From 8cab8e07bc94fb2fea8e1421d2d2e6de6398b69b Mon Sep 17 00:00:00 2001
From: Kyle Huey <khuey@kylehuey.com>
Date: Fri, 21 Mar 2025 17:34:45 -0700
Subject: [PATCH] Don't produce debug information for compiler-introduced-vars
 when desugaring assignments.

An assignment such as

(a, b) = (b, c);

desugars to the HIR

{ let (lhs, lhs) = (b, c); a = lhs; b = lhs; };

The repeated `lhs` leads to multiple Locals assigned to the same DILocalVariable. Rather than
attempting to fix that, get rid of the debug info for these bindings that don't even exist
in the program to begin with.

Fixes #138198
---
 .../src/builder/matches/mod.rs                | 58 ++++++++++++++-----
 tests/codegen/assign-desugar-debuginfo.rs     | 18 ++++++
 2 files changed, 60 insertions(+), 16 deletions(-)
 create mode 100644 tests/codegen/assign-desugar-debuginfo.rs

diff --git a/compiler/rustc_mir_build/src/builder/matches/mod.rs b/compiler/rustc_mir_build/src/builder/matches/mod.rs
index ea341b604e0be..cf216ff73ca27 100644
--- a/compiler/rustc_mir_build/src/builder/matches/mod.rs
+++ b/compiler/rustc_mir_build/src/builder/matches/mod.rs
@@ -13,13 +13,13 @@ use std::sync::Arc;
 use rustc_abi::VariantIdx;
 use rustc_data_structures::fx::FxIndexMap;
 use rustc_data_structures::stack::ensure_sufficient_stack;
-use rustc_hir::{BindingMode, ByRef};
+use rustc_hir::{BindingMode, ByRef, LetStmt, LocalSource, Node};
 use rustc_middle::bug;
 use rustc_middle::middle::region;
 use rustc_middle::mir::{self, *};
 use rustc_middle::thir::{self, *};
 use rustc_middle::ty::{self, CanonicalUserTypeAnnotation, Ty};
-use rustc_span::{BytePos, Pos, Span, Symbol};
+use rustc_span::{BytePos, Pos, Span, Symbol, sym};
 use tracing::{debug, instrument};
 
 use crate::builder::ForGuard::{self, OutsideGuard, RefWithinGuard};
@@ -2796,13 +2796,15 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             )))),
         };
         let for_arm_body = self.local_decls.push(local);
-        self.var_debug_info.push(VarDebugInfo {
-            name,
-            source_info: debug_source_info,
-            value: VarDebugInfoContents::Place(for_arm_body.into()),
-            composite: None,
-            argument_index: None,
-        });
+        if self.should_emit_debug_info_for_binding(name, var_id) {
+            self.var_debug_info.push(VarDebugInfo {
+                name,
+                source_info: debug_source_info,
+                value: VarDebugInfoContents::Place(for_arm_body.into()),
+                composite: None,
+                argument_index: None,
+            });
+        }
         let locals = if has_guard.0 {
             let ref_for_guard = self.local_decls.push(LocalDecl::<'tcx> {
                 // This variable isn't mutated but has a name, so has to be
@@ -2815,13 +2817,15 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     BindingForm::RefForGuard,
                 ))),
             });
-            self.var_debug_info.push(VarDebugInfo {
-                name,
-                source_info: debug_source_info,
-                value: VarDebugInfoContents::Place(ref_for_guard.into()),
-                composite: None,
-                argument_index: None,
-            });
+            if self.should_emit_debug_info_for_binding(name, var_id) {
+                self.var_debug_info.push(VarDebugInfo {
+                    name,
+                    source_info: debug_source_info,
+                    value: VarDebugInfoContents::Place(ref_for_guard.into()),
+                    composite: None,
+                    argument_index: None,
+                });
+            }
             LocalsForNode::ForGuard { ref_for_guard, for_arm_body }
         } else {
             LocalsForNode::One(for_arm_body)
@@ -2829,4 +2833,26 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         debug!(?locals);
         self.var_indices.insert(var_id, locals);
     }
+
+    /// Some bindings are introduced when producing HIR from the AST and don't
+    /// actually exist in the source. Skip producing debug info for those when
+    /// we can recognize them.
+    fn should_emit_debug_info_for_binding(&self, name: Symbol, var_id: LocalVarId) -> bool {
+        // For now we only recognize the output of desugaring assigns.
+        if name != sym::lhs {
+            return true;
+        }
+
+        let tcx = self.tcx;
+        for (_, node) in tcx.hir_parent_iter(var_id.0) {
+            // FIXME(khuey) at what point is it safe to bail on the iterator?
+            // Can we stop at the first non-Pat node?
+            if matches!(node, Node::LetStmt(&LetStmt { source: LocalSource::AssignDesugar(_), .. }))
+            {
+                return false;
+            }
+        }
+
+        true
+    }
 }
diff --git a/tests/codegen/assign-desugar-debuginfo.rs b/tests/codegen/assign-desugar-debuginfo.rs
new file mode 100644
index 0000000000000..77ee8758b3b3b
--- /dev/null
+++ b/tests/codegen/assign-desugar-debuginfo.rs
@@ -0,0 +1,18 @@
+//@ compile-flags: -g -Zmir-opt-level=0
+
+#![crate_type = "lib"]
+
+#[inline(never)]
+fn swizzle(a: u32, b: u32, c: u32) -> (u32, (u32, u32)) {
+    (b, (c, a))
+}
+
+pub fn work() {
+    let mut a = 1;
+    let mut b = 2;
+    let mut c = 3;
+    (a, (b, c)) = swizzle(a, b, c);
+    println!("{a} {b} {c}");
+}
+
+// CHECK-NOT: !DILocalVariable(name: "lhs",