Usually opening a file handle with access set to metadata only will always succeed, even if the file is locked. However some special system files, such as `C:\hiberfil.sys`, are locked by the system in a way that denies even that. So as a fallback we try reading the cached metadata from the directory.

… r=thomcc

Add assertion that `transmute_copy`'s U is not larger than T

This is called out as a safety requirement in the docs, but because knowing this can be done at compile time and constant folded (just like the `align_of` branch is removed), we can just panic here.

I've looked at the asm (using `cargo-asm`) of a function that both is correct and incorrect, and the panic is completely removed, or is unconditional, without needing build-std.

I don't expect this to cause much breakage in the wild. I scanned through https://miri.saethlin.dev/ub for issues that would look like this (error: Undefined Behavior: memory access failed: alloc1768 has size 1, so pointer to 8 bytes starting at offset 0 is out-of-bounds), but couldn't find any.

That doesn't rule out it happening in crates tested that fail earlier for some other reason, though, but it indicates that doing this is rare, if it happens at all. A crater run for this would need to be build and test, since this is a runtime thing.

Also added a few more transmute_copy tests.

Windows: Use `FindFirstFileW` for getting the metadata of locked system files

Fixes rust-lang#96980

Usually opening a file handle with access set to metadata only will always succeed, even if the file is locked. However some special system files, such as `C:\hiberfil.sys`, are locked by the system in a way that denies even that. So as a fallback we try reading the cached metadata from the directory.

Note that the test is a bit iffy. I don't know if `hiberfil.sys` actually exists in the CI.

r? rust-lang/libs

add missing null ptr check in alloc example

`alloc` can return null on OOM, if I understood correctly. So we should never just deref a pointer we get from `alloc`.

…al-def-id, r=oli-obk

Use `LocalDefId` in `OpaqueTypeKey`

Addresses a `// FIXME(oli-obk): make this a LocalDefId`

r? ```@oli-obk```

interpret/visitor: add missing early return

I forgot to add this when adding the special `Box` handling branch.

r? `@oli-obk`