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Rollup merge of #152729 - Enselic:single_use_consts-not-required, r=cjgillot

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compiler: Don't mark `SingleUseConsts` MIR pass as "required for soundness"

I don't think this MIR pass is required for soundness. The reasons are:
* Something like it was not enabled by default before PR rust-lang/rust#107404 which was the precursor to `SingleUseConsts` (see rust-lang/rust#125910 for the switch).
* By following the advice from https://github.com/rust-lang/rust/pull/128657#discussion_r1705114015 we can conclude it is not required for soundness since it has only ever run on MIR opt level > 0.
* Its [`MirPass::can_be_overridden()`](0ee7d96253/compiler/rustc_mir_transform/src/pass_manager.rs (L98-L102)) is unchanged and thus returns `true`, indicating that it is not a required MIR pass.
* PR CI pass in rust-lang/rust#151426 which stops enabling it by default in non-optimized builds.

As shown in the updated test `tests/mir-opt/optimize_none.rs`, `#[optimize(none)]` functions become even less optimized, as expected and desired.

Unblocks https://github.com/rust-lang/rust/pull/151426.
This commit is contained in:
Stuart Cook 2026-02-18 17:29:43 +11:00 • committed by GitHub
commit dbc2193d37
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2
compiler/rustc_mir_transform/src/single_use_consts.rs
View file

@ -85,7 +85,7 @@ impl<'tcx> crate::MirPass<'tcx> for SingleUseConsts {
}
fn is_required(&self) -> bool {
true
false
}
}

7
tests/mir-opt/optimize_none.rs
View file

@ -15,13 +15,14 @@ pub fn add_noopt() -> i32 {
#[optimize(none)]
pub fn const_branch() -> i32 {
// CHECK-LABEL: fn const_branch(
// CHECK: switchInt(const true) -> [0: [[FALSE:bb[0-9]+]], otherwise: [[TRUE:bb[0-9]+]]];
// CHECK: [[BOOL:_[0-9]+]] = const true;
// CHECK: switchInt(move [[BOOL]]) -> [0: [[BB_FALSE:bb[0-9]+]], otherwise: [[BB_TRUE:bb[0-9]+]]];
// CHECK-NEXT: }
// CHECK: [[FALSE]]: {
// CHECK: [[BB_FALSE]]: {
// CHECK-NEXT: _0 = const 0
// CHECK-NEXT: goto
// CHECK-NEXT: }
// CHECK: [[TRUE]]: {
// CHECK: [[BB_TRUE]]: {
// CHECK-NEXT: _0 = const 1
// CHECK-NEXT: goto
// CHECK-NEXT: }