next solver: FN unused variable

#![allow(unconditional_recursion)]
fn fun<T>(x: T) -> impl Sized {
    let x = 0;
    fun(std::env::temp_dir())
}

fn main() {}

old solver:

error[E0720]: cannot resolve opaque type
 --> a.rs:2:20
  |
2 | fn fun<T>(x: T) -> impl Sized {
  |                    ^^^^^^^^^^

warning: unused variable: `x`
 --> a.rs:2:11
  |
2 | fn fun<T>(x: T) -> impl Sized {
  |           ^ help: if this is intentional, prefix it with an underscore: `_x`
  |
  = note: `#[warn(unused_variables)]` (part of `#[warn(unused)]`) on by default

warning: unused variable: `x`
 --> a.rs:3:9
  |
3 |     let x = 0;
  |         ^ help: if this is intentional, prefix it with an underscore: `_x`

error: aborting due to 1 previous error; 2 warnings emitted

For more information about this error, try `rustc --explain E0720`.

next solver:

error[E0282]: type annotations needed
 --> a.rs:2:20
  |
2 | fn fun<T>(x: T) -> impl Sized {
  |                    ^^^^^^^^^^ cannot infer type
  |
  = note: cannot infer type of hidden type of opaque

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0282`.

the next solve eats the warnings about the unused variables somehow.

rustc 1.100.0-nightly (923c95cdf 2026-09-16)
binary: rustc
commit-hash: 923c95cdf5ba65cea505aa2ea829f578e1506ed8
commit-date: 2026-09-16
host: x86_64-unknown-linux-gnu
release: 1.100.0-nightly
LLVM version: 23.1.1

Start by compiling the provided reproducer with the old and next solvers using the reported nightly version, then compare the diagnostics. Trace how the next solver handles the opaque-type inference failure and unused-variable warnings. Done means the inference error remains while both unused-variable warnings are emitted with their usual suggestions.