Wrong underlined argument in tuple returned by function

[GuillaumeGomez]

Code

fn get_arguments(data: &[u8]) -> (Vec<String>, &[u8]) {
    (Vec::new(), data)
}

let x = true;
let cmd = &[1, 2, 3, 4];
let (cmd, proc_args) = if x {
    get_arguments(cmd)
} else {
    (Vec::new(), &[])
};

Current output

error[E0277]: the size for values of type `[_]` cannot be known at compilation time
   --> src/unix/apple/macos/process.rs:581:10
    |
581 |     let (cmd, proc_args) = if x {
    |          ^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `[_]`
    = note: all local variables must have a statically known size
    = help: unsized locals are gated as an unstable feature

Desired output

error[E0277]: the size for values of type `[_]` cannot be known at compilation time
   --> src/unix/apple/macos/process.rs:581:10
    |
581 |     let (cmd, proc_args) = if x {
    |               ^^^^^^^^^ doesn't have a size known at compile-time
    |
    = help: the trait `Sized` is not implemented for `[_]`
    = note: all local variables must have a statically known size
    = help: unsized locals are gated as an unstable feature

Rationale and extra context

It's underlining the wrong argument.

Other cases

No response

Anything else?

No response

[bjorn3]

I'm getting an entirely different and much more helpful error:

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=8ef43273677bf72e3c82fb026cf4d1b8

error[E0308]: `if` and `else` have incompatible types
  --> src/main.rs:11:5
   |
8  |   let (cmd, proc_args) = if x {
   |  ________________________-
9  | |     get_arguments(cmd)
   | |     ------------------ expected because of this
10 | | } else {
11 | |     (Vec::new(), &[])
   | |     ^^^^^^^^^^^^^^^^^ expected `(Vec<String>, &[u8])`, found `(Vec<_>, &[_; 0])`
12 | | };
   | |_- `if` and `else` have incompatible types
   |
   = note: expected tuple `(Vec<String>, &[u8])`
              found tuple `(Vec<_>, &[_; 0])`

[compiler-errors]

Are you sure you minimized the code correctly? Not sure how to repro the example you shared, @GuillaumeGomez.

[GuillaumeGomez]

Arf, sorry about that... Also important to note, I'm using:

$ rustc --version
rustc 1.76.0-nightly (2c1b65ee1 2023-11-11)

I pushed the code with this issue here. However it requires a mac to enter this code.

Just running cargo check is enough to trigger it.

[compiler-errors]

This should be re-minimized by someone who has a Mac then.

@rustbot label +E-needs-mcve

[kpreid]

Minimization:

fn issue_117846_repro() {
    let (cmd, _) = if true {
        produce() //
    } else {
        (Vec::new(), &[])
    };

    accept(&cmd);
}

fn produce() -> (Vec<String>, &'static [u8]) {
    todo!()
}

fn accept(cmd: &[String]) {}

Note that:

• in all cases (including when I ran the original code), there is now a more sensible type error, and
• the unsized local error is not actually pointing at the wrong thing, it's talking about the type of the part of the tuple unrelated to the if/else type mismatch.

It seems like the type mismatch is causing the compiler to fail to properly perform the deref coercion of the Vec<String>, even though it's not the part of the tuple that has the type mismatch.

error[E0308]: `if` and `else` have incompatible types
 --> src/lib.rs:5:9
  |
2 |       let (cmd, _) = if true {
  |  ____________________-
3 | |         produce() //
  | |         --------- expected because of this
4 | |     } else {
5 | |         (Vec::new(), &[])
  | |         ^^^^^^^^^^^^^^^^^ expected `(Vec<String>, &[u8])`, found `(Vec<_>, &[_; 0])`
6 | |     };
  | |_____- `if` and `else` have incompatible types
  |
  = note: expected tuple `(Vec<String>, &[u8])`
             found tuple `(Vec<_>, &[_; 0])`

error[E0277]: the size for values of type `[String]` cannot be known at compilation time
 --> src/lib.rs:2:10
  |
2 |     let (cmd, _) = if true {
  |          ^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `[String]`
  = note: all local variables must have a statically known size
  = help: unsized locals are gated as an unstable feature

Tested on versions:

rustc 1.77.0-nightly (bf8716f1c 2023-12-24)
binary: rustc
commit-hash: bf8716f1cd6416266807706bcae0ecb2e51c9d4a
commit-date: 2023-12-24
host: x86_64-apple-darwin
release: 1.77.0-nightly
LLVM version: 17.0.6

rustc 1.75.0 (82e1608df 2023-12-21)
binary: rustc
commit-hash: 82e1608dfa6e0b5569232559e3d385fea5a93112
commit-date: 2023-12-21
host: x86_64-apple-darwin
release: 1.75.0
LLVM version: 17.0.6

[kpreid]

@rustbot label -E-needs-mcve