main.rs

// A problem with type inference. A closure object is bound to cb

let cb = Box::new(move |state| {
    let callgraph = cell.replace(None).unwrap();
    visit::after_analysis(state, callgraph)
});

// And right after, cb is set to callback.
control.after_analysis.callback = cb;

//////////////////////////////////////////////////////////////////

// This works:
control.after_analysis.callback = Box::new(move |state| {
    let callgraph = cell.replace(None).unwrap();
    visit::after_analysis(state, callgraph)
});

//////////////////////////////////////////////////////////////////
// This works too.

let cb: Box<Fn(&mut CompileState) + 'a> = Box::new(move |state| {
    let callgraph = cell.replace(None).unwrap();
    visit::after_analysis(state, callgraph)
});
control.after_analysis.callback = cb;

//////////////////////////////////////////////////////////////////
// The error message:

error[E0631]: type mismatch in closure arguments
   --> src/main.rs:108:43
    |
104 |         let cb = Box::new(move |state| {
    |                           ------------ found signature of `fn(&mut rustc_driver::driver::CompileState<'_, '_>) -> _`
...
108 |         control.after_analysis.callback = cb;
    |                                           ^^ expected signature of `for<'r, 's, 't0> fn(&'r mut rustc_driver::driver::CompileState<'s, 't0>) -> _`
    |
    = note: required for the cast to the object type `for<'r, 's, 't0> std::ops::Fn(&'r mut rustc_driver::driver::CompileState<'s, 't0>)`

error[E0271]: type mismatch resolving `for<'r, 's, 't0> <[closure@src/main.rs:104:27: 107:10 cell:_] as std::ops::FnOnce<(&'r mut rustc_driver::driver::CompileState<'s, 't0>,)>>::Output == ()`
   --> src/main.rs:108:43
    |
108 |         control.after_analysis.callback = cb;
    |                                           ^^ expected bound lifetime parameter, found concrete lifetime
    |
    = note: required for the cast to the object type `for<'r, 's, 't0> std::ops::Fn(&'r mut rustc_driver::driver::CompileState<'s, 't0>)`