SimplifyComparisonIntegral introduces access to a dead local variable

#![feature(custom_mir, core_intrinsics)]
extern crate core;
use core::intrinsics::mir::*;

#[custom_mir(dialect = "runtime")]
pub fn f(a: bool) {
    mir! {
        let b: u32;
        let c: bool;
        {
            StorageLive(b);
            Goto(bb1)
        }
        bb1 = {
            b = a as u32;
            c = b == 42;
            StorageDead(b);
            StorageLive(b);
            match Move(c) {
                true => bb1,
                _    => bb2,
            }
        }
        bb2 = {
            StorageDead(b);
            Return()
        }
    }
}

rustc a.rs --crate-type=lib -Zmir-opt-level=0 -Zmir-enable-passes=+SimplifyComparisonIntegral -Zunpretty=mir

fn f(_1: bool) -> () {
    let mut _0: ();
    let mut _2: u32;
    let mut _3: bool;

    bb0: {
        StorageLive(_2);
        goto -> bb1;
    }

    bb1: {
        StorageDead(_2);
        _2 = copy _1 as u32 (IntToInt); // undefined behavior: _2 is dead
        nop;
        nop;
        StorageLive(_2);
        switchInt(copy _2) -> [42: bb1, otherwise: bb2];
    }

    bb2: {
        StorageDead(_2);
        StorageDead(_2);
        return;
    }
}

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rustc --version --verbose:

rustc 1.98.0-nightly (8b6558a02 2026-06-20)

From what I can tell, the pass's logic for shuffling StorageDeads around tries to identify StorageDeads that would interfere with extending the lifetime of the comparison result, and sink them into successor blocks (i.e., after the switchInt). In this case, the block is its own successor, so the StorageDead gets hoisted unintentionally. Presumably this problem can occur for any block involved in a loop.

I think the StorageLive(_2) in bb1 is also problematic, since StorageLive is defined to free the local and re-allocate it with. Probably the whole StorageLive/StorageDead handling needs to be overhauled completely.

@rustbot

@rustbot claim

Assigning P-high (discussion on Zulip).

Rollup merge of rust-lang#159220 - qaijuang:issue-158231, r=hanna-kruppe

Unrolled build for #159220

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SimplifyComparisonIntegral introduces access to a dead local variable #158231

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SimplifyComparisonIntegral introduces access to a dead local variable #158231

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hanna-kruppe commented on Jun 21, 2026

hanna-kruppe commented on Jun 21, 2026

qaijuang commented on Jul 13, 2026

rustbot commented on Jul 19, 2026

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