Provide suggestion when trying to use method on numeric literal

[estebank]

New output:

error[E0688]: can't call method `powi` on ambiguous numeric type `{float}`
  --> $DIR/method-on-ambiguous-numeric-type.rs:12:17
   |
12 |     let x = 2.0.powi(2);
   |                 ^^^^
help: you must specify a concrete type for this numeric value, like `f32`
   |
12 |     let x = 2.0_f32.powi(2);
   |             ^^^^^^^

Previous output:

error[E0599]: no method named `powi` found for type `{float}` in the current scope
  --> src/main.rs:12:17
   |
12 |     let x = 2.0.powi(2);
   |                 ^^^^
   |
   = help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope, perhaps add a `use` for it:
   |
11 | use core::num::Float;
   |

Fix #40985.

[rust-highfive]

r? @nikomatsakis

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[leonardo-m]

The help message is not good. The number of "as" cast should be minimized in good Rust code. So instead of this:

let x = (2.0 as f32).powi(2);

It's better to suggest something like:

let x = 2.0_f32.powi(2);

[nikomatsakis]

The code seems fine; I agree that 2.0_f32 is probably a better suggestion though.

[estebank]

This error can also be triggered by

fn main() {
    let y = 2.0;
    let x = y.powi(2);
    println!("{:?}", x);
}

Granted, in that case the suggestion let y: f32 = 2.0; would be more appropriate. I'll have to take a look at it.

[estebank]

Both literals and local bindings are now handled correctly.

[estebank]

@bors r=nikomatsakis

[bors]

📌 Commit f7aed3e has been approved by nikomatsakis

[bors]

⌛ Testing commit f7aed3e with merge b950587...

[bors]

💔 Test failed - status-appveyor

[kennytm]

@bors retry #46903

[bors]

⌛ Testing commit f7aed3e with merge 9c663205a2b984f635c577d17a5729c9e1754843...

[bors]

💔 Test failed - status-appveyor

[estebank]

@bors retry

[bors]

⌛ Testing commit f7aed3e with merge a29461f...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: nikomatsakis
Pushing a29461f to master...