boolean to integer conversion multiple passes and error

[KonradHoeffner]

Description

Not sure if this is a bug in one of the lints or in their interplay or normal behavior but I found it strange that it requires multiple runs of cargo clippy --fix, does not perform it's own suggestion and then outputs an error.

Applying cargo clippy on an unoptimized bool to integer conversion version 1...

#![warn(clippy::pedantic)] 
fn main() {
    let _ = |b| match b { true => 1, false => 0};
}

Warns about:

warning: `match` on a boolean expression
 --> src/main.rs:3:17
  |
3 |     let _ = |b| match b { true => 1, false => 0};
  |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using an `if`/`else` expression: `if b { 1 } else { 0 }`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#match_bool

Applying cargo clippy --fixleads to this (only showing the relevant line from now on):

let _ = |b| if b { 1 } else { 0 };

However this still causes warnings with cargo clippy:

warning: boolean to int conversion using if
 --> src/main.rs:3:17
  |
3 |     let _ = |b| if b { 1 } else { 0 };
  |                 ^^^^^^^^^^^^^^^^^^^^^ help: replace with from: `i32::from(b)`
  |
  = note: `b as i32` or `b.into()` can also be valid options
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#bool_to_int_with_i

So lets cargo clippy --fix again, but somehow it does not apply it's own suggestion to use i32::from(b), instead we get an error:

error[E0283]: type annotations needed
 --> src/main.rs:3:14
  |
3 |     let _ = |b| i32::from(b);
  |              ^  --- type must be known at this point
  |
  = note: multiple `impl`s satisfying `i32: std::convert::From<_>` found in the `core` crate:
          - impl std::convert::From<bool> for i32;
          - impl std::convert::From<i16> for i32;
          - impl std::convert::From<i8> for i32;
          - impl std::convert::From<u16> for i32;
          - impl std::convert::From<u8> for i32;
help: consider giving this closure parameter an explicit type
  |
3 |     let _ = |b: /* Type */| i32::from(b);
  |               ++++++++++++

error: aborting due to 1 previous error

OK so we manually change it to let _ : fn(bool) -> u8 = |b| if b { 1 } else { 0 }; and cargo clippy --fix again and finally arrive at a stable solution where clippy does not give any warnings anymore:

let _ : fn(bool) -> u8 = |b| u8::from(b);

Version

rustc 1.90.0-nightly (a84ab0ce6 2025-07-06)
binary: rustc
commit-hash: a84ab0ce6c4557a2f01a3a6c3fdb0f92098db78d
commit-date: 2025-07-06
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.7

Additional Labels

No response

[y21]

IMO it isn't inherently a bug if the suggestion of one lint is further warned on by another lint to incrementally improve code, so long as there isn't a contradiction. rustfix also normally handles this fine IIRC and repeatedly runs cargo clippy and applies suggestions, unless it runs into an error like what happened here I guess.

So this seems like it's solely about the bool_to_int_with_if lint, if it suggests changing if b { 1 } else { 0 } to i32::from(b) if the only source of type information for b comes from the use in the if condition. Seems hard to fix in the general case though.

Did this come up in real code? The closure here is unused, but in normal code I'd expect that closures have their argument types unified in another place, like when called

[KonradHoeffner]

The example here is simplified, the real code was:

 self.dict.bit_vector().iter().take(500).map(|b| match b { true => 1, false => 0,})