Unimplement unsized_locals #141811

mejrs · 2025-05-31T11:46:40Z

Tracking issue here: #111942

Note that this just removes the feature, not the implementation, and does not touch unsized_fn_params. This is because it is required to support Box<dyn FnOnce()>: FnOnce().

There may be more that should be removed (possibly in follow up prs)

the forget_unsized function and forget intrinsic.
the unsized_locals test directory; I've just fixed up the tests for now
various codegen support for unsized values and allocas

cc @JakobDegen @oli-obk @Noratrieb @programmerjake @bjorn3

@rustbot label F-unsized_locals

Fixes #79409

rustbot · 2025-05-31T11:46:45Z

r? @SparrowLii

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rustbot · 2025-05-31T11:46:47Z

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol

The Miri subtree was changed

cc @rust-lang/miri

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @davidtwco, @vakaras

compiler/rustc_borrowck/src/type_check/mod.rs

@@ -1951,7 +1950,7 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
                    );
                }

-                // When `unsized_fn_params` and `unsized_locals` are both not enabled,
+                // When `unsized_fn_params` is both not enabled,


RalfJung · 2025-05-31T21:02:56Z

Implements rust-lang/compiler-team#630

That's 2 years ago and so it's not indicative of the team position today. Some stuff happened since then since some people want to preserve the feature.

This will need a new MCP I think.

Noratrieb · 2025-05-31T21:25:12Z

I don't think anything changed about the biggest issues (MIR support being lacking), the unsoundness around alignment was just one small factor I'd say. No one has stepped up to actually resolve any of these issues in two years, which doesn't indicate a lot of interest in the feature to me. That said, if you think a re-MCP should be done, I'd be happy to help drafting that up (I imagine it would mostly just copy from the old MCP).
To me, two years is not that long in this case and I'm not aware of any significant changes in any area that would make the result different now (other than the one fix), so I don't think it warrants a new MCP.

RalfJung · 2025-06-01T06:32:01Z

The MIR representation of unsized locals still needs to be re-done from scratch, that is true.

Noratrieb · 2025-06-01T08:40:34Z

compiler/rustc_feature/src/removed.rs

@@ -247,6 +247,8 @@ declare_features! (
    /// Allows unnamed fields of struct and union type
    (removed, unnamed_fields, "1.83.0", Some(49804), Some("feature needs redesign")),
    (removed, unsafe_no_drop_flag, "1.0.0", None, None),
+    /// Allows unsized rvalues at arguments and parameters.
+    (removed, unsized_locals, "CURRENT_RUSTC_VERSION", Some(48055), Some("removed due to soundness issues; see https://github.com/rust-lang/rust/issues/111942")),


Suggested change

(removed, unsized_locals, "CURRENT_RUSTC_VERSION", Some(48055), Some("removed due to soundness issues; see https://github.com/rust-lang/rust/issues/111942")),

(removed, unsized_locals, "CURRENT_RUSTC_VERSION", Some(48055), Some("removed due to implementation concerns; see https://github.com/rust-lang/rust/issues/111942")),

As pointed out by Ralf, it should be sound, just not sufficiently implemented

scottmcm · 2025-06-01T19:31:10Z

Note that this just removes the feature, not the implementation

Maybe a dumb question, but why not remove the implementation? If the implementation is going to be there anyway, feels like having the feature is fine.

(My preference would be to remove the implementation too.)

Noratrieb · 2025-06-01T19:34:41Z

The implementation should be removed too but not in this PR^^

flip1995 · 2025-06-02T09:30:14Z

src/tools/clippy/tests/ui/large_stack_frames.rs

+
+
+
+


Please remove all those newlines and run x.py test src/tools/clippy --bless once.

The blank lines are intentional for now, I will address them later. I wanted to check that CI got green but that command is broken and I didn't have the time to debug clippy at the time.

https://rust-lang.zulipchat.com/#narrow/channel/257328-clippy/topic/clippy.20ui.20tests.20broken/with/521708167

SparrowLii · 2025-06-04T00:38:07Z

This change makes sense. I know little about unsized locals, so I'd like to have someone more experienced take a look.
r? compiler

rustbot assigned SparrowLii May 31, 2025

rustbot added the F-unsized_locals `#![feature(unsized_locals)]` label May 31, 2025