Rust: Two-level type inference throws unhelpful E0308

[friendzis]

I have tried two level type inference from vector literal and got totally unhelpful E0308. While I would expect this to work, I file this report due to unhelpfulness of the error.
I expected vec2 alocation to succeed or complain about inability to to coerce types.
Actual error:

src\main.rs:14:25: 14:26 error: mismatched types [E0308]
src\main.rs:14     let vec2 = Baz(vec![1, 2, 3]);
                                       ^
src\main.rs:14:20: 14:33 note: in this expansion of vec! (defined in <std macros>)
src\main.rs:14:25: 14:26 help: run `rustc --explain E0308` to see a detailed explanation
src\main.rs:14:25: 14:26 note: expected type `_`
src\main.rs:14:25: 14:26 note:    found type `_`
<std macros>:3:25: 3:46 error: mismatched types [E0308]
<std macros>:3 < [ _ ] > :: into_vec ( box [ $ ( $ x ) , * ] ) ) ; ( $ ( $ x : expr , ) * )
                                       ^~~~~~~~~~~~~~~~~~~~~
src\main.rs:14:20: 14:33 note: in this expansion of vec! (defined in <std macros>)
<std macros>:3:25: 3:46 help: run `rustc --explain E0308` to see a detailed explanation
<std macros>:3:25: 3:46 note: expected type `Box<[Foo]>`
<std macros>:3:25: 3:46 note:    found type `Box<[_; 3]>`
error: aborting due to 2 previous errors
error: Could not compile `s1c1`.

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

"cargo build" completed with code 101
It took approximately 0.379 seconds

Code sample

#[derive(Debug)]
struct Foo(u8);

#[derive(Debug)]
struct Bar(Vec<u8>);

#[derive(Debug)]
struct Baz(Vec<Foo>);

fn main() {
    let vec1 = Bar(vec!(1, 2, 3));
    println!("{:?}", vec1);

    let vec2 = Baz(vec!(1, 2, 3));  // Offending line
    println!("{:?}", vec2);

    let vec3 = Baz(vec!(Foo(1), Foo(2), Foo(3)));
    println!("{:?}", vec3);
}

Meta

rustc --version --verbose
rustc 1.11.0 (9b21dcd6a 2016-08-15)
binary: rustc
commit-hash: 9b21dcd6a89f38e8ceccb2ede8c9027cb409f6e3
commit-date: 2016-08-15
host: x86_64-pc-windows-gnu
release: 1.11.0

[steveklabnik]

To me, the

src\main.rs:14:25: 14:26 note: expected type `_`
src\main.rs:14:25: 14:26 note:    found type `_`

line is the worst part, though at least it does give the real error here:

<std macros>:3:25: 3:46 note: expected type `Box<[Foo]>`
<std macros>:3:25: 3:46 note:    found type `Box<[_; 3]>`

[frewsxcv]

(incoming tangential comment)

Regarding:

src\main.rs:14:25: 14:26 note: expected type `_`
src\main.rs:14:25: 14:26 note:    found type `_`

Maybe we should add a debug assertion that both of the types in this error message are not equal. I don't see a valid scenario where we should display the same type on both lines.

[killercup]

cc @GuillaumeGomez because of #37388

FYI, error with recent nightly is (see playground):

rustc 1.14.0-nightly (cae6ab1c4 2016-11-05)
error[E0308]: mismatched types
  --> <anon>:14:25
   |
14 |     let vec2 = Baz(vec!(1, 2, 3));  // Offending line
   |                         ^ expected struct `Foo`, found integral variable
   |
   = note: expected type `_`
   = note:    found type `{integer}`

error[E0308]: mismatched types
  --> <anon>:14:20
   |
14 |     let vec2 = Baz(vec!(1, 2, 3));  // Offending line
   |                    ^^^^^^^^^^^^^ expected slice, found array of 3 elements
   |
   = note: expected type `Box<[Foo]>`
   = note:    found type `Box<[{integer}; 3]>`
   = note: this error originates in a macro outside of the current crate

error: aborting due to 2 previous errors

[GuillaumeGomez]

Hum, that might deserve another update once my current PR is merged.

[killercup]

14 |     let vec2 = Baz(vec!(1, 2, 3));  // Offending line
   |                         ^ expected struct `Foo`, found integral variable

Maybe change the "variable" here to "value" (or "literal") if it's, you know, a literal value? (Should probably be a new issue if it doesn't exist yet.)

[Mark-Simulacrum]

Closing as fixed, I think the current error represents the problem well:

error[E0308]: mismatched types
  --> test.rs:14:25
   |
14 |     let vec2 = Baz(vec!(1, 2, 3));  // Offending line
   |                         ^ expected struct `Foo`, found integral variable
   |
   = note: expected type `Foo`
              found type `{integer}`

error: aborting due to previous error(s)