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Don't try to remove drop_in_place calls in RemoveUnneededDrops #155044

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Don't try to remove drop_in_place calls in RemoveUnneededDrops #155044

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21 changes: 2 additions & 19 deletions compiler/rustc_mir_transform/src/remove_unneeded_drops.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -4,13 +4,7 @@
//! useful because (unlike MIR building) it runs after type checking, so it can make use of
//! `TypingMode::PostAnalysis` to provide more precise type information, especially about opaque
//! types.
//!
//! When we're optimizing, we also remove calls to `drop_in_place<T>` when `T` isn't `needs_drop`,
//! as those are essentially equivalent to `Drop` terminators. While the compiler doesn't insert
//! them automatically, preferring the built-in instead, they're common in generic code (such as
//! `Vec::truncate`) so removing them from things like inlined `Vec<u8>` is helpful.

use rustc_hir::LangItem;
use rustc_middle::mir::*;
use rustc_middle::ty::TyCtxt;
use tracing::{debug, trace};
Expand All @@ -27,19 +21,8 @@ impl<'tcx> crate::MirPass<'tcx> for RemoveUnneededDrops {
let mut should_simplify = false;
for block in body.basic_blocks.as_mut() {
let terminator = block.terminator_mut();
let (ty, target) = match terminator.kind {
TerminatorKind::Drop { place, target, .. } => {
(place.ty(&body.local_decls, tcx).ty, target)
}
TerminatorKind::Call { ref func, target: Some(target), .. }
if tcx.sess.mir_opt_level() > 0
&& let Some((def_id, generics)) = func.const_fn_def()
&& tcx.is_lang_item(def_id, LangItem::DropInPlace) =>
{
(generics.type_at(0), target)
}
_ => continue,
};
let TerminatorKind::Drop { place, target, .. } = terminator.kind else { continue };
let ty = place.ty(&body.local_decls, tcx).ty;

if ty.needs_drop(tcx, typing_env) {
continue;
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12 changes: 7 additions & 5 deletions tests/mir-opt/remove_unneeded_drop_in_place.rs
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Original file line number Diff line number Diff line change
@@ -1,12 +1,14 @@
//@ test-mir-pass: RemoveUnneededDrops
//@ test-mir-pass: Inline
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@scottmcm scottmcm Apr 9, 2026

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Suggestion: now that this isn't a different pass, maybe move it into https://github.com/rust-lang/rust/tree/main/tests/mir-opt/inline? Could rename it to inline_empty_drop_glue or something too, to make it clearer what the goal is.

//@ needs-unwind
//@ compile-flags: -Z mir-opt-level=1

// EMIT_MIR remove_unneeded_drop_in_place.slice_in_place.RemoveUnneededDrops.diff
// EMIT_MIR remove_unneeded_drop_in_place.slice_in_place.Inline.diff
unsafe fn slice_in_place(ptr: *mut [char]) {
// CHECK-LABEL: fn slice_in_place(_1: *mut [char])
// CHECK: bb0: {
// CHECK-NEXT: return;
// CHECK: bb0: {
// CHECK-NEXT: StorageLive(_2);
// CHECK-NEXT: _2 = copy _1;
// CHECK-NEXT: StorageDead(_2);
// CHECK-NEXT: return;
// CHECK-NEXT: }
std::ptr::drop_in_place(ptr)
}
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@WaffleLapkin WaffleLapkin Apr 9, 2026 •
edited by rustbot
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View changes since the review

idk how this used to pass, given that the diff is unchanged...

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@scottmcm scottmcm Apr 9, 2026

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You removed the compile-flags, so that probably impacted it? The filecheck validations are on final mir always...

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6 changes: 4 additions & 2 deletions ...e.slice_in_place.RemoveUnneededDrops.diff → ..._drop_in_place.slice_in_place.Inline.diff
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Original file line number Diff line number Diff line change
@@ -1,10 +1,12 @@
- // MIR for `slice_in_place` before RemoveUnneededDrops
+ // MIR for `slice_in_place` after RemoveUnneededDrops
- // MIR for `slice_in_place` before Inline
+ // MIR for `slice_in_place` after Inline

fn slice_in_place(_1: *mut [char]) -> () {
debug ptr => _1;
let mut _0: ();
let mut _2: *mut [char];
+ scope 1 (inlined drop_in_place::<[char]> - shim(None)) {
+ }

bb0: {
StorageLive(_2);
Expand Down
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