Type expectation appears to be reversed

[camelid]

The expected type and the found type seem to be reversed here. The error says it expected () and found i32, but it seems to me that it expected i32 and found ().

fn do_stuff(x: i32) -> i32 {
    if x < 0 {
        return 0;
    } else {
        if 1 == 1 {
            return 1;
        }
    }
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0317]: `if` may be missing an `else` clause
 --> src/lib.rs:5:9
  |
5 | /         if 1 == 1 {
6 | |             return 1;
7 | |         }
  | |_________^ expected `()`, found `i32`
  |
  = note: `if` expressions without `else` evaluate to `()`
  = help: consider adding an `else` block that evaluates to the expected type

error: aborting due to previous error

For more information about this error, try `rustc --explain E0317`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

[JohnTitor]

The diagnostics look correct to me, actually, the note points out why the error happens.

note: `if` expressions without `else` evaluate to `()`

[camelid]

But then it says:

expected `()`, found `i32`

What does that mean? Where's the i32 coming from?

[JohnTitor]

What does that mean? Where's the i32 coming from?

Note that the error talks about the if expression, not the function!
if expressions without else evaluate to (), we expect the body returns (). But when rustc looked at it, they found i32 (i.e. 1). Does this make sense for you?

[camelid]

I understand that it evaluates to (), but where did rustc find i32? In the function return type? That would seem like an expectation to me.

[JohnTitor]

I understand that it evaluates to (), but where did rustc find i32? In the function return type?

Yep, rustc infers 1's type following the return type. You could change it to u32, for example, and see the message :)

That would seem like an expectation to me.

Again, note that the error talks about the if expression only.

[camelid]

But then shouldn't the message be "expected i32 [because of the return type], found () [because if without else has type ()]", which is the reverse of the current message? I still feel confused :/

[JohnTitor]

Rustc only talks about line 5-7 in that error. In that scope, we expect to be returned () from the if expression since it doesn't have else.
If you add else with a wrong return type (https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=f4df73521f639fcf5939d8051207a3fe), now rustc looks at the entire function and finds the bool is incorrect.

[camelid]

In that scope, we expect to be returned () from the if expression since it doesn't have else.

So why doesn't the if expression return () here?