Reachable panic in `check_expr_handle_mut_arg`

[Marsman1996]

PoC

use vstd::prelude::*;
verus!{
proof fn p(){
    let _f = |x:int| -> int { return x; };
}
fn main(){}
}

Panic Log

verus/source/vir/src/modes.rs

Line 2896 in 128f4d1

(_, None) => panic!("internal error: missing return type"),

thread 'rustc' (42) panicked at vir/src/modes.rs:2891:30:
internal error: missing return type
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Env

Verus Version: 2026.02.11 c780c07

Playground Link

https://play.verus-lang.org/?version=stable&mode=basic&edition=2021&gist=82beda3c9809ccf0d59539c8b381193b

[Marsman1996]

With #2180, it returns

error: Verus Internal Error: missing return type
 --> ./test/retype.rs:4:31
  |
4 |     let _f = |x:int| -> int { return x; };
  |                               ^^^^^^^^

error: aborting due to 1 previous error