Obtuse diagnostic: overflow evaluating the requirement `&_ well-formed`

[meithecatte]

Code

fn main() {
    let v = vec![];
    loop {
        if todo!() {
            let x = v.last().unwrap();
            v.push(x); // NOTE: should be v.push(*x);
        } else {
            v.push(0);
        }
    }
}

Current output

error[E0275]: overflow evaluating the requirement `&_ well-formed`
 --> src/main.rs:5:21
  |
5 |             let x = v.last().unwrap();
  |                     ^^^^^^^^

Desired output

error[E0308]: mismatched types
    --> src/lib.rs:6:20
     |
6    |             v.push(x); // NOTE: should be v.push(*x);
     |               ---- ^ expected `{?T}`, found `&{?T}`
     |               |
     |               arguments to this method are incorrect
     |
note: method defined here
    --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:1994:12
     |
1994 |     pub fn push(&mut self, value: T) {
     |            ^^^^
help: consider dereferencing the borrow
     |
6    |             v.push(*x); // NOTE: should be v.push(*x);
     |                    +

Rationale and extra context

The current error message uses jargon that is probably only understandable by compiler developers, type theory enthusiasts, and possibly some genders of Haskell junkies – in an area of the language very easily encountered by newcomers.

Other cases

Swapping the order of the branches yields very good output:

error[E0308]: mismatched types
    --> src/lib.rs:8:20
     |
5    |             v.push(0);
     |             -      - this argument has type `{integer}`...
     |             |
     |             ... which causes `v` to have type `Vec<{integer}>`
...
8    |             v.push(x);
     |               ---- ^ expected integer, found `&{integer}`
     |               |
     |               arguments to this method are incorrect
     |
note: method defined here
    --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/vec/mod.rs:1994:12
     |
1994 |     pub fn push(&mut self, value: T) {
     |            ^^^^

Rust Version

rustc 1.82.0-nightly (92c6c0380 2024-07-21)
binary: rustc
commit-hash: 92c6c03805408a1a261b98013304e9bbf59ee428
commit-date: 2024-07-21
host: x86_64-unknown-linux-gnu
release: 1.82.0-nightly
LLVM version: 18.1.7

Anything else?

No response

[compiler-errors]

This is another fallout from removing sub_relations, cc @lcnr #119989