`seq!` for tuples errors when capturing locally-defined parsers.

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rust version

rustc 1.77.0 (aedd173a2 2024-03-17)

winnow version

0.6.5

Minimal reproducible code

use winnow::combinator::seq;
use winnow::prelude::*;

pub fn doesnt_work(input: &mut &str) -> PResult<(i32, i32)> {
    let mut foo = "foo".map(|_| 10);
    let mut bar = "bar".map(|_| 20);
    seq!(foo, bar).parse_next(input)
}

Steps to reproduce the bug with the above code

cargo check

Actual Behaviour

error[E0525]: expected a closure that implements the `FnMut` trait, but this closure only implements `FnOnce`
  --> good-crates/parser/examples/mcve.rs:18:5
   |
18 |     seq!(foo, bar).parse_next(input)
   |     ^^^^^---^^^^^^
   |     |    |
   |     |    closure is `FnOnce` because it moves the variable `foo` out of its environment
   |     this closure implements `FnOnce`, not `FnMut`
   |     the requirement to implement `FnMut` derives from here
   |     required by a bound introduced by this call
   |
   = note: required for `{closure@/home/alona/.cargo/registry/src/index.crates.io-6f17d22bba15001f/winnow-0.6.5/src/macros/seq.rs:88:44: 88:64}` to implement `winnow::Parser<&str, (i32, i32), ContextError>`
note: required by a bound in `trace`
  --> /home/alona/.cargo/registry/src/index.crates.io-6f17d22bba15001f/winnow-0.6.5/src/combinator/debug/mod.rs:45:18
   |
43 | pub fn trace<I: Stream, O, E>(
   |        ----- required by a bound in this function
44 |     name: impl crate::lib::std::fmt::Display,
45 |     parser: impl Parser<I, O, E>,
   |                  ^^^^^^^^^^^^^^^ required by this bound in `trace`
   = note: this error originates in the macro `$crate::seq` which comes from the expansion of the macro `seq` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0525`.

Expected Behaviour

This should compile.

If seq! is used to make a struct (instead of a tuple) it does compile.

pub struct Res {
    pub a: i32,
    pub b: i32,
}

pub fn does_work(input: &mut &str) -> PResult<Res> {
    let mut foo = "foo".map(|_| 10);
    let mut bar = "bar".map(|_| 20);
    seq! { Res{ a: foo, b: bar } }.parse_next(input)
}

works fine.

Additional Context

Manually expanding the seq! macro (with RA) for the broken example gives:

pub fn doesnt_work(input: &mut &str) -> PResult<(i32, i32)> {
    let mut foo = "foo".map(|_| 10);
    let mut bar = "bar".map(|_| 20);
    {
        winnow::combinator::trace("tuple", move |input: &mut _| {
            use winnow::Parser;
            (foo, bar).map(|t| ((t.0), (t.1))).parse_next(input)
        })
    }
    .parse_next(input)
}

This gives a more descriptive error:


error[E0525]: expected a closure that implements the `FnMut` trait, but this closure only implements `FnOnce`
  --> good-crates/parser/examples/mcve.rs:19:44
   |
19 |           winnow::combinator::trace("tuple", move |input: &mut _| {
   |           -------------------------          -^^^^^^^^^^^^^^^^^^^
   |           |                                  |
   |  _________|__________________________________this closure implements `FnOnce`, not `FnMut`
   | |         |
   | |         required by a bound introduced by this call
20 | |             use winnow::Parser;
21 | |             (foo, bar).map(|t| ((t.0), (t.1))).parse_next(input)
   | |              --- closure is `FnOnce` because it moves the variable `foo` out of its environment
22 | |         })
   | |_________- the requirement to implement `FnMut` derives from here
   |
   = note: required for `{closure@good-crates/parser/examples/mcve.rs:19:44: 19:64}` to implement `winnow::Parser<&str, (i32, i32), ContextError>`
note: required by a bound in `trace`
  --> /home/alona/.cargo/registry/src/index.crates.io-6f17d22bba15001f/winnow-0.6.5/src/combinator/debug/mod.rs:45:18
   |
43 | pub fn trace<I: Stream, O, E>(
   |        ----- required by a bound in this function
44 |     name: impl crate::lib::std::fmt::Display,
45 |     parser: impl Parser<I, O, E>,
   |                  ^^^^^^^^^^^^^^^ required by this bound in `trace`

For more information about this error, try `rustc --explain E0525`.

Manually expanding the example that does work gives:

pub fn does_work(input: &mut &str) -> PResult<Res> {
    let mut foo = "foo".map(|_| 10);
    let mut bar = "bar".map(|_| 20);
    {
        winnow::combinator::trace("Res", move |input: &mut _| {
            use winnow::Parser;
            let a = foo.parse_next(input)?;
            let b = bar.parse_next(input)?;
            #[allow(clippy::redundant_field_names)]
            Ok(Res { a: a, b: b })
        })
    }
    .parse_next(input)
}

I think the reason this works is that foo and bar don't need to be captured by value here, as .parse_next takes &mut self.

Adding .by_ref() to the tuple example version may fix it.

winnow-rs / winnow