Transmute doesn't work on arrays of generic types

[mbartlett21]

I tried transmuting an array of [T; 10] to itself, but rustc gives an error.

fn transmute_to_self<T: Sized>(v: [T; 10]) -> [T; 10] {
    unsafe { core::mem::transmute(v) }
}

Error:

error[E0512]: cannot transmute between types of different sizes, or dependently-sized types
 --> src/lib.rs:4:14
  |
4 |     unsafe { transmute(v) }
  |              ^^^^^^^^^
  |
  = note: `[T; 10]` does not have a fixed size

Meta

rustc --version --verbose:

rustc 1.51.0-nightly (e22670468 2020-12-30)
binary: rustc
commit-hash: e2267046859c9ceb932abc983561d53a117089f6
commit-date: 2020-12-30
host: x86_64-pc-windows-msvc
release: 1.51.0-nightly

[camelid]

Hmm, that does seem weird.

[camelid]

This compiles fine, so it's not that rustc thinks [T; 10] is unsized:

fn transmute_to_self<T: Sized>(v: [T; 10]) -> impl Sized {
    v
}

[mbartlett21]

The error also comes from transmuting generics:

fn transmute_to_self<T>(t: T) -> T {
    unsafe { core::mem::transmute(t) }
}

gives the same error.

[camelid]

Looks like the code that does this check might not work on generic parameters:

rust/compiler/rustc_passes/src/intrinsicck.rs

Lines 71 to 73 in 26d451f

	fn check_transmute(&self, span: Span, from: Ty<'tcx>, to: Ty<'tcx>) {
	let sk_from = SizeSkeleton::compute(from, self.tcx, self.param_env);
	let sk_to = SizeSkeleton::compute(to, self.tcx, self.param_env);

It does say that it's conservative:

rust/compiler/rustc_middle/src/ty/layout.rs

Lines 1792 to 1810 in 26d451f

	/// Type size "skeleton", i.e., the only information determining a type's size.
	/// While this is conservative, (aside from constant sizes, only pointers,
	/// newtypes thereof and null pointer optimized enums are allowed), it is
	/// enough to statically check common use cases of transmute.
	#[derive(Copy, Clone, Debug)]
	pub enum SizeSkeleton<'tcx> {
	/// Any statically computable Layout.
	Known(Size),
	/// A potentially-fat pointer.
	Pointer {
	/// If true, this pointer is never null.
	non_zero: bool,
	/// The type which determines the unsized metadata, if any,
	/// of this pointer. Either a type parameter or a projection
	/// depending on one, with regions erased.
	tail: Ty<'tcx>,
	},
	}

[DutchGhost]

See #61956 for a previously opened issue

[mbartlett21]

If and when the Project Safe Transmute's ext-layout-traits rfc goes through the process, transmute could be changed to have a bound where the types are SizeEq, which should stop this problem.

[JohnTitor]

Closing as duplicate of #61956 and removing I-prioritize.