Proposal: "late" fields on tracked structs

[nikomatsakis]

TL;DR

Permit fields to be tagged as "late"

#[salsa::tracked]
struct Variable {
    name: Id<'db>,
    #[late] ty: Ty<'db>,
}

When you create a tracked struct, you specify only the early fields:

let v = Variable::new(name);

but you can later specify the late fields, so long as it is within the same tracked function (query):

v.set_ty(db, ty);

Panics occur if:

• there is an attempt to access v.ty(db) but it has not yet been set
• you reach the end of the tracked function without setting ty

Background

Salsa currently supports a rather obscure feature called "specified" functions that lets you emulate this:

#[salsa::tracked(specify)]
fn ty<'db>(db: &'db dyn Database, v: Variable<'db>) -> Ty<'db> {
    panic!("ty not specified");
}

This lets you specify the value of ty for some v so long as v was created in the current query (and ty has not yet been called on s):

ty::specify(db, v, value)

Supporting the late pattern directly would

• let us remove specifiable functions, simplifying salsa in my opinion;
• be mildly more efficient, since we could create space in the fields struct for the field instead of storing the value in the memo table
• let us check at the end of the query that all late fields were assigned