Conditional borrow with RPIT

Code

fn foo<'a, 'b>(one: &'a (), two: &'b ()) -> impl Sized {
    if true { one } else { two }
}

Output

    Checking zk v0.1.0 (/private/var/folders/9d/tqvdlqrd6fz7t8746q0jdjzr0000gp/T/tmp.zk1OwsfTpg/zk)
error[E0700]: hidden type for `impl Sized` captures lifetime that does not appear in bounds
   ╭▸ src/lib.rs:25:28
   │
24 │ fn foo<'a, 'b>(one: &'a (), two: &'b ()) -> impl Sized {
   │                                             ────────── opaque type defined here
25 │     if true { one } else { two }
   │                            ━━━
   │
   ╰ note: hidden type `&()` captures lifetime `'_`

For more information about this error, try `rustc --explain E0700`.
error: could not compile `zk` (lib) due to 1 previous error

Desired output

I'm not sure what I want the compiler to tell me here because I'm still a little confused as to why this doesn't compile. Currently the output confusingly mentions '_ which appears nowhere in our function signature. If '_ refers to a lifetime that comes from the body of the function, that we cannot name, I'd expect it to be called something like '0 in the diagnostic.

These two snippets compile:

fn foo<'a, 'b>(one: &'a (), two: &'b ()) -> impl Sized {
    one
}

and

fn foo<'a, 'b>(one: &'a (), two: &'b ()) -> impl Sized {
    two
}

Under edition 2024 capture rules, I expect the return type to implicitly be impl Sized + use<'a, 'b>. Since we can see both one and two satisfy that type in the two snippets above, I'd expect variance to handle the original case where I return conditionally.

Rust version

rustc 1.95.0-nightly (3c9faa0d0 2026-02-16)
binary: rustc
commit-hash: 3c9faa0d037b9eecda4a440cc482ff7f960fb8a5
commit-date: 2026-02-16
host: aarch64-apple-darwin
release: 1.95.0-nightly
LLVM version: 22.1.0

@rustbot label +A-diagnostics +T-compiler +A-impl-trait