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Misleading compile error message when checking match arm types. #78124

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supermank17 opened this issue Oct 20, 2020 · 0 comments · Fixed by #114819
Closed

Misleading compile error message when checking match arm types. #78124

supermank17 opened this issue Oct 20, 2020 · 0 comments · Fixed by #114819
Labels
A-diagnostics Area: Messages for errors, warnings, and lints A-typesystem Area: The type system C-enhancement Category: An issue proposing an enhancement or a PR with one. D-confusing Diagnostics: Confusing error or lint that should be reworked. D-incorrect Diagnostics: A diagnostic that is giving misleading or incorrect information. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@supermank17
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supermank17 commented Oct 20, 2020

I ran into a case where the compiler is complaining that match arms have incompatible types, as seen in this code snippet below.
However, the line it is pointing to is not necessarily incorrect: the real issue is that the outer "shouldwe" Some => match-arm is not returning a u32 as expected. (In other words, this can be made to compile by adding a return after the match shouldwe2 statement).
I eventually realized the actual issue, but this could possibly be clarified by pointing at the outer shouldwe match instead of the inner shouldwe2 match.

#![allow(unused)]

fn test(shouldwe: Option<u32>, shouldwe2: Option<u32>) -> u32 {
    match shouldwe {
        Some(val) => {
            match shouldwe2 {
                Some(val) => {
                    return val;
                }
                None => (),
            }
        }
        None => return 12,
    }
}

fn main() {
    println!("returned {}", test(None, Some(5)));
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0308]: `match` arms have incompatible types
  --> src/main.rs:10:25
   |
6  | /             match shouldwe2 {
7  | |                 Some(val) => {
8  | |                     return val;
   | |                     ----------- this is found to be of type `u32`
9  | |                 }
10 | |                 None => (),
   | |                         ^^ expected `u32`, found `()`
11 | |             }
   | |_____________- `match` arms have incompatible types

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: could not compile `playground`.

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

@jonas-schievink jonas-schievink added A-diagnostics Area: Messages for errors, warnings, and lints A-typesystem Area: The type system C-enhancement Category: An issue proposing an enhancement or a PR with one. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 20, 2020
@JohnTitor JohnTitor added the D-confusing Diagnostics: Confusing error or lint that should be reworked. label Oct 20, 2020
@estebank estebank added the D-incorrect Diagnostics: A diagnostic that is giving misleading or incorrect information. label Aug 3, 2023
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Aug 15, 2023
…rrors

Point at return type when it influences non-first `match` arm

When encountering code like

```rust
fn foo() -> i32 {
    match 0 {
        1 => return 0,
        2 => "",
        _ => 1,
    }
}
```

Point at the return type and not at the prior arm, as that arm has type `!` which isn't influencing the arm corresponding to arm `2`.

Fix rust-lang#78124.
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Aug 15, 2023
…rrors

Point at return type when it influences non-first `match` arm

When encountering code like

```rust
fn foo() -> i32 {
    match 0 {
        1 => return 0,
        2 => "",
        _ => 1,
    }
}
```

Point at the return type and not at the prior arm, as that arm has type `!` which isn't influencing the arm corresponding to arm `2`.

Fix rust-lang#78124.
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Aug 15, 2023
…rrors

Point at return type when it influences non-first `match` arm

When encountering code like

```rust
fn foo() -> i32 {
    match 0 {
        1 => return 0,
        2 => "",
        _ => 1,
    }
}
```

Point at the return type and not at the prior arm, as that arm has type `!` which isn't influencing the arm corresponding to arm `2`.

Fix rust-lang#78124.
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Aug 15, 2023
…rrors

Point at return type when it influences non-first `match` arm

When encountering code like

```rust
fn foo() -> i32 {
    match 0 {
        1 => return 0,
        2 => "",
        _ => 1,
    }
}
```

Point at the return type and not at the prior arm, as that arm has type `!` which isn't influencing the arm corresponding to arm `2`.

Fix rust-lang#78124.
@bors bors closed this as completed in 55f8c66 Aug 15, 2023
flip1995 pushed a commit to flip1995/rust that referenced this issue Aug 24, 2023
…rrors

Point at return type when it influences non-first `match` arm

When encountering code like

```rust
fn foo() -> i32 {
    match 0 {
        1 => return 0,
        2 => "",
        _ => 1,
    }
}
```

Point at the return type and not at the prior arm, as that arm has type `!` which isn't influencing the arm corresponding to arm `2`.

Fix rust-lang#78124.
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Labels
A-diagnostics Area: Messages for errors, warnings, and lints A-typesystem Area: The type system C-enhancement Category: An issue proposing an enhancement or a PR with one. D-confusing Diagnostics: Confusing error or lint that should be reworked. D-incorrect Diagnostics: A diagnostic that is giving misleading or incorrect information. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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