"not all control paths return a value" should indicate which control paths don't return a value

[emberian]

It's very mysterious when this error occurs in a big function and it's not one of the obvious extra-semicolon errors.

[steveklabnik]

Triage: no change. As an attempt to reproduce this, here's a big function:

fn main() {
    let _ = foo();
}

fn foo() -> i32 {
    let x = 5;
    match x {
        1 => {
            1
        },
        2 => {
            2
        },
        3 => {
            3
        },
        4 => {
            4
        },
        5 => {
            5
        },
        _ => {
            0
        },
    };
    match x {
        1 => {
            1
        },
        2 => {
            2
        },
        3 => {
            3
        },
        4 => {
            4
        },
        5 => {
            5
        },
        _ => {
            0
        },
    }; // <- here is the culprit!
}

Here's the error:

hello.rs:5:1: 47:2 error: not all control paths return a value [E0269]
hello.rs: 5 fn foo() -> i32 {
hello.rs: 6     let x = 5;
hello.rs: 7     match x {
hello.rs: 8         1 => {
hello.rs: 9             1
hello.rs:10         },
            ...
hello.rs:5:1: 47:2 help: run `rustc --explain E0269` to see a detailed explanation
error: aborting due to previous error

Doesn't point out the second match at all.

[cyplo]

Hi ! Maybe worth bumping and linking to the recent big push for better error messages ? (Not sure/haven't found yet which issue to link specifically)

[zackmdavis]

Triage: now fixed? Playground says:

error[E0269]: not all control paths return a value
  --> <anon>:5:1
   |
5  | fn foo() -> i32 {
   | ^
   |
help: consider removing this semicolon:
  --> <anon>:46:6
   |
46 |     }; // <- here is the culprit!
   |      ^

error: aborting due to previous error

[eddyb]

It's now:

error[E0308]: mismatched types
  --> <anon>:5:17
   |
5  | fn foo() -> i32 {
   |                 ^ expected i32, found ()
   |
   = note: expected type `i32`
              found type `()`
help: consider removing this semicolon:
  --> <anon>:46:6
   |
46 |     }; // <- here is the culprit!
   |      ^

error: aborting due to previous error

[Mark-Simulacrum]

#39968 is a clear case where this goes wrong. I'm tempted to close this (not sure how helpful it is)...

[estebank]

"Not all control paths" error is no longer being displayed, but rather we're catching type errors and extraneous semicolons and providing help for them:

error[E0308]: mismatched types
 --> src/main.rs:1:27
  |
1 |   fn add_one(x: i32) -> i32 {
  |  ___________________________^
2 | |     x + 1;
  | |          - help: consider removing this semicolon
3 | | }
  | |_^ expected i32, found ()
  |
  = note: expected type `i32`
             found type `()`

[nikomatsakis]

I think we can close this... I think the interesting case is now covered by #39968. That is, I modified the original example to something like this:

fn main() {
    let _ = foo();
}

fn foo() -> i32 {
    let x = 5;
    match x {
        1 => {
            1
        },
        2 => {
            2
        },
        3 => {
            3
        },
        4 => {
            4
        },
        5 => {
            5
        },
        _ => {
            0
        },
    };
    match x {
        1 => {
            return 1
        },
        2 => {
            return 2
        },
        3 => {
            return 3
        },
        4 => {
            return 4
        },
        5 => {
            return 5
        },
        _ => { }
    }
}

But now you get:

error[E0308]: mismatched types
  --> src/main.rs:43:14
   |
43 |         _ => { }
   |              ^^^ expected i32, found ()
   |
   = note: expected type `i32`
              found type `()`

which is not the original example here.

[nikomatsakis]

Going to close.