Display correct unused field suggestion for nested struct patterns

[varkor]

Extends #47922 by checking more sophisticated patterns (e.g. references, slices, etc.).
Before:

warning: unused variable: `bar`
  --> src/main.rs:37:21
   |
37 |         &Foo::Bar { bar } => true,
   |                     ^^^ help: consider using `_bar` instead
   |
   = note: #[warn(unused_variables)] on by default

After:

warning: unused variable: `bar`
  --> src/main.rs:37:21
   |
37 |         &Foo::Bar { bar } => true,
   |                     ^^^ help: try ignoring the field: `bar: _`
   |
   = note: #[warn(unused_variables)] on by default

Fixes #50303.

r? @estebank

[estebank]

@bors r+ rollup

[bors]

📌 Commit 2eb8343 has been approved by estebank

[b-jonas0]

The previous suggestion might have been incorrect, but I don't like the new suggestion either, because ignoring the field can change semantics. Consider the following snippet.

fn main() {
    struct K;
    impl Drop for K { fn drop(&mut self) { println!("K dropped"); } }
    struct M {k: K}
    let m = Some(M{k: K});
    if let Some(M{k}) = m { }
    println!("hello");
}

The output is:

K dropped
hello

The warning I get is:

warning: unused variable: `k`
 --> src/main.rs:7:19
  |
7 |     if let Some(M{k}) = m { }
  |                   ^ help: try ignoring the field: `k: _`
  |
  = note: #[warn(unused_variables)] on by default

If I follow that suggestion, then the output changes to:

hello
K dropped

A correct suggestion would be something like:

warning: unused variable: `k`
 --> src/main.rs:7:19
  |
7 |     if let Some(M{k}) = m { }
  |                   ^ help: consider using `k: _k` instead
  |
  = note: #[warn(unused_variables)] on by default