Indentation for `fn` body misaligned when `(` is on next line

The fn body here indents as shown:

pub fn build_lane_table_states<'grammar>
    (grammar: &'grammar Grammar,
     start: NonterminalString)
     -> Result<Vec<LR1State<'grammar>>, LR1TableConstructionError<'grammar>> {
        let (lr0_states, lr0_conflicts) = match build::build_lr0_states(grammar, start) {
            Ok(s) => (s, vec![]),
            Err(e) => (e.states, e.conflicts),
        };

        unimplemented!()
    }

But I would expect:

pub fn build_lane_table_states<'grammar>
    (grammar: &'grammar Grammar,
     start: NonterminalString)
     -> Result<Vec<LR1State<'grammar>>, LR1TableConstructionError<'grammar>> {
    let (lr0_states, lr0_conflicts) = match build::build_lr0_states(grammar, start) {
        Ok(s) => (s, vec![]),
        Err(e) => (e.states, e.conflicts),
    };

    unimplemented!()
}

The trigger is definitely having the open paren on the next line. Admittedly, I wouldn't ordinarily write a fn this way, but it is produced by rustfmt in some cases.

Reproduce the reported indentation with the Rust snippet in rust-mode, focusing on a function whose opening parenthesis is on the next line. No implementation file or test is named; trace the indentation entry point and add coverage showing the body aligns with the expected example.