Identifiers introduced via `if let` binding can't be debugged

[wesleywiser]

Identifiers introduced via an if let binding can't be inspected in the debugger. For instance:

pub fn do_it(o: Option<i32>) {
    if let Some(x) = o {
        zzz(x); // #break
    }
}

fn zzz(_: i32) { }

Stopping the debugger on line 3 shows no variables in scope other than the function argument o. I tested this in both gdb and WinDbg.

This appears to be because rustc generates incorrect scoping information for the call to zzz. Looking at the LLVM IR, I see

...
  %x = load i32, i32* %7, align 4, !dbg !36
  store i32 %x, i32* %x.dbg.spill, align 4, !dbg !36
  call void @llvm.dbg.declare(metadata i32* %x.dbg.spill, metadata !31, metadata !DIExpression()), !dbg !37
  call void @_ZN7example3zzz17h59648a546f323dcaE(i32 %x), !dbg !38
...
!5 = distinct !DISubprogram(name: "do_it", linkageName: "_ZN7example5do_it17h549531f5caefefa2E", scope: !7, file: !6, line: 1, type: !8, scopeLine: 1, flags: DIFlagPrototyped, spFlags: DISPFlagDefinition, unit: !3, templateParams: !19, retainedNodes: !29)
!32 = distinct !DILexicalBlock(scope: !5, file: !6, line: 2, column: 12)
!37 = !DILocation(line: 2, column: 17, scope: !32)
!38 = !DILocation(line: 3, column: 9, scope: !5)

Which puts the call to the zzz in a different scope than the one x is in and thus it isn't visible.

This reproduces on all current versions of Rust:

$ rustc +stable --version
rustc 1.61.0 (fe5b13d68 2022-05-18)
$ rustc +beta --version
rustc 1.62.0-beta.3 (a5cf77ca6 2022-06-02)
$ rustc +nightly --version
rustc 1.63.0-nightly (fee3a459d 2022-06-05)

[xldenis]

I think I'm encountering the same issue, I've noticed that the variables inside an if let block don't have nested scopes in their VarDebugInfo, that is:

if let Some(x /* has scope 1 */) = y /* has scope 0 */ {
   
   let z = expr; // has scope 2

}

has the following scope tree:

0 <---- 1
^
|
+----- 2

instead of

0 <---- 1 <----- 2

[wesleywiser]

Yeah, that looks like the same issue. I hadn't dug in further to see how deep the problem ran. Based on what you're seeing, this might be happening during MIR building or perhaps even earlier in (T)HIR.

[xldenis]

Based on my understanding of the compilation my initial suspect is THIR since by the time we get to MIR building the if let will have been replaced by a match

[xldenis]

Ok, a little more digging has led to me confirming the proximal cause of this issue: if let statements turn their bindings into match guards rather than match patterns. My guess is that the scope rules are handled differently between those two cases.

This can be confirmed by replacing your example with the following:

#![feature(if_let_guard)]
...
match o {
  y if let Some(x) = y => { zzz(x) }
}