fix fallback impl for select_unpredictable intrinsic #151042

RedDaedalus · 2026-01-13T02:10:07Z

intrinsics::select_unpredictable does not drop the value that is not selected, but the fallback impl did not consider this behavior. This creates an observable difference between Miri and actual execution, and possibly does not play well with the core::hint version which has extra logic to drop the value that was not selected.

#![feature(core_intrinsics)]

fn main() {
    core::intrinsics::select_unpredictable(true, LoudDrop(0), LoudDrop(1));
    // cargo run: "0"; cargo miri run: "1 0"
}

struct LoudDrop(u8);

impl Drop for LoudDrop {
    fn drop(&mut self) {
        print!("{} ", self.0);
    }
}

This change let me remove the T: [const] Destruct bound as well, since the destructor is no longer relevant.

rustbot · 2026-01-13T02:10:11Z

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

rustbot · 2026-01-13T02:10:13Z

r? @joboet

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RalfJung · 2026-01-13T09:22:46Z

Good catch!

Could you add a Miri test?

rustbot · 2026-01-13T14:59:29Z

The Miri subtree was changed

cc @rust-lang/miri

rustbot · 2026-01-13T14:59:31Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

RedDaedalus · 2026-01-13T17:10:09Z

@RalfJung test added!

src/tools/miri/tests/pass/intrinsics/select-unpredictable-drop.rs

tgross35

Libs bits LGTM, but since this is mostly miri:

r? @RalfJung

rustbot · 2026-01-22T06:30:49Z

RalfJung is not on the review rotation at the moment.
They may take a while to respond.

RalfJung · 2026-01-22T07:50:48Z

@bors r+ rollup

rust-bors · 2026-01-22T07:50:51Z

📌 Commit 7d3bf37 has been approved by RalfJung

It is now in the queue for this repository.

…le-fallback, r=RalfJung fix fallback impl for select_unpredictable intrinsic `intrinsics::select_unpredictable` does not drop the value that is not selected, but the fallback impl did not consider this behavior. This creates an observable difference between Miri and actual execution, and possibly does not play well with the `core::hint` version which has extra logic to drop the value that was not selected. ```rust #![feature(core_intrinsics)] fn main() { core::intrinsics::select_unpredictable(true, LoudDrop(0), LoudDrop(1)); // cargo run: "0"; cargo miri run: "1 0" } struct LoudDrop(u8); impl Drop for LoudDrop { fn drop(&mut self) { print!("{} ", self.0); } } ``` This change let me remove the `T: [const] Destruct` bound as well, since the destructor is no longer relevant.

@ghost

…uwer Rollup of 8 pull requests Successful merges: - #148206 (Deduplicated float tests and unified in floats/mod.rs) - #151042 (fix fallback impl for select_unpredictable intrinsic) - #151220 (option: Use Option::map in Option::cloned) - #151260 (Handle unevaluated ConstKind in in_operand) - #151441 (Fix ICE: Don't try to evaluate type_consts when eagerly collecting items) - #151465 (codegen: clarify some variable names around function calls) - #151469 (llvm: Tolerate dead_on_return attribute changes) - #151476 (Avoid `-> ()` in derived functions.) r? @ghost

@ghost

…uwer Rollup of 8 pull requests Successful merges: - #148206 (Deduplicated float tests and unified in floats/mod.rs) - #151042 (fix fallback impl for select_unpredictable intrinsic) - #151220 (option: Use Option::map in Option::cloned) - #151260 (Handle unevaluated ConstKind in in_operand) - #151441 (Fix ICE: Don't try to evaluate type_consts when eagerly collecting items) - #151465 (codegen: clarify some variable names around function calls) - #151469 (llvm: Tolerate dead_on_return attribute changes) - #151476 (Avoid `-> ()` in derived functions.) r? @ghost

@ghost

…uwer Rollup of 8 pull requests Successful merges: - #148206 (Deduplicated float tests and unified in floats/mod.rs) - #151042 (fix fallback impl for select_unpredictable intrinsic) - #151220 (option: Use Option::map in Option::cloned) - #151260 (Handle unevaluated ConstKind in in_operand) - #151441 (Fix ICE: Don't try to evaluate type_consts when eagerly collecting items) - #151465 (codegen: clarify some variable names around function calls) - #151469 (llvm: Tolerate dead_on_return attribute changes) - #151476 (Avoid `-> ()` in derived functions.) r? @ghost

@ghost

…uwer Rollup of 11 pull requests Successful merges: - #151001 (rustdoc: render doc(hidden) as a code attribute) - #151042 (fix fallback impl for select_unpredictable intrinsic) - #151220 (option: Use Option::map in Option::cloned) - #151260 (Handle unevaluated ConstKind in in_operand) - #151296 (MGCA: Fix incorrect pretty printing of valtree arrays) - #151423 (Move assert_matches to planned stable path) - #151441 (Fix ICE: Don't try to evaluate type_consts when eagerly collecting items) - #151465 (codegen: clarify some variable names around function calls) - #151468 (fix `f16` doctest FIXMEs) - #151469 (llvm: Tolerate dead_on_return attribute changes) - #151476 (Avoid `-> ()` in derived functions.) r? @ghost

