Preserve argument indexes when inlining MIR

compiler/rustc_codegen_ssa/src/mir/debuginfo.rs

@@ -442,11 +442,10 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 let (var_ty, var_kind) = match var.value {
                     mir::VarDebugInfoContents::Place(place) => {
                         let var_ty = self.monomorphized_place_ty(place.as_ref());
-                        let var_kind = if self.mir.local_kind(place.local) == mir::LocalKind::Arg
+                        let var_kind = if let Some(arg_index) = var.argument_index
                             && place.projection.is_empty()
-                            && var.source_info.scope == mir::OUTERMOST_SOURCE_SCOPE
                         {
-                            let arg_index = place.local.index() - 1;
+                            let arg_index = arg_index as usize;
                             if target_is_msvc {
                                 // ScalarPair parameters are spilled to the stack so they need to
                                 // be marked as a `LocalVariable` for MSVC debuggers to visualize
@@ -455,13 +454,12 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                                 if let Abi::ScalarPair(_, _) = var_ty_layout.abi {
                                     VariableKind::LocalVariable
                                 } else {
-                                    VariableKind::ArgumentVariable(arg_index + 1)
+                                    VariableKind::ArgumentVariable(arg_index)
                                 }
                             } else {
                                 // FIXME(eddyb) shouldn't `ArgumentVariable` indices be
                                 // offset in closures to account for the hidden environment?
-                                // Also, is this `+ 1` needed at all?
-                                VariableKind::ArgumentVariable(arg_index + 1)
+                                VariableKind::ArgumentVariable(arg_index)
                             }
                         } else {
                             VariableKind::LocalVariable

compiler/rustc_middle/src/mir/mod.rs

@@ -1115,6 +1115,11 @@ pub struct VarDebugInfo<'tcx> {
 
     /// Where the data for this user variable is to be found.
     pub value: VarDebugInfoContents<'tcx>,
+
+    /// When present, indicates what argument number this variable is in the function that it
+    /// originated from (starting from 1). Note, if MIR inlining is enabled, then this is the
+    /// argument number in the original function before it was inlined.
+    pub argument_index: Option<u16>,
 }
 
 ///////////////////////////////////////////////////////////////////////////

compiler/rustc_middle/src/mir/visit.rs

@@ -832,6 +832,7 @@ macro_rules! make_mir_visitor {
                     name: _,
                     source_info,
                     value,
+                    argument_index: _,
                 } = var_debug_info;
 
                 self.visit_source_info(source_info);

compiler/rustc_mir_build/src/build/matches/mod.rs

@@ -2242,6 +2242,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             name,
             source_info: debug_source_info,
             value: VarDebugInfoContents::Place(for_arm_body.into()),
+            argument_index: None,
         });
         let locals = if has_guard.0 {
             let ref_for_guard = self.local_decls.push(LocalDecl::<'tcx> {
@@ -2260,6 +2261,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 name,
                 source_info: debug_source_info,
                 value: VarDebugInfoContents::Place(ref_for_guard.into()),
+                argument_index: None,
             });
             LocalsForNode::ForGuard { ref_for_guard, for_arm_body }
         } else {

compiler/rustc_mir_build/src/build/mod.rs

@@ -811,6 +811,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     name,
                     source_info: SourceInfo::outermost(captured_place.var_ident.span),
                     value: VarDebugInfoContents::Place(use_place),
+                    argument_index: None,
                 });
 
                 let capture = Capture { captured_place, use_place, mutability };
@@ -827,7 +828,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         expr: &Expr<'tcx>,
     ) -> BlockAnd<()> {
         // Allocate locals for the function arguments
-        for param in arguments.iter() {
+        for (argument_index, param) in arguments.iter().enumerate() {
             let source_info =
                 SourceInfo::outermost(param.pat.as_ref().map_or(self.fn_span, |pat| pat.span));
             let arg_local =
@@ -839,6 +840,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     name,
                     source_info,
                     value: VarDebugInfoContents::Place(arg_local.into()),
+                    argument_index: Some(argument_index as u16 + 1),
                 });
             }
         }

compiler/rustc_mir_transform/src/generator.rs

@@ -1556,6 +1556,13 @@ impl<'tcx> MirPass<'tcx> for StateTransform {
         body.arg_count = 2; // self, resume arg
         body.spread_arg = None;
 
+        // The original arguments to the function are no longer arguments, mark them as such.
+        // Otherwise they'll conflict with our new arguments, which although they don't have
+        // argument_index set, will get emitted as unnamed arguments.
+        for var in &mut body.var_debug_info {
+            var.argument_index = None;
+        }
+
         body.generator.as_mut().unwrap().yield_ty = None;
         body.generator.as_mut().unwrap().generator_layout = Some(layout);
 

tests/codegen/inline-function-args-debug-info.rs

@@ -0,0 +1,20 @@
+// This test checks that debug information includes function argument indexes even if the function
+// gets inlined by MIR inlining. Without function argument indexes, `info args` in gdb won't show
+// arguments and their values for the current function.
+
+// compile-flags: -Zinline-mir=yes -Cdebuginfo=2 --edition=2021
+
+#![crate_type = "lib"]
+
+pub fn outer_function(x: usize, y: usize) -> usize {
+    inner_function(x, y) + 1
+}
+
+#[inline]
+fn inner_function(aaaa: usize, bbbb: usize) -> usize {
+    // CHECK: !DILocalVariable(name: "aaaa", arg: 1
+    // CHECK-SAME: line: 14
+    // CHECK: !DILocalVariable(name: "bbbb", arg: 2
+    // CHECK-SAME: line: 14
+    aaaa + bbbb
+}

tests/ui/async-await/task-context-arg.rs

@@ -0,0 +1,25 @@
+// Checks that we don't get conflicting arguments in our debug info with a particular async function
+// structure.
+
+// edition:2021
+// compile-flags: -Cdebuginfo=2
+// build-pass
+
+#![crate_type = "lib"]
+
+use std::future::Future;
+
+// The compiler produces a closure as part of this function. That closure initially takes an
+// argument _task_context. Later, when the MIR for that closure is transformed into a generator
+// state machine, _task_context is demoted to not be an argument, but just part of an unnamed
+// argument. If we emit debug info saying that both _task_context and the unnamed argument are both
+// argument number 2, then LLVM will fail with "conflicting debug info for argument". See
+// https://github.com/rust-lang/rust/pull/109466#issuecomment-1500879195 for details.
+async fn recv_unit() {
+    std::future::ready(()).await;
+}
+
+pub fn poll_recv() {
+    // This box is necessary in order to reproduce the problem.
+    let _: Box<dyn Future<Output = ()>> = Box::new(recv_unit());
+}