-Znext-solver=globally breaks TransmuteFrom<&T>

I tried this code:

#![feature(transmutability)]

use std::mem::TransmuteFrom;

fn main() {
    let bytes: &[u8; 4] = unsafe { TransmuteFrom::<&i32>::transmute(&1i32) };
}

I expected to see this happen: code compiles (it does with -Znext-solver=coherence)

Instead, this happened: code doesn't compile

Meta

rustc --version --verbose:

rustc 1.100.0-nightly (67854e511 2026-08-15)
binary: rustc
commit-hash: 67854e511de21d881bb16426996cd4259d44aa2e
commit-date: 2026-08-15
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.0

Backtrace

error[E0277]: `&i32` cannot be safely transmuted into `&[u8; 4]`
  |
6 |     let bytes: &[u8; 4] = unsafe { TransmuteFrom::<&i32>::transmute(&1i32) };
  |                                    -------------------------------- ^^^^^ the nightly-only, unstable trait `TransmuteFrom<&i32, Assume { alignment: false, lifetimes: false, safety: false, validity: false }>` is not implemented for `&[u8; 4]`
  |                                    |
  |                                    required by a bound introduced by this call

For more information about this error, try `rustc --explain E0277`.

Responsible lines:

rust/compiler/rustc_trait_selection/src/solve/delegate.rs

Lines 497 to 501 in 5321a4f

    
           // FIXME(transmutability): This really should be returning nested goals for `Answer::If*` 
        
           match rustc_transmute::TransmuteTypeEnv::new(self.0.tcx).is_transmutable(src, dst, assume) { 
        
               rustc_transmute::Answer::Yes => Ok(Certainty::Yes), 
        
               rustc_transmute::Answer::No(_) | rustc_transmute::Answer::If(_) => Err(NoSolution), 
        
           }

Rollup merge of rust-lang#163946 - lsunsi:next-transmute, r=lcnr,Shoy…

Unrolled build for #163946 in rollup 164216

Navigation Menu

Repository navigation

`-Znext-solver=globally` breaks `TransmuteFrom<&T>` #161251

Meta

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

	// FIXME(transmutability): This really should be returning nested goals for `Answer::If*`
	match rustc_transmute::TransmuteTypeEnv::new(self.0.tcx).is_transmutable(src, dst, assume) {
	rustc_transmute::Answer::Yes => Ok(Certainty::Yes),
	rustc_transmute::Answer::No(_) \| rustc_transmute::Answer::If(_) => Err(NoSolution),
	}

-Znext-solver=globally breaks TransmuteFrom<&T> #161251

Description

Meta

Activity

ShoyuVanilla commented on Aug 31, 2026

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions

`-Znext-solver=globally` breaks `TransmuteFrom<&T>` #161251