Terrible error message when trying to use borrowed self in move closure

[cramertj]

The code:

struct Foo {
    name: String,
}

impl Foo {
    fn foo(&self) -> Box<Fn()> {
        Box::new(move || println!("{}", self.name))
    }
}

Expected behavior: prints error that looks something like this:

self.name
^^^^ cannot use self in 'static move closure because `self` is borrowed

Actual behavior: prints this error message:

error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
 --> src/main.rs:8:9
  |
8 |         Box::new(move || println!("{}", self.name))
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 7:5...
 --> src/main.rs:7:5
  |
7 | /     fn foo(&self) -> Box<Fn()> {
8 | |         Box::new(move || println!("{}", self.name))
9 | |     }
  | |_____^
note: ...so that the type `[closure@src/main.rs:8:18: 8:51 self:&Foo]` will meet its required lifetime bounds
 --> src/main.rs:8:9
  |
8 |         Box::new(move || println!("{}", self.name))
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: but, the lifetime must be valid for the static lifetime...
note: ...so that expression is assignable (expected std::boxed::Box<std::ops::Fn() + 'static>, found std::boxed::Box<std::ops::Fn()>)
 --> src/main.rs:8:9
  |
8 |         Box::new(move || println!("{}", self.name))
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[cramertj]

This issue isn't limited to self, either, but this was the case in which the error originally occurred. While I understand it might be difficult to identify exactly which variable in the closure has too short of a lifetime, it should be possible to at least give a better error message like "value with non-'static lifetime referenced in static closure" and provide the name of the lifetime (if there is one).

[estebank]

Triage: very little change:

error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
 --> src/main.rs:7:9
  |
7 |         Box::new(move || println!("{}", self.name))
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 6:5...
 --> src/main.rs:6:5
  |
6 | /     fn foo(&self) -> Box<Fn()> {
7 | |         Box::new(move || println!("{}", self.name))
8 | |     }
  | |_____^
note: ...so that the type `[closure@src/main.rs:7:18: 7:51 self:&Foo]` will meet its required lifetime bounds
 --> src/main.rs:7:9
  |
7 |         Box::new(move || println!("{}", self.name))
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  = note: but, the lifetime must be valid for the static lifetime...
  = note: ...so that the expression is assignable:
          expected std::boxed::Box<std::ops::Fn() + 'static>
             found std::boxed::Box<std::ops::Fn()>

Error should point at self inside the closure, and to the closure.

[estebank]

Ast output:

error[E0495]: cannot infer an appropriate lifetime due to conflicting requirements
 --> file.rs:7:18
  |
7 |         Box::new(move || println!("{}", self.name))
  |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: first, the lifetime cannot outlive the anonymous lifetime #1 defined on the method body at 6:5...
 --> file.rs:6:5
  |
6 | /     fn foo(&self) -> Box<Fn()> {
7 | |         Box::new(move || println!("{}", self.name))
8 | |     }
  | |_____^
  = note: ...so that the types are compatible:
          expected &Foo
             found &Foo
  = note: but, the lifetime must be valid for the static lifetime...
  = note: ...so that the expression is assignable:
          expected std::boxed::Box<(dyn std::ops::Fn() + 'static)>
             found std::boxed::Box<dyn std::ops::Fn()>

MIR output:

error: lifetime may not live long enough
 --> file.rs:7:9
  |
6 |     fn foo(&self) -> Box<Fn()> {
  |            - let's call the lifetime of this reference `'1`
7 |         Box::new(move || println!("{}", self.name))
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ returning this value requires that `'1` must outlive `'static`

[estebank]

No change in the output. The compiler should suggest a signature of fn foo(&self) -> Box<dyn Fn() + '_> when we infer a 'static lifetime, as it is rarely what we desire. We already provide that suggestion for impl Traits in the return type.

[cramertj]

@estebank FWIW in the case this was originally minimized from, the solution was actually to call self.name.clone() and then move the resulting name into the closure, rather than moving self in . I don't think a suggestion is what's needed here-- I think the primary thing that would help would be a hint that the use of self.name in the closure is what's causing the lifetime to be restricted to '1.

[estebank]

Closing in favor of the newer ticket, which will be closed by #106641.

Looking at the code it now compiles (after applying some suggested changes), and the new suggestion will not help for closures being returned.