From 4e9725cd2f5327f65f881c59d6bcbae46308318e Mon Sep 17 00:00:00 2001
From: Kyle Huey <khuey@kylehuey.com>
Date: Mon, 19 Aug 2024 17:13:30 -0700
Subject: [PATCH] Add a comment.

---
 compiler/rustc_codegen_llvm/src/debuginfo/mod.rs | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs b/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs
index 66dd653bb21..3fbaebe28aa 100644
--- a/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs
+++ b/compiler/rustc_codegen_llvm/src/debuginfo/mod.rs
@@ -570,6 +570,11 @@ impl<'ll, 'tcx> DebugInfoMethods<'tcx> for CodegenCx<'ll, 'tcx> {
         inlined_at: Option<&'ll DILocation>,
         span: Span,
     ) -> &'ll DILocation {
+        // When emitting debugging information, DWARF (i.e. everything but MSVC)
+        // treats line 0 as a magic value meaning that the code could not be
+        // attributed to any line in the source. That's also exactly what dummy
+        // spans are. Make that equivalence here, rather than passing dummy spans
+        // to lookup_debug_loc, which will return line 1 for them.
         let (line, col) = if span.is_dummy() && !self.sess().target.is_like_msvc {
             (0, 0)
         } else {