Assign memo ingredients per salsa-struct-ingredient, not globally

[nikomatsakis]

Today every tracked function gets assigned a distinct MemoIngredientIndex. These are used to index into the memo table attached to a salsa struct. But every function can be attached to salsa structs of exactly one kind, so indices only have to be distinct per salsa struct, not globally. This would save a lot of space in larger projects.

[nikomatsakis]

Mentoring instructions

Currently 'memo ingredient indices' are a vector; the values in the vector are the fn ingredient indices that would be storing the memo (this should be documented!).

salsa/src/zalsa.rs

Lines 122 to 123 in 710691d

	/// Number of memo ingredient indices created by calls to [`next_memo_ingredient_index`](`Self::next_memo_ingredient_index`)
	memo_ingredients: Mutex<Vec<IngredientIndex>>,

This should be changed into a map of vectors, indexed by the "salsa struct ingredient index" (the ingredient index on which the memo is stored):

/// Map from the [`IngredientIndex`][] of a salsa struct to a list of
/// [ingredient-indices](`IngredientIndex`)for tracked functions that have this salsa struct as input.
memo_ingredient_indices: RwLock<Map<IngredientIndex, Vec<IngredientIndex>>>

then we modify next_memo_ingredient_index to take the tracked struct ingredient index as an argument

salsa/src/zalsa.rs

Lines 300 to 305 in 710691d

	fn next_memo_ingredient_index(&self, ingredient_index: IngredientIndex) -> MemoIngredientIndex {
	let mut memo_ingredients = self.memo_ingredients.lock();
	let mi = MemoIngredientIndex(u32::try_from(memo_ingredients.len()).unwrap());
	memo_ingredients.push(ingredient_index);
	mi
	}

This is called my the various code in salsa-macro-rules, so that will have to be modified to pass in the ingredient index for the salsa struct on which the function is invoked.

We also modify ingredient_index_for_memo similarly

salsa/src/zalsa.rs

Lines 291 to 296 in 710691d

	pub(crate) fn ingredient_index_for_memo(
	&self,
	memo_ingredient_index: MemoIngredientIndex,
	) -> IngredientIndex {
	self.memo_ingredients.lock()[memo_ingredient_index.as_usize()]
	}

and change the one caller of that last method:

https://github.com/salsa-rs/salsa/blob/master/src/tracked_struct.rs#L504

to pass in the self.ingredient_index.

[MichaReiser]

@ibraheemdev you expressed interest in this task. Let me know if you start working on it so that I can assign it to you.

[davidbarsky]

We had some interest/demand from rust-analyzer contributors to contribute to Salsa and @ShoyuVanilla expressed interested in this work. I also believe that he has bandwidth to work on this sooner than @ibraheemdev and memory usage is a concern for rust-analyzer, so for those reasons, I'd like to assign him to this issue.