"lifetime `'static` required" - poor diagnostics

[rukai]

Given the following code:

use std::cell::RefCell;
use std::io::Read;

fn main() {
    let foo: &[u8] = &[0, 1];
    inner(foo);
}

fn inner(mut foo: &[u8]) {
    //let mut foo: &[u8] = &[0, 1]; // uncommenting this line gives an improved error message
    let refcell = RefCell::new(&mut foo);
    let read = &refcell as &RefCell<dyn Read>;

    read_thing(read);
}

fn read_thing(refcell: &RefCell<dyn Read>) {
}

The current output is:

   Compiling foo_rust v0.1.0 (/home/rukai/Foo/foo_rust)
error[E0621]: explicit lifetime required in the type of `foo`
  --> src/main.rs:14:16
   |
14 |     read_thing(read);
   |                ^^^^ lifetime `'static` required

Ideally the output should look like:

error[E0621]: explicit lifetime required in the type of `foo`
  --> $DIR/issue-90600.rs:10:16
   |
LL | fn inner(mut foo: &[u8]) {
   |                   ----- the type of `foo` does not have lifetime `'static'`
...
   |
LL |     let refcell = RefCell::new(&mut foo);
   |                   ---------------------- the type of `refcell` takes the lifetime of type of `foo` here
...
LL |     read_thing(read);
   |                ^^^^ lifetime `'static` required

Problem occurs on both latest stable and latest nightly. (1.56 and 2021-11-04}

[rukai]

I did some investigation and noticed that this issue is at least partially reproduced here
https://github.com/rust-lang/rust/blob/bc6330d47a7a81e25f492a837822121a3cd88158/src/test/ui/issues/issue-46983.stderr
https://github.com/rust-lang/rust/blob/be5fe051a843cfbbc1ba4fcd347b641417181b8f/src/test/ui/issues/issue-46983.rs
I'll investigate further and see if I can put together a PR to improve the situation.

[rukai]

I came across this issue #86759 which is describing the same problem but has what seems to me a better solution

[rukai]

I experimented with skipping try_report_named_anon_conflict when static is required and its giving much better diagnostics, so ill make a PR for that once #90667 is merged.

[estebank]

Current output:

error[E0597]: `foo` does not live long enough
  --> src/main.rs:11:32
   |
 9 | fn inner(mut foo: &[u8]) {
   |          ------- binding `foo` declared here
10 |     //let mut foo: &[u8] = &[0, 1]; // uncommenting this line gives an improved error message
11 |     let refcell = RefCell::new(&mut foo);
   |                                ^^^^^^^^ borrowed value does not live long enough
...
14 |     read_thing(read);
   |     ---------------- argument requires that `foo` is borrowed for `'static`
15 | }
   |  - `foo` dropped here while still borrowed

error[E0521]: borrowed data escapes outside of function
  --> src/main.rs:14:5
   |
 9 | fn inner(mut foo: &[u8]) {
   |          -------  - let's call the lifetime of this reference `'1`
   |          |
   |          `foo` is a reference that is only valid in the function body
...
14 |     read_thing(read);
   |     ^^^^^^^^^^^^^^^^
   |     |
   |     `foo` escapes the function body here
   |     argument requires that `'1` must outlive `'static`
   |
   = note: requirement occurs because of the type `RefCell<(dyn std::io::Read + 'static)>`, which makes the generic argument `(dyn std::io::Read + 'static)` invariant
   = note: the struct `RefCell<T>` is invariant over the parameter `T`
   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance

error[E0597]: `foo` does not live long enough
  --> src/main.rs:11:32
   |
10 |     let mut foo: &[u8] = &[0, 1]; // uncommenting this line gives an improved error message
   |         ------- binding `foo` declared here
11 |     let refcell = RefCell::new(&mut foo);
   |                                ^^^^^^^^ borrowed value does not live long enough
...
14 |     read_thing(read);
   |     ---------------- argument requires that `foo` is borrowed for `'static`
15 | }
   | - `foo` dropped here while still borrowed