Extract into function should preserve existing parameter order

[emilk]

Problem

I often have a list of similar "context" parameters to my functions, and I try to be consistent with their order. For example:

fn foo_ui(ctx: &Context, ui: &mut Ui, foo: &Foo) {
    …
}

fn bar_ui(ctx: &Context, ui: &mut Ui, bar: &Bar, option: Option) {
    …
}

When calling "Extract to function" on some code, Rust analyzer ignores this parameter order and I often end up with functions where I need to manually rearrange the parameter order to my liking.

Suggested solution

When "Extract to function" decides the parameter order of the new function, it should use the following algorithm:

A) Categorize the arguments based on wether or not they were parameters to the already existing function
B) If yes, put them first, and in the same order as in the existing function
C) if no, put them last, in whatever order (I have no strong opinion here - can use same heuristic as the current code)

Effect

fn existing(a: A, b: B, c: C) {
    let x = 32;
    let y = 1.0;

    long_code_block_that_uses(x,b,a,y) // <--- call "Extract to function" on this code block
}

Results in ->

fn existing(a: A, b: B, c: C) {
    let x = 32;
    let y = 1.0;
    fun(a, b, x, y);
}

fn fun(a: A, b: B, x: i32, y: f32) {
    long_code_block_that_uses(x,b,a,y)
}

[Veykril]

C) if no, put them last, in whatever order (I have no strong opinion here - can use same heuristic as the current code)

The current emitted ordering is dependent on appearance order in the extracted snippet, so we probably want to use that for C) here as well. Generally agree though, the assist should preserve parent function parameter ordering where possible!

[roife]

Since we would lower params in order, can we sort locals by binding_id here?

rust-analyzer/crates/hir/src/lib.rs

Lines 3622 to 3626 in 99a6ecd

	#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
	pub struct Local {
	pub(crate) parent: DefWithBodyId,
	pub(crate) binding_id: BindingId,
	}

[Veykril]

We could probably do that