`overflowing_literals` error is confusing because of type inference

[shreepads]

While experimenting with the Rust By Example playground on the Inference section at https://doc.rust-lang.org/stable/rust-by-example/types/inference.html I came across what I believe is an error in the error produced by the compiler.

I do not expect this example to work but the error literal out of range for 'i8' is reported against line 4 let e2 = 230; which is a perfectly valid statement. The problem is lower down where two different types are pushed onto the same Vector but possibly due to the way the inference engine works this is being incorrectly reported against line 4. If I comment out the line vec.push(e2);, the example works fine, apart from a warning: unused variable: 'e2'.

Code

fn main() {
    // Because of the annotation, the compiler knows that `elem` has type i8.
    let elem = 6i8;
    let e2 = 230;

    // Create an empty vector (a growable array).
    let mut vec = Vec::new();
    // At this point the compiler doesn't know the exact type of `vec`, it
    // just knows that it's a vector of something (`Vec<_>`).

    // Insert `elem` in the vector.
    vec.push(e2);
    vec.push(elem);
    // Aha! Now the compiler knows that `vec` is a vector of `u8`s (`Vec<u8>`)
    // TODO ^ Try commenting out the `vec.push(elem)` line

    println!("{:?}", vec);
}

Meta

rustc --version --verbose: I am unable to determine the exact compiler version in the Rust By Example playground. I have run the same example on my Fedora 32 Workstation machine and with the same result. The rustc version details for this machine are as follows

rustc 1.48.0
binary: rustc
commit-hash: unknown
commit-date: unknown
host: x86_64-unknown-linux-gnu
release: 1.48.0
LLVM version: 10.0

Error output

   Compiling playground v0.0.1 (/playground)
error: literal out of range for `i8`
 --> src/main.rs:4:14
  |
4 |     let e2 = 230;
  |              ^^^
  |
  = note: `#[deny(overflowing_literals)]` on by default
  = note: the literal `230` does not fit into the type `i8` whose range is `-128..=127`

error: aborting due to previous error

error: could not compile `playground`

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

Backtrace

Sorry I don't know exactly how to provide this.

<backtrace>

[jonas-schievink]

e2 also has type i8, because you push it onto the same Vec as elem, so the error message is correct – the literal is out of range for the target type i8

[shreepads]

While that is true (and as I mentioned, I do not expect this to work) the problem is the error reported by the compiler doesn't help the user resolve the problem. In a small example like this it's easy to see where the problem lies but in a larger one the compiler error would send the user in the wrong direction.

[jonas-schievink]

okay, marking as a diagnostics enhancement

[camelid]

I'm guessing this would be hard to implement since we'd need to somehow track the spot(s) that caused us to infer a particular type.