Make get_return_block() return Some only for HIR nodes in body

[gurry]

Fixes #114918

The issue occurred while compiling the following input:

fn uwu() -> [(); { () }] {
    loop {}
}

It was caused by the code below trying to suggest a missing return type which resulted in a const eval cycle:

rust/compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

Lines 68 to 75 in 1bd0430

	if let Some((fn_id, fn_decl, can_suggest)) = self.get_fn_decl(blk_id) {
	pointing_at_return_type = self.suggest_missing_return_type(
	err,
	&fn_decl,
	expected,
	found,
	can_suggest,
	fn_id,

The root cause was get_return_block() returning an Fn node for a node in the return type (i.e. the second () in the return type [(); { () }] of the input) although it is supposed to do so only for nodes that lie in the body of the function and return None otherwise (at least as per my understanding).

The PR fixes the issue by fixing this behaviour of get_return_block().

[rustbot]

r? @wesleywiser

(rustbot has picked a reviewer for you, use r? to override)

[compiler-errors]

Please squash this into one commit, then I can approve.

[gurry]

Squashed

[compiler-errors]

@bors r+

[bors]

📌 Commit 136f057 has been approved by compiler-errors

It is now in the queue for this repository.