Wrong span for E0277 error message "trait bound is not satisfied"

[sharifhsn]

Problem

I had an error in my code where I tried to convert a u32 to an enum using try_into without implementing it first. My code essentially looked like this:

enum Thing {
    First,
    Second,
}

struct BigThing {
    value: u32,
    other: u16,
    thing: Thing,
}

impl BigThing {
    fn new(value: u32, other: u16, thing: Thing) -> Self {
        BigThing {value, other,  thing}
    }
}

//-------------------------------------------VVV note that this is u32, not Thing
fn make_thing(value: u32, other: u16, thing: u32) -> BigThing {
    BigThing::new(value, other, thing.try_into().unwrap())
}

This code generates the following error message:

error[E0277]: the trait bound `Thing: From<u32>` is not satisfied
  --> src/starlet/iso/iso.rs:67:19
   |
67 |     BigThing::new(value, other, thing.try_into().unwrap())
   |     ------------- ^^^^^ the trait `From<u32>` is not implemented for `Thing`
   |     |
   |     required by a bound introduced by this call
   |
   = note: required because of the requirements on the impl of `std::convert::Into<Thing>` for `u32`
   = note: required because of the requirements on the impl of `TryFrom<u32>` for `Thing`
   = note: required because of the requirements on the impl of `TryInto<Thing>` for `u32`

The compiler correctly notes that I have not implemented TryInto on Thing for u32, but it tells me that I haven't satisfied that trait bound for value, not thing. This problem does not occur if I use a struct initializer, only if I use a function.

Steps

1. Copy the above code.
2. Run cargo check

Possible Solution(s)

My guess here is that when showing an error for trait bound in a function argument, the checker defaults to the first argument. I'm not familiar with cargo code though so that's just speculation.

Notes

I am using the nightly compiler, but the same problem occurs on stable.

Version

cargo 1.64.0-nightly (a5e08c470 2022-06-23)
release: 1.64.0-nightly
commit-hash: a5e08c4703f202e30cdaf80ca3e7c00baa59c496
commit-date: 2022-06-23
host: x86_64-unknown-linux-gnu
libgit2: 1.4.2 (sys:0.14.2 vendored)
libcurl: 7.83.1-DEV (sys:0.4.55+curl-7.83.1 vendored ssl:OpenSSL/1.1.1n)
os: Arch Linux Rolling Release [64-bit]

[ehuss]

Thanks for the report! I have transferred this to the rust-lang/rust repository as this is a rustc issue.

[ehuss]

I bisected this to nightly-2021-10-27. There are several PRs which are candidates there 29b1248...e269e6b

The most likely look like either #90282, #90299, or #90075.

The previous error message looked like:

error[E0277]: the trait bound `Thing: From<u32>` is not satisfied
  --> src/main.rs:21:33
   |
21 |     BigThing::new(value, other, thing.try_into().unwrap())
   |     -------------               ^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `From<u32>` is not implemented for `Thing`
   |     |
   |     required by a bound introduced by this call
   |
   = note: required because of the requirements on the impl of `Into<Thing>` for `u32`
   = note: required because of the requirements on the impl of `TryFrom<u32>` for `Thing`
   = note: required because of the requirements on the impl of `TryInto<Thing>` for `u32`

For more information about this error, try `rustc --explain E0277`.

[compiler-errors]

This is probably #90181

[compiler-errors]

I was looking into point_at_arg_instead_of_call_if_possible recently in #98607 so I can look at this too.

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