method has an incompatible type for trait due to lifetimes has too wide a span

[nagisa]

For

trait A<'a> {
    fn a<'b>(self) -> Box<A<'b>>;
}

struct C;

impl<'a> A<'a> for C {
    fn a<'b>(self) -> Box<A<'a>>{}
}

it would be much more useful if we spanned the offending lifetime rather than the whole method: i.e. like this

error: method `a` has an incompatible type for trait:
 expected bound lifetime parameter 'b,
    found concrete lifetime [--explain E0053]
 --> <anon>:8:5
8 |>     fn a<'b>(self) -> Box<A<'a>>{}
  |>                             ^^

instead of

error: method `a` has an incompatible type for trait:
 expected bound lifetime parameter 'b,
    found concrete lifetime [--explain E0053]
 --> <anon>:8:5
8 |>     fn a<'b>(self) -> Box<A<'a>>{}
  |>     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[estebank]

Current output:

error[E0195]: lifetime parameters or bounds on method `a` do not match the trait declaration
 --> src/main.rs:8:5
  |
2 |     fn a<'b>(self) -> Box<A<'b>>;
  |     ----------------------------- lifetimes in impl do not match this method in trait
...
8 |     fn a<'b>(self) -> Box<A<'a>> {}
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ lifetimes do not match method in trait

[estebank]

Current output:

error[E0195]: lifetime parameters or bounds on method `a` do not match the trait declaration
 --> src/main.rs:8:9
  |
2 |     fn a<'b>(self) -> Box<A<'b>>;
  |         ---- lifetimes in impl do not match this method in trait
...
8 |     fn a<'b>(self) -> Box<A<'a>> {}
  |         ^^^^ lifetimes do not match method in trait

It fails to point at the 'a in the second case :-/

[estebank]

Triage: no change.

[RodBurman]

The output from:

% cargo -V -v
cargo 1.84.0 (66221abde 2024-11-19)
release: 1.84.0
commit-hash: 66221abdeca2002d318fde6efff516aab091df0e
commit-date: 2024-11-19
host: aarch64-apple-darwin
libgit2: 1.8.1 (sys:0.19.0 vendored)
libcurl: 8.7.1 (sys:0.4.74+curl-8.9.0 system ssl:(SecureTransport) LibreSSL/3.3.6)
ssl: OpenSSL 1.1.1w  11 Sep 2023
os: Mac OS 15.2.0 [64-bit]

is:

% cargo build
   Compiling toowidespan v0.1.0 (/Users/rod/code/rust/triage/toowidespan)
error[E0195]: lifetime parameters or bounds on method `a` do not match the trait declaration
 --> src/lib.rs:8:9
  |
2 |     fn a<'b>(self) -> Box<A<'b>>;
  |         ---- lifetimes in impl do not match this method in trait
...
8 |     fn a<'b>(self) -> Box<A<'a>>{}
  |         ^^^^ lifetimes do not match method in trait

error[E0782]: expected a type, found a trait
 --> src/lib.rs:2:27
  |
2 |     fn a<'b>(self) -> Box<A<'b>>;
  |                           ^^^^^
  |
help: you can add the `dyn` keyword if you want a trait object
  |
2 |     fn a<'b>(self) -> Box<dyn A<'b>>;
  |                           +++

error[E0782]: expected a type, found a trait
 --> src/lib.rs:8:27
  |
8 |     fn a<'b>(self) -> Box<A<'a>>{}
  |                           ^^^^^
  |
help: you can add the `dyn` keyword if you want a trait object
  |
8 |     fn a<'b>(self) -> Box<dyn A<'a>>{}
  |                           +++

Some errors have detailed explanations: E0195, E0782.
For more information about an error, try `rustc --explain E0195`.
error: could not compile `toowidespan` (lib) due to 3 previous errors

The spans indicated are now much shorter but none seem to indicate the 'a that was originally suggested, thus I am not sure if we can agree to close the issue.

[estebank]

Yeah, the ticket should not be closed as now the shorter span is actively not pointing at where it should be pointing. I'm happy with the output you show above in general compared to what it used to be, but it can be better.