Compiler hang: infinite type expansion for trait impl with recursive double-reference bound (`&'a &'a T`)

[elimayano]

I encountered a compiler hang when compiling a code snippet with a recursive trait implementation involving double references.

Instead of hitting the recursion_limit or reporting a conflicting implementation error, the compiler continues to consume memory. Maybe related to #151632

trait UseRef {}
impl<T> UseRef for T {}
impl<'a, T> UseRef for & T
where
    T: UseRef,
    &'a &'a T: UseRef,
{}
fn main(){}

Meta

rustc --version --verbose:

rustc 1.93.0 (254b59607 2026-01-19)
binary: rustc
commit-hash: 254b59607d4417e9dffbc307138ae5c86280fe4c
commit-date: 2026-01-19
host: x86_64-unknown-linux-gnu
release: 1.93.0
LLVM version: 21.1.8

Backtrace

<backtrace>

[cyrgani]

This also hangs with -Znext-solver=globally, but not with -Znext-solver=no.