@ghost

…uwer Rollup of 11 pull requests Successful merges: - #151001 (rustdoc: render doc(hidden) as a code attribute) - #151042 (fix fallback impl for select_unpredictable intrinsic) - #151220 (option: Use Option::map in Option::cloned) - #151260 (Handle unevaluated ConstKind in in_operand) - #151296 (MGCA: Fix incorrect pretty printing of valtree arrays) - #151423 (Move assert_matches to planned stable path) - #151441 (Fix ICE: Don't try to evaluate type_consts when eagerly collecting items) - #151465 (codegen: clarify some variable names around function calls) - #151468 (fix `f16` doctest FIXMEs) - #151469 (llvm: Tolerate dead_on_return attribute changes) - #151476 (Avoid `-> ()` in derived functions.) r? @ghost

@ghost

…uwer Rollup of 11 pull requests Successful merges: - #151001 (rustdoc: render doc(hidden) as a code attribute) - #151042 (fix fallback impl for select_unpredictable intrinsic) - #151220 (option: Use Option::map in Option::cloned) - #151260 (Handle unevaluated ConstKind in in_operand) - #151296 (MGCA: Fix incorrect pretty printing of valtree arrays) - #151423 (Move assert_matches to planned stable path) - #151441 (Fix ICE: Don't try to evaluate type_consts when eagerly collecting items) - #151465 (codegen: clarify some variable names around function calls) - #151468 (fix `f16` doctest FIXMEs) - #151469 (llvm: Tolerate dead_on_return attribute changes) - #151476 (Avoid `-> ()` in derived functions.) r? @ghost

Rollup merge of #151042 - RedDaedalus:fix-select-unpredictable-fallback, r=RalfJung fix fallback impl for select_unpredictable intrinsic `intrinsics::select_unpredictable` does not drop the value that is not selected, but the fallback impl did not consider this behavior. This creates an observable difference between Miri and actual execution, and possibly does not play well with the `core::hint` version which has extra logic to drop the value that was not selected. ```rust #![feature(core_intrinsics)] fn main() { core::intrinsics::select_unpredictable(true, LoudDrop(0), LoudDrop(1)); // cargo run: "0"; cargo miri run: "1 0" } struct LoudDrop(u8); impl Drop for LoudDrop { fn drop(&mut self) { print!("{} ", self.0); } } ``` This change let me remove the `T: [const] Destruct` bound as well, since the destructor is no longer relevant.

@ghost

…uwer Rollup of 11 pull requests Successful merges: - rust-lang/rust#151001 (rustdoc: render doc(hidden) as a code attribute) - rust-lang/rust#151042 (fix fallback impl for select_unpredictable intrinsic) - rust-lang/rust#151220 (option: Use Option::map in Option::cloned) - rust-lang/rust#151260 (Handle unevaluated ConstKind in in_operand) - rust-lang/rust#151296 (MGCA: Fix incorrect pretty printing of valtree arrays) - rust-lang/rust#151423 (Move assert_matches to planned stable path) - rust-lang/rust#151441 (Fix ICE: Don't try to evaluate type_consts when eagerly collecting items) - rust-lang/rust#151465 (codegen: clarify some variable names around function calls) - rust-lang/rust#151468 (fix `f16` doctest FIXMEs) - rust-lang/rust#151469 (llvm: Tolerate dead_on_return attribute changes) - rust-lang/rust#151476 (Avoid `-> ()` in derived functions.) r? @ghost

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rustbot added has-merge-commits PR has merge commits, merge with caution. S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jan 13, 2026

RedDaedalus force-pushed the fix-select-unpredictable-fallback branch from a69e8e6 to 44ae04e Compare January 13, 2026 15:10

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tgross35 reviewed Jan 16, 2026

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src/tools/miri/tests/pass/intrinsics/select-unpredictable-drop.rs Show resolved Hide resolved

fix fallback impl for select_unpredictable intrinsic

7d3bf37

RedDaedalus force-pushed the fix-select-unpredictable-fallback branch from 44ae04e to 7d3bf37 Compare January 16, 2026 13:46

tgross35 approved these changes Jan 22, 2026

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JonathanBrouwer mentioned this pull request Jan 22, 2026

Rollup of 5 pull requests #151484

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Rollup of 8 pull requests #151485

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Rollup of 11 pull requests #151490

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rust-bors bot merged commit ddd8965 into rust-lang:main Jan 22, 2026
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rustbot added this to the 1.95.0 milestone Jan 22, 2026

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