Writing `float::pow(int)` causes rustc to suggest using `powf`, not `powi`

[ash2x3zb9cy]

While trying to raise a float to an integer power, if the user tries to write float.pow(int), rustc suggests using powf, which then results in a type error. rustc then suggests casting the int to a float with as. The correct fix would instead be to use powi.

Given the following code: [playground]

fn main() {
    let x: f32 = 10.0;
    let p: i32 = 3;
    let x_cubed = x.pow(p);
    println!("{} cubed is {}", x, x_cubed);
}

The current output is:

error[[E0599]](https://doc.rust-lang.org/stable/error-index.html#E0599): no method named `pow` found for type `f32` in the current scope
 --> src/main.rs:4:21
  |
4 |     let x_cubed = x.pow(p);
  |                     ^^^ help: there is an associated function with a similar name: `powf`

For more information about this error, try `rustc --explain E0599`.

When using the suggested fix, the output is:

error[[E0308]](https://doc.rust-lang.org/stable/error-index.html#E0308): mismatched types
 --> src/main.rs:4:26
  |
4 |     let x_cubed = x.powf(p);
  |                     ---- ^ expected `f32`, found `i32`
  |                     |
  |                     arguments to this function are incorrect
  |
note: associated function defined here
help: you can convert an `i32` to an `f32`, producing the floating point representation of the integer, rounded if necessary
  |
4 |     let x_cubed = x.powf(p as f32);
  |                            ++++++

For more information about this error, try `rustc --explain E0308`.

Ideally the output should look like:

error[[E0599]](https://doc.rust-lang.org/stable/error-index.html#E0599): no method named `pow` found for type `f32` in the current scope
 --> src/main.rs:4:21
  |
4 |     let x_cubed = x.pow(p);
  |                     ^^^ help: there is an associated function with a similar name: `powi`

For more information about this error, try `rustc --explain E0599`.

The ideal output provides a working fix, and is likely to be more correct to the user's intended function.

[chenyukang]

Currently, the Levenshtein based suggestions do not checking the type matching of arguments for candidates,
and only will give one final suggestion.

  fn candidate_method_names(&self) -> Vec<Ident> {
        let mut set = FxHashSet::default();
        let mut names: Vec<_> = self
            .inherent_candidates
            .iter()
            .chain(&self.extension_candidates)
            .filter(|candidate| {
                if let Some(return_ty) = self.return_type {
                    self.matches_return_type(&candidate.item, None, return_ty)
                } else {
                    true
                }
            })
            .map(|candidate| candidate.item.ident(self.tcx))
            .filter(|&name| set.insert(name))
            .collect();

        // Sort them by the name so we have a stable result.
        names.sort_by(|a, b| a.as_str().partial_cmp(b.as_str()).unwrap());
        names
    }