Bad MIR spans

In #99780 (comment), I tried to add an assertion in the span formatting function to validate that the span for the mir::Body.span span always contains all spans of MIR statements/terminators.

diff --git a/compiler/rustc_span/src/source_map.rs b/compiler/rustc_span/src/source_map.rs
index 28381157d50..ad44a0366b5 100644
--- a/compiler/rustc_span/src/source_map.rs
+++ b/compiler/rustc_span/src/source_map.rs
@@ -477,6 +477,8 @@ pub fn span_to_relative_line_string(&self, sp: Span, relative_to: Span) -> Strin
             return self.span_to_embeddable_string(sp);
         }

+        assert!(relative_to.contains(sp));
+
         let lo_line = lo.line.saturating_sub(offset.line);
         let hi_line = hi.line.saturating_sub(offset.line);

But this assertion failed for many tests. One such case was for associated constants.

We should investigate all the cases where the MIR body span is not fully correct, and add the assertion back in.

This is trivially false in the face of macros and MIR function inlining, right?

#[inline(always)]
fn inline_me() -> u32 {
    42
}

pub fn root() -> u32 {
    inline_me()
}

when compiled with nightly and optimizations enabled gives the following MIR:

fn root() -> u32 {
    let mut _0: u32;                     // return place in scope 0 at src/lib.rs:6:18: 6:21
    scope 1 (inlined inline_me) {        // at src/lib.rs:7:5: 7:16
    }

    bb0: {
        _0 = const 42_u32;               // scope 1 at src/lib.rs:3:5: 3:7
        return;                          // scope 0 at src/lib.rs:8:2: 8:2
    }
}

_0 = const 42_u32; here is not contained inside root.

@oli-obk

Good point, I did not consider this. I guess then the assertion should not be added, and the spans from outside should be better dealt with (I am not sure how that would look like though).

cc @oli-obk

Yea, I think we should not use relative offsets unless the span is within the function's span. Basically fallback to just the original span in that case

That sounds good, I'll improve that.

There are still a few issues that should be improved: #99908 (comment)

@rustbot

@rustbot label +A-mir

Navigation Menu

Repository navigation

Bad MIR spans #99854

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Bad MIR spans #99854

Description

Activity

bjorn3 commented on Jul 28, 2022

Noratrieb commented on Jul 28, 2022

oli-obk commented on Jul 29, 2022

Noratrieb commented on Jul 29, 2022

Noratrieb commented on Aug 22, 2022

clubby789 commented on Mar 31, 2023

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions