Mismatched lifetime on trait impl shows a misleading error

The rust compiler gives a very poor error message when there is a lifetime expectation mismatch on trait usage.

I used this code: (playground link: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=581fa159f8ff99f2df447308fccd4bbc )

use std::cell::*;

#[derive(Default)]
struct Test {
    pub foo: u32,
}

trait FooSetter {
    fn set_foo(&mut self, value: u32);
}

impl FooSetter for Test {
    fn set_foo(&mut self, value: u32) {
        self.foo = value;
    }
}

trait BaseSetter{
    fn set(&mut self, value: u32);
}
impl BaseSetter for dyn FooSetter{
    fn set(&mut self, value: u32){
        self.set_foo(value);
    }
}

struct TestHolder<'a> {
    pub holder: Option<RefCell<&'a mut dyn FooSetter>>,
}

impl <'a>TestHolder<'a>{
    pub fn test_foo(&self){
       self.holder.as_ref().unwrap().borrow_mut().set(20); 
    }
}



fn main() {
    let mut test = Test::default();
    test.foo = 10;
    {
        let holder = TestHolder { holder: Some(RefCell::from(&mut test))};
        
        holder.test_foo();
    }
    test.foo = 30;
}

I expected to see a useful error message, such as "expected trait BaseSetter<'a>, found trait: BaseSetter<'static>"

Instead, I was greeted with this almost useless error message: "expected trait BaseSetter, found trait BaseSetter"

The same error occurs on stable and nightly compilers

For reference, this error message is resolved by changing

impl BaseSetter for dyn FooSetter

To this explicit lifetime scoped version:

impl <'a>BaseSetter for (dyn FooSetter + 'a)

@rustbot

@rustbot modify labels: A-diagnostics A-traits C-bug D-confusing D-papercut T-compiler A-lifetimes

Current error:

error[[E0521]](https://doc.rust-lang.org/stable/error_codes/E0521.html): borrowed data escapes outside of associated function
  --> src/main.rs:33:8
   |
31 | impl <'a>TestHolder<'a>{
   |       -- lifetime `'a` defined here
32 |     pub fn test_foo(&self){
   |                     ----- `self` is a reference that is only valid in the associated function body
33 |        self.holder.as_ref().unwrap().borrow_mut().set(20); 
   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |        |
   |        `self` escapes the associated function body here
   |        argument requires that `'a` must outlive `'static`
   |
   = note: requirement occurs because of a mutable reference to `dyn FooSetter`
   = note: mutable references are invariant over their type parameter
   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance

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Mismatched lifetime on trait impl shows a misleading error #80701

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Mismatched lifetime on trait impl shows a misleading error #80701

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Earlz commented on Jan 4, 2021

ThePuzzlemaker commented on Jan 7, 2021

Dylan-DPC commented on Apr 18, 2023

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