fix[154166]: closure debug capture print

[joseph-isaacs]

Found and remove the // FIXME(project-rfc-2229#48) see in mir/pretty.rs used to Debug print mir.

Fix was to replace the old tcx.upvars_mentioned with tcx.closure_captures.
Added a test to check for printing of both in 2018 and 2021

[rustbot]

Thanks for the pull request, and welcome! The Rust Project has assigned @folkertdev (or someone else) to review your changes, you should hear from them (or someone else) within the next two weeks.

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[folkertdev]

r? types

[spastorino]

I think this is correct but can you share the before and after MIR of #154166

[joseph-isaacs]

 $ diff -u mir_dump/issue-162074.mir.before mir_dump/issue-162074.mir.after
--- mir_dump/issue-162074.mir.before    2026-09-03 00:15:41
+++ mir_dump/issue-162074.mir.after     2026-09-03 00:12:38
@@ -12,7 +12,7 @@
 
     bb0: {
         _3 = &(_1.1: std::string::String);
-        _2 = {closure@tests/mir-opt/issues/issue_154166.rs:13:20: 13:22} { x: move (_1.0: std::string::String) };
+        _2 = {closure@tests/mir-opt/issues/issue_154166.rs:13:20: 13:22} { x__0: move (_1.0: std::string::String), x__1: move _3 };
         drop(_2) -> [return: bb1, unwind: bb4];
     }

Compiled using

rustc \                                          
    tests/mir-opt/issues/issue_154166.rs \
    --edition=2021 \
    -Zunpretty=mir

after MIR

 $ cat mir_dump/issue-162074.mir.after
// WARNING: This output format is intended for human consumers only
// and is subject to change without notice. Knock yourself out.
// HINT: See also -Z dump-mir for MIR at specific points during compilation.
fn foo(_1: (String, String)) -> () {
    debug x => _1;
    let mut _0: ();
    let _2: {closure@tests/mir-opt/issues/issue_154166.rs:13:20: 13:22};
    let mut _3: &std::string::String;
    scope 1 {
        debug _closure => _2;
    }

    bb0: {
        _3 = &(_1.1: std::string::String);
        _2 = {closure@tests/mir-opt/issues/issue_154166.rs:13:20: 13:22} { x__0: move (_1.0: std::string::String), x__1: move _3 };
        drop(_2) -> [return: bb1, unwind: bb4];
    }

    bb1: {
        drop((_1.1: std::string::String)) -> [return: bb2, unwind continue];
    }

    bb2: {
        return;
    }

    bb3 (cleanup): {
        resume;
    }

    bb4 (cleanup): {
        drop((_1.1: std::string::String)) -> [return: bb3, unwind terminate(cleanup)];
    }
}

fn foo::{closure#0}(_1: {closure@tests/mir-opt/issues/issue_154166.rs:13:20: 13:22}) -> () {
    debug x__0 => (_1.0: std::string::String);
    debug x__1 => (*(_1.1: &std::string::String));
    let mut _0: ();
    let mut _2: bool;
    let _3: std::string::String;
    let mut _4: &std::string::String;
    let mut _5: bool;
    scope 1 {
        debug _a => _4;
    }
    scope 2 {
        debug _b => _3;
    }

    bb0: {
        _5 = const false;
        _5 = const true;
        _2 = std::hint::black_box::<bool>(const true) -> [return: bb1, unwind: bb9];
    }

    bb1: {
        switchInt(move _2) -> [0: bb3, otherwise: bb2];
    }

    bb2: {
        _4 = no_retag copy (_1.1: &std::string::String);
        goto -> bb4;
    }

    bb3: {
        _5 = const false;
        _3 = move (_1.0: std::string::String);
        drop(_3) -> [return: bb4, unwind: bb9];
    }

    bb4: {
        switchInt(copy _5) -> [0: bb5, otherwise: bb7];
    }

    bb5: {
        return;
    }

    bb6 (cleanup): {
        resume;
    }

    bb7: {
        drop((_1.0: std::string::String)) -> [return: bb5, unwind continue];
    }

    bb8 (cleanup): {
        drop((_1.0: std::string::String)) -> [return: bb6, unwind terminate(cleanup)];
    }

    bb9 (cleanup): {
        switchInt(copy _5) -> [0: bb6, otherwise: bb8];
    }
}

[spastorino]

@bors r+

[rust-bors]

📌 Commit 8c4486b has been approved by spastorino

It is now in the queue for this repository.