redundant_closure must not expand macro in suggestion

[matthiaskrgr]

Lint name: redundant_closure

warning: redundant closure found
   --> src/iter/test.rs:121:13
    |
121 |             || vec![],
    |             ^^^^^^^^^ help: remove closure as shown: `$crate::vec::Vec::new`
    |
    = note: `#[warn(clippy::redundant_closure)]` on by default
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure

This is obviously not going to work with $crate::vec...

The suggestion should just say Vec::new()

How to fix: find out where the lint generates the snippet and use snippet_with_macro_callsite instead of just snippet.

( this type of bug popped up a couple of times already #6801 (comment)
maybe we should lint for that?)

[mgacek8]

If we go here: https://github.com/rust-lang/rust-clippy/blob/master/clippy_lints/src/eta_reduction.rs#L111 and change that snipped_opt to snippet_with_macro_callsite, then clippy would suggest the following:

+error: redundant closure found
+  --> $DIR/eta.rs:207:42
+   |
+LL |     let _ : Option<Vec<i32>> = true.then(||vec![]);
+   |                                          ^^^^^^^^ help: remove closure as shown: `vec![]`

which causes the compiler error.

We can strip the $crate::vec prefix, but it will work only for the vec![] macro.
This issue is similar: #6732 but with the quote!() macro
Maybe there is no sense to trigger redundant closure lint for macros ?

[camsteffen]

In the general case, || macro!() should not fire redundant_closure since the suggestion will force the user to inline the macro and expose its implementation. But vec![] -> Vec::new is a special case. So that special case can use a hard-coded suggestion or (better) obtain the minimal path to Vec depending on whether it is imported.