Type inference failure for `f32`

[P1n3appl3]

After this rollup we started seeing the following compile failure:

fn main() {
    let x = f32::from(3.14);
}

error[E0277]: the trait bound `f32: From<f64>` is not satisfied
  --> /tmp/repro.rs:2:13
   |
16 |     let x = f32::from(3.14);
   |             ^^^ the trait `From<f64>` is not implemented for `f32`
   |
   = help: the following other types implement trait `From<T>`:
             <f32 as From<bool>>
             <f32 as From<f16>>
             <f32 as From<i16>>
             <f32 as From<i8>>
             <f32 as From<u16>>
             <f32 as From<u8>>

error: aborting due to 1 previous error

I repro'd this locally on aa6a697 but we have CI that runs on every commit so we're fairly sure that it started with 4435924. Of the changes in that rollup #122470 is the only one that seemed to touch float stuff, but I'm not sure how adding some extra impls for f16 and f128 would break inference in this case.

[jieyouxu]

cc @tgross35: is it possible that adding the the From impls for f16/f128 may have caused type inference regression here?

[tgross35]

It must be, but I don't know why since https://github.com/rust-lang/rust/pull/122470/files#diff-154dff23a2c2f4fb97915229fd9c27d73293940250088020bfd2074094186ce4R171 wasn't touched. Also reproduced locally

[Nemo157]

When there is only one applicable impl it can guide inference, as soon as there are multiple it can't use those and instead goes to default-numeric-fallback which appears to be f64 for floating point.

[workingjubilee]

Correct.

[tgross35]

Just coming to the same conclusion - the literal is getting guided to f32 on stable since that's the only type that fits the from, but when there are more options it gets the default float literal type of f64

fn main() {
    let a = 3.14;
    dbg!(std::any::type_name_of_val(&a)); // prints f32 on stable
    let x = f32::from(a);
}

[tgross35]

I'll put up a PR to remove these impls for now but what is the correct longer term fix?

cc @rust-lang/libs-api