diagnostics: avoid ICE in confusable_method_name for associated functions

[Unique-Usman]

Avoid unconditionally slicing inputs()[1..], which assumes a self parameter. Use is_method() to conditionally skip the receiver, preventing out-of-bounds access and fixing suggestions for associated functions.

[rustbot]

r? @fmease

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[Unique-Usman]

Initially this will ICE

#![feature(rustc_attrs)]

struct S;

impl S {
    #[rustc_confusables("bar")]
    fn foo() {}
}

fn main() {
    S::bar();
}

but, with this pr, it does not

warning: the feature `rustc_attrs` is internal to the compiler or standard library
 --> foo.rs:1:12
  |
1 | #![feature(rustc_attrs)]
  |            ^^^^^^^^^^^
  |
  = note: using it is strongly discouraged
  = note: `#[warn(internal_features)]` on by default

error[E0599]: no function or associated item named `bar` found for struct `S` in the current scope
  --> foo.rs:11:8
   |
 3 | struct S;
   | -------- function or associated item `bar` not found for this struct
...
11 |     S::bar();
   |        ^^^ function or associated item not found in `S`
   |
help: you might have meant to use `foo`
   |
11 -     S::bar();
11 +     S::foo();
   |

[Kivooeo]

r? me @bors r+ rollup

[rust-bors]

📌 Commit fa3e85a has been approved by Kivooeo

It is now in the queue for this repository.