7617: Add getter/setter assists r=Veykril a=yoshuawuyts
This patch makes progress towards the design outlined in https://github.com/rust-analyzer/rust-analyzer/issues/5943, and includes a small refactor which closes https://github.com/rust-analyzer/rust-analyzer/issues/7607. All together this patch does 4 things:
- Adds a `generate_getter` assist.
- Adds a `generate_getter_mut` assist.
- Adds a `generate_setter` assist.
- Moves the `generate_impl_text` function from `generate_new` into `utils` (which closes#7607).
## Design Notes
I've chosen to follow the [Rust API guidelines on getters](https://rust-lang.github.io/api-guidelines/naming.html#getter-names-follow-rust-convention-c-getter) as closely as possible. This deliberately leaves "builder pattern"-style setters out of scope.
Also, similar to https://github.com/rust-analyzer/rust-analyzer/pull/7570 this assist generates doc comments. I think this should work well in most cases, and for the few where it doesn't it's probably easily edited. This makes it slightly less correct than the #7570 implementation, but I think this is still useful enough to include for many of the same reasons.
The reason why this PR contains 3 assists, rather than 1, is because each of them is so similar to the others that it felt more noisy to do them separately than all at once. The amount of code added does not necessarily reflect that, but hope that still makes sense.
## Examples
**Input**
```rust
struct Person {
name: String, // <- cursor on "name"
}
```
**generate getter**
```rust
struct Person {
name: String,
}
impl Person {
/// Get a reference to the person's name.
fn name(&self) -> &String {
&self.name
}
}
```
**generate mut getter**
```rust
struct Person {
name: String,
}
impl Person {
/// Get a mutable reference to the person's name.
fn name_mut(&mut self) -> &mut String {
&mut self.name
}
}
```
**generate setter**
```rust
struct Person {
name: String,
}
impl Person {
/// Set the person's name.
fn set_name(&mut self, name: String) {
self.name = name;
}
}
```
Co-authored-by: Yoshua Wuyts <yoshuawuyts+github@gmail.com>
Assist vs UnresolvedAssist split doesn't really pull its weight. This
is especially bad if we want to include `Assist` as a field of
diagnostics, where we'd have to make the thing generic.
