.align.abi,
+ }
+ }
+
  pub fn storage_live<Bx: BuilderMethods<'a, 'tcx, Value = V>>(&self, bx: &mut Bx) {
  bx.lifetime_start(self.llval, self.layout.size);
  }

src/test/debuginfo/var-captured-in-nested-closure.rs

@@ -83,6 +83,55 @@
 // lldbr-check:(isize) closure_local = 8
 // lldb-command:continue
 
+
+// === CDB TESTS ===================================================================================
+
+// cdb-command: g
+
+// cdb-command: dx variable
+// cdb-check:variable : 1 [Type: [...]]
+// cdb-command: dx constant
+// cdb-check:constant : 2 [Type: [...]]
+// cdb-command: dx a_struct
+// cdb-check:a_struct [Type: var_captured_in_nested_closure::Struct]
+// cdb-check: [+0x[...]] a : -3 [Type: [...]]
+// cdb-check: [+0x[...]] b : 4.500000 [Type: [...]]
+// cdb-check: [+0x[...]] c : 0x5 [Type: unsigned [...]]
+// cdb-command: dx struct_ref
+// cdb-check:struct_ref : 0x[...] [Type: var_captured_in_nested_closure::Struct *]
+// cdb-check: [+0x[...]] a : -3 [Type: [...]]
+// cdb-check: [+0x[...]] b : 4.500000 [Type: [...]]
+// cdb-check: [+0x[...]] c : 0x5 [Type: unsigned [...]]
+// cdb-command: dx owned
+// cdb-check:owned : 0x[...] : 6 [Type: [...] *]
+// cdb-check: 6 [Type: [...]]
+// cdb-command: dx closure_local
+// cdb-check:closure_local : 8 [Type: [...]]
+// cdb-command: dx nested_closure
+// cdb-check:nested_closure [Type: var_captured_in_nested_closure::main::{{closure}}::closure-0]
+
+// cdb-command: g
+
+// cdb-command: dx variable
+// cdb-check:variable : 1 [Type: [...]]
+// cdb-command: dx constant
+// cdb-check:constant : 2 [Type: [...]]
+// cdb-command: dx a_struct
+// cdb-check:a_struct [Type: var_captured_in_nested_closure::Struct]
+// cdb-check: [+0x[...]] a : -3 [Type: [...]]
+// cdb-check: [+0x[...]] b : 4.500000 [Type: [...]]
+// cdb-check: [+0x[...]] c : 0x5 [Type: unsigned [...]]
+// cdb-command: dx struct_ref
+// cdb-check:struct_ref : 0x[...] [Type: var_captured_in_nested_closure::Struct *]
+// cdb-check: [+0x[...]] a : -3 [Type: [...]]
+// cdb-check: [+0x[...]] b : 4.500000 [Type: [...]]
+// cdb-check: [+0x[...]] c : 0x5 [Type: unsigned [...]]
+// cdb-command: dx owned
+// cdb-check:owned : 0x[...] : 6 [Type: [...] *]
+// cdb-check: 6 [Type: [...]]
+// cdb-command: dx closure_local
+// cdb-check:closure_local : 8 [Type: [...]]
+
 #![allow(unused_variables)]
 #![feature(box_syntax)]
 #![feature(omit_gdb_pretty_printer_section)]

src/test/debuginfo/var-captured-in-stack-closure.rs

@@ -73,6 +73,48 @@
 // lldbg-check:[...]$9 = 6
 // lldbr-check:(isize) *owned = 6
 
+
+// === CDB TESTS ===================================================================================
+
+// cdb-command: g
+
+// cdb-command: dx variable
+// cdb-check:variable : 1 [Type: [...]]
+// cdb-command: dx constant
+// cdb-check:constant : 2 [Type: [...]]
+// cdb-command: dx a_struct
+// cdb-check:a_struct [Type: var_captured_in_stack_closure::Struct]
+// cdb-check: [+0x[...]] a : -3 [Type: [...]]
+// cdb-check: [+0x[...]] b : 4.500000 [Type: [...]]
+// cdb-check: [+0x[...]] c : 0x5 [Type: unsigned [...]]
+// cdb-command: dx struct_ref
+// cdb-check:struct_ref : 0x[...] [Type: var_captured_in_stack_closure::Struct *]
+// cdb-check: [+0x[...]] a : -3 [Type: [...]]
+// cdb-check: [+0x[...]] b : 4.500000 [Type: [...]]
+// cdb-check: [+0x[...]] c : 0x5 [Type: unsigned [...]]
+// cdb-command: dx owned
+// cdb-check:owned : 0x[...] : 6 [Type: [...] *]
+
+
+// cdb-command: g
+
+// cdb-command: dx variable
+// cdb-check:variable : 2 [Type: [...]]
+// cdb-command: dx constant
+// cdb-check:constant : 2 [Type: [...]]
+// cdb-command: dx a_struct
+// cdb-check:a_struct [Type: var_captured_in_stack_closure::Struct]
+// cdb-check: [+0x[...]] a : -3 [Type: [...]]
+// cdb-check: [+0x[...]] b : 4.500000 [Type: [...]]
+// cdb-check: [+0x[...]] c : 0x5 [Type: unsigned [...]]
+// cdb-command: dx struct_ref
+// cdb-check:struct_ref : 0x[...] [Type: var_captured_in_stack_closure::Struct *]
+// cdb-check: [+0x[...]] a : -3 [Type: [...]]
+// cdb-check: [+0x[...]] b : 4.500000 [Type: [...]]
+// cdb-check: [+0x[...]] c : 0x5 [Type: unsigned [...]]
+// cdb-command: dx owned
+// cdb-check:owned : 0x[...] : 6 [Type: [...] *]
+
 #![feature(box_syntax)]
 #![allow(unused_variables)]
 #![feature(omit_gdb_pretty_printer_section)]