[MIR-opt] conflicting debug info for argument

[bjorn3]

$ git clone https://github.com/ebobby/simple-raytracer.git
$ cd simple-raytracer
$ RUSTFLAGS="-Zmir-opt-level=2" cargo build
[...]
conflicting debug info for argument
  call void @llvm.dbg.declare(metadata { [0 x i8]*, i64 }* %22, metadata !534, metadata !DIExpression()), !dbg !541
!532 = !DILocalVariable(name: "code", arg: 1, scope: !528, file: !5, line: 14, type: !327)
!534 = !DILocalVariable(name: "self", arg: 1, scope: !535, file: !536, line: 2244, type: !327)
conflicting debug info for argument
  call void @llvm.dbg.declare(metadata { [0 x i8]*, i64 }* %22, metadata !534, metadata !DIExpression()), !dbg !541
!532 = !DILocalVariable(name: "code", arg: 1, scope: !528, file: !5, line: 14, type: !327)
!534 = !DILocalVariable(name: "self", arg: 1, scope: !535, file: !536, line: 2244, type: !327)
LLVM ERROR: Broken module found, compilation aborted!
error: could not compile `num-traits`.
warning: build failed, waiting for other jobs to finish...
conflicting debug info for argument
  call void @llvm.dbg.declare(metadata { [0 x i8]*, i64 }* %22, metadata !779, metadata !DIExpression()), !dbg !785
!777 = !DILocalVariable(name: "code", arg: 1, scope: !774, file: !5, line: 58, type: !480)
!779 = !DILocalVariable(name: "self", arg: 1, scope: !780, file: !706, line: 2244, type: !480)
conflicting debug info for argument
  call void @llvm.dbg.declare(metadata { [0 x i8]*, i64 }* %22, metadata !779, metadata !DIExpression()), !dbg !785
!777 = !DILocalVariable(name: "code", arg: 1, scope: !774, file: !5, line: 58, type: !480)
!779 = !DILocalVariable(name: "self", arg: 1, scope: !780, file: !706, line: 2244, type: !480)
LLVM ERROR: Broken module found, compilation aborted!
error: could not compile `arrayvec`.
warning: build failed, waiting for other jobs to finish...
conflicting debug info for argument
  call void @llvm.dbg.declare(metadata { [0 x i8]*, i64 }* %5, metadata !291, metadata !DIExpression()), !dbg !294
!289 = !DILocalVariable(name: "target", arg: 1, scope: !286, file: !5, line: 82, type: !38)
!291 = !DILocalVariable(name: "self", arg: 1, scope: !292, file: !221, line: 2934, type: !38)
conflicting debug info for argument
  call void @llvm.dbg.declare(metadata { [0 x i8]*, i64 }* %5, metadata !291, metadata !DIExpression()), !dbg !294
!289 = !DILocalVariable(name: "target", arg: 1, scope: !286, file: !5, line: 82, type: !38)
!291 = !DILocalVariable(name: "self", arg: 1, scope: !292, file: !221, line: 2934, type: !38)
LLVM ERROR: Broken module found, compilation aborted!
error: could not compile `proc-macro2`.
warning: build failed, waiting for other jobs to finish...
error: build failed

@rustbot modify labels: +A-mir +C-bug +requires-nightly

[Centril]

cc @eddyb @michaelwoerister

[eddyb]

I wonder if this is caused by MIR inlining or by other passes like copy propagation.
The conflict with "self" would seem to suggest inlining (of a method call).

Anyway, we can improve this for now by treating anything not in the outermost SourceScope as not an argument user variable (even when backed by an argument MIR local).

[eddyb]

Minimal repro, with -Zmir-opt-level=2 -Cdebuginfo=2 (https://godbolt.org/z/UdZsnS):

pub fn foo(bar: usize) -> usize {
    std::convert::identity(bar)
}