"this closure does not fulfill the lifetime requirements" could be more helpful #102877

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1. mejrs

   added

   A-diagnosticsArea: Messages for errors, warnings, and lintsArea: Messages for errors, warnings, and lints

   T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

   on Oct 10, 2022
2. kadiwa4 commented on Mar 1, 2024

   

   kadiwa4

   on Mar 1, 2024

   Last edited by kadiwa4

   Contributor

   More actions

   [Outdated] Since #105888, rustc provides you with a working suggestion:

   Outdated output (2024-03-01)

   error[E0308]: mismatched types
    --> src/main.rs:5:5
     |
   5 |     foo(&mut fun);
     |     ^^^^^^^^^^^^^ one type is more general than the other
     |
     = note: expected trait `for<'a> FnMut(&'a str)`
                found trait `FnMut(&str)`
   note: this closure does not fulfill the lifetime requirements
    --> src/main.rs:4:19
     |
   4 |     let mut fun = |_arg| {};
     |                   ^^^^^^
   note: the lifetime requirement is introduced here
    --> src/main.rs:1:21
     |
   1 | fn foo(_: &mut impl FnMut(&str)) {}
     |                     ^^^^^^^^^^^
   help: consider specifying the type of the closure parameters
     |
   4 |     let mut fun = |_arg: &_| {};
     |                   ~~~~~~~~~~
   
   error: implementation of `FnOnce` is not general enough
    --> src/main.rs:5:5
     |
   5 |     foo(&mut fun);
     |     ^^^^^^^^^^^^^ implementation of `FnOnce` is not general enough
     |
     = note: closure with signature `fn(&'2 str)` must implement `FnOnce<(&'1 str,)>`, for any lifetime `'1`...
     = note: ...but it actually implements `FnOnce<(&'2 str,)>`, for some specific lifetime `'2`
   
   For more information about this error, try `rustc --explain E0308`.
   error: could not compile `playground` (bin "playground") due to 2 previous errors
   

   Test is in tests/ui/lifetimes/issue-79187.

   Update: The suggestion was removed again in #119849.
   Updated, not so helpful output (2024-09-06):

   error: implementation of `FnMut` is not general enough
    --> src/main.rs:5:5
     |
   5 |     foo(&mut fun);
     |     ^^^^^^^^^^^^^ implementation of `FnMut` is not general enough
     |
     = note: closure with signature `fn(&'2 str)` must implement `FnMut<(&'1 str,)>`, for any lifetime `'1`...
     = note: ...but it actually implements `FnMut<(&'2 str,)>`, for some specific lifetime `'2`
   
   error: implementation of `FnOnce` is not general enough
    --> src/main.rs:5:5
     |
   5 |     foo(&mut fun);
     |     ^^^^^^^^^^^^^ implementation of `FnOnce` is not general enough
     |
     = note: closure with signature `fn(&'2 str)` must implement `FnOnce<(&'1 str,)>`, for any lifetime `'1`...
     = note: ...but it actually implements `FnOnce<(&'2 str,)>`, for some specific lifetime `'2`
   
   error: could not compile `playground` (bin "playground") due to 2 previous errors