Confusing "cannot infer an appropriate lifetime" error message

Consider example:

trait Bar {
    fn bar(&self, r: &mut Re);
}

struct Re<'a> {
    data: &'a u16,
}

struct Foo;

impl Bar for Foo {
    fn bar<'a, 'b>(&'a self, r: &'b mut Re<'a>){}
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0495]: cannot infer an appropriate lifetime for lifetime parameter 'a in generic type due to conflicting requirements
  --> src/lib.rs:12:5
   |
12 |     fn bar<'a, 'b>(&'a self, r: &'b mut Re<'a>){}
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 12:5...
  --> src/lib.rs:12:5
   |
12 |     fn bar<'a, 'b>(&'a self, r: &'b mut Re<'a>){}
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
note: ...but the lifetime must also be valid for the anonymous lifetime #3 defined on the method body at 12:5...
  --> src/lib.rs:12:5
   |
12 |     fn bar<'a, 'b>(&'a self, r: &'b mut Re<'a>){}
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: ...so that the method type is compatible with trait:
           expected fn(&Foo, &mut Re<'_>)
              found fn(&Foo, &mut Re<'_>)

error: aborting due to previous error

For more information about this error, try `rustc --explain E0495`.
error: could not compile `playground`.

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

All 3 notes of this error message is confusing:

note1, note2: Message not point where are the anonymous lifetime #1 and the anonymous lifetime #3. Also it's not obviously why there are two different anonymous lifetimes at one place (at 12:5).

=note3: Expected result looks totally equal to founded. So it's not obviously why it considered as different.

It is difficult to figure out what was going on.
May be some change of this message needed.

I am here due to the same issue. This annoying error bothers me again and again. And I spend hours and hours to figure out whats wrong with my lifetimes. This is very demotivating and frustrating.

I was trying to implement something like nom parser, there is a lot of generic code, macrorules and so on. And this lifetimes just makes me crazy.

Every time I see this message it makes my head explode.

So I think my journey with rust will end today. Rust is great and writing some simple programs is easy. But when I try to implement something harder then hello world, things got very messy.

Current output:

error: `impl` item signature doesn't match `trait` item signature
  --> src/lib.rs:12:5
   |
 2 |     fn bar(&self, r: &mut Re);
   |     -------------------------- expected `fn(&'1 Foo, &'2 mut Re<'3>)`
...
12 |     fn bar<'a, 'b>(&'a self, r: &'b mut Re<'a>){}
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found `fn(&'1 Foo, &'2 mut Re<'1>)`
   |
   = note: expected signature `fn(&'1 Foo, &'2 mut Re<'3>)`
              found signature `fn(&'1 Foo, &'2 mut Re<'1>)`
   = help: the lifetime requirements from the `impl` do not correspond to the requirements in the `trait`
   = help: verify the lifetime relationships in the `trait` and `impl` between the `self` argument, the other inputs and its output

We should likely detect that the first Re has a lifetime param that isn't being specified and point that out. Beyond that, I don't know how we could improve this case any further.

Rollup merge of rust-lang#152637 - JohnTitor:issue-65866, r=estebank

Unrolled build for #152637

Rollup merge of #152637 - JohnTitor:issue-65866, r=estebank

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Confusing "cannot infer an appropriate lifetime" error message #65866

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Confusing "cannot infer an appropriate lifetime" error message #65866

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zim32 commented on Aug 28, 2021

estebank commented on Nov 3, 2025

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