Inline direct calls to `transmute_copy` in the frontend

[pcwalton]

When we see a direct call to mem::transmute_copy, we should just inline it in the frontend. This has two important benefits, one obvious and one not-so-obvious: (1) it reduces compilation time; (2) it can sometimes avoid one memcpy by translating the destination in place if the argument is an rvalue.

[nox]

Does this still need to be done? Did anyone investigate it? The issue is not currently very actionable.

Cc @rust-lang/compiler @rust-lang/wg-codegen @rust-lang/wg-compiler-performance

[oli-obk]

are there enough transmute_copy calls to actually make any difference? I agree that this issue needs some numbers or we should just close it until transmute_copy shows up in other perf discussions

[bjorn3]

#91743 should cause transmute_copy to be inlined before codegen.

[workingjubilee]

That PR was merged so I am closing this on the presumption that it is happening just fine now and that if we need followups on that PR it can be tracked in another issue.

[bjorn3]

It doesn't get inlined. I think the body of transmute_copy is too big. https://play.rust-lang.org/?version=nightly&mode=release&edition=2021&gist=1bb23b8a0df94e2aef6ea204baf6237d

[workingjubilee]

Welp, my bad! Thank you for catching that, @bjorn3.