Bind unused parameter assistant
This PR introduces a new **Bind unused parameter assistant**.
While we do have a QuickFix from `rustc` (prefixing the parameter with an underscore), it's sometimes more convenient to suppress the warning using the following approach:
```rust
fn some_function(unused: i32) {}
```
->
```rust
fn some_function(unused: i32) {
let _ = unused;
}
```
minor : Deunwrap generate_derive
#15398 subtask 1. Since the editing closure has arms, I did something *experimental* ( in this case just a clever term for bad code ) to bypass creating an `Option` but I am ready to change this.
the "add missing members" assists: implemented substitution of default values of const params
To achieve this, I've made `hir::ConstParamData` store the default values
internal : rewrite DeMorgan assist
fixes#15239 , #15240 . This PR is a rewrite of the DeMorgan assist that essentially rids of all the string manipulation and modifies syntax trees to apply demorgan on a binary expr. The main reason for the rewrite is that I wanted to use `Expr::needs_parens_in` method to see if the expr on which the assist is applied would still need the parens it had once the parent expression's operator had equal precedence with that of the expression. I used `.clone_(subtree|for_update)` left and right and probably more than I should have, so I would also be happy to hear how I could have prevented redundant cloning.
internal: Turn unresolved proc macro expansions into missing expressions
Reduces the amount of type related errors one gets when proc macro expansion is disabled.
Add ExternCrateDecl to HIR
Adding these doesn't really require much design effort as they represent a single import, unlike use trees which are one item that represent 0 or more imports.
We only resolve to this definition when actually resolving on the name or alias of an `extern crate name as alias` item, not usages yet as that requires far more changes that won't lead anywhere without giving it more thought. Nevertheless the changes slightly improve IDE things, an example being hover on the decl showing the merged doc comments for example.
cc https://github.com/rust-lang/rust-analyzer/issues/14079
Added remove unused imports assist
This resolves the most important part of #5131. I needed to make a couple of cosmetic changes to the search infrastructure to do this.
A few open questions:
* Should imports that don't resolve to anything be considered unused? I figured probably not, but it would be a trivial change to make if we want it.
* Is there a cleaner way to make the edits to the use list?
* Is there a cleaner way to get the list of uses that intersect the current selection?
* Is the performance acceptable? When testing this on itself, it takes a good couple seconds to perform the assist.
* Is there a way to hide the rustc diagnostics that overlap with this functionality?
