nll should preserve user types in type ascription

[nikomatsakis]

Spawned off from #47184. The following test case compiles, but it should not (playground):

#![allow(warnings)]
#![feature(nll)]
#![feature(type_ascription)]

fn main() {
    let x = 22_u32;
    let y: &u32 = &x: &'static u32;
}

The problem is that the "type ascription" expression is not preserving the full type that the user gave (&'static u32).

[nikomatsakis]

The first step to fix this is that we have to extend the HAIR to support user annotations. Currently we just remove ExprType ("type ascription") nodes when we create the HAIR:

rust/src/librustc_mir/hair/cx/expr.rs

Line 721 in 79fcc58

hir::ExprKind::Type(ref source, _) => return source.make_mirror(cx),

So that means extending the HAIR ExprKind type:

rust/src/librustc_mir/hair/mod.rs

Line 140 in 79fcc58

pub enum ExprKind<'tcx> {

I imagine we would add a new variant like this:

/// Type ascription
ExprType {
    /// Type that the user gave to this expression
    user_ty: CanonicalTy<'tcx>,
}

We would then lower hir::ExprKind::Type to one of those nodes.

In order to populate it, we'll need to "preserve" the user-given type from the first round of type-checking. This is done via the user_provided_tys field in typeck-tables. An example where we do this is for local variables:

rust/src/librustc_typeck/check/mod.rs

Line 951 in 79fcc58

self.fcx.tables.borrow_mut().user_provided_tys_mut().insert(ty.hir_id, c_ty);

We would want to add similar code the ExprKind::Type case:

rust/src/librustc_typeck/check/mod.rs

Lines 4169 to 4173 in 79fcc58

	hir::ExprKind::Type(ref e, ref t) => {
	let ty = self.to_ty(&t);
	self.check_expr_eq_type(&e, ty);
	ty
	}

Here, we want to add something like

let c_ty = self.fcx.inh.infcx.canonicalize_response(&ty);
self.fcx.tables.borrow_mut().user_provided_tys_mut().insert(t.hir_id, c_ty);

When we lower the ExprKind::Type case, then, we can fetch the canonical type from the user_provided_tys using some code like this:

rust/src/librustc_mir/hair/cx/block.rs

Line 89 in 79fcc58

if let Some(user_ty) = cx.tables.user_provided_tys().get(ty.hir_id) {

Here, the key would be the hir_id of the type.

Then we have to modify the NLL type checker sort of like this code does:

rust/src/librustc_mir/borrow_check/nll/type_check/mod.rs

Lines 931 to 947 in 79fcc58

	if let Some(user_ty) = self.rvalue_user_ty(rv) {
	if let Err(terr) = self.relate_type_and_user_type(
	rv_ty,
	ty::Variance::Invariant,
	user_ty,
	location.boring(),
	) {
	span_mirbug!(
	self,
	stmt,
	"bad user type on rvalue ({:?} = {:?}): {:?}",
	user_ty,
	rv_ty,
	terr
	);
	}
	}

(In fact, I think we can then remove the user_ty field from ExprKind::Adt. We can instead represent it as an Adt wrapped in a Type. But that can happen later.)