Why does Rust allow code with the wrong return type, but only with a trailing semicolon?

The return type in the first code block is actually ! (called never) because you have a loop that never exits (so rust gives you a warning saying it's unreachable). The full type would be:

fn f() -> !

I suspect ! is more like the 'bottom' type in Rust than anything else. In the second case, your function likely errors out in an earlier stage during type checking because of the mismatch between i32 and () before the compiler gets to the 'unreachability' analysis, like it does in the first example.

edit: as suggested, here is the relevant part of the rust book https://doc.rust-lang.org/book/ch19-04-advanced-types.html#the-never-type-that-never-returns