Auto merge of #76928 - lcnr:opaque-types-cache, r=tmandry

cache types during normalization

partially fixes #75992

reduces the following test from 14 to 3 seconds locally.

cc `@Mark-Simulacrum` would it make sense to add that test to `perf`?
```rust
#![recursion_limit="2048"]
#![type_length_limit="112457564"]

pub async fn h0(v: &String, x: &u64) { println!("{} {}", v, x) }
pub async fn h1(v: &String, x: &u64) { h0(v, x).await }
pub async fn h2(v: &String, x: &u64) { h1(v, x).await }
pub async fn h3(v: &String, x: &u64) { h2(v, x).await }
pub async fn h4(v: &String, x: &u64) { h3(v, x).await }
pub async fn h5(v: &String, x: &u64) { h4(v, x).await }
pub async fn h6(v: &String, x: &u64) { h5(v, x).await }
pub async fn h7(v: &String, x: &u64) { h6(v, x).await }
pub async fn h8(v: &String, x: &u64) { h7(v, x).await }
pub async fn h9(v: &String, x: &u64) { h8(v, x).await }

pub async fn h10(v: &String, x: &u64) { h9(v, x).await }
pub async fn h11(v: &String, x: &u64) { h10(v, x).await }
pub async fn h12(v: &String, x: &u64) { h11(v, x).await }
pub async fn h13(v: &String, x: &u64) { h12(v, x).await }
pub async fn h14(v: &String, x: &u64) { h13(v, x).await }
pub async fn h15(v: &String, x: &u64) { h14(v, x).await }
pub async fn h16(v: &String, x: &u64) { h15(v, x).await }
pub async fn h17(v: &String, x: &u64) { h16(v, x).await }
pub async fn h18(v: &String, x: &u64) { h17(v, x).await }
pub async fn h19(v: &String, x: &u64) { h18(v, x).await }

macro_rules! async_recursive {
    (29, $inner:expr) => { async { async_recursive!(28, $inner) }.await };
    (28, $inner:expr) => { async { async_recursive!(27, $inner) }.await };
    (27, $inner:expr) => { async { async_recursive!(26, $inner) }.await };
    (26, $inner:expr) => { async { async_recursive!(25, $inner) }.await };
    (25, $inner:expr) => { async { async_recursive!(24, $inner) }.await };
    (24, $inner:expr) => { async { async_recursive!(23, $inner) }.await };
    (23, $inner:expr) => { async { async_recursive!(22, $inner) }.await };
    (22, $inner:expr) => { async { async_recursive!(21, $inner) }.await };
    (21, $inner:expr) => { async { async_recursive!(20, $inner) }.await };
    (20, $inner:expr) => { async { async_recursive!(19, $inner) }.await };

    (19, $inner:expr) => { async { async_recursive!(18, $inner) }.await };
    (18, $inner:expr) => { async { async_recursive!(17, $inner) }.await };
    (17, $inner:expr) => { async { async_recursive!(16, $inner) }.await };
    (16, $inner:expr) => { async { async_recursive!(15, $inner) }.await };
    (15, $inner:expr) => { async { async_recursive!(14, $inner) }.await };
    (14, $inner:expr) => { async { async_recursive!(13, $inner) }.await };
    (13, $inner:expr) => { async { async_recursive!(12, $inner) }.await };
    (12, $inner:expr) => { async { async_recursive!(11, $inner) }.await };
    (11, $inner:expr) => { async { async_recursive!(10, $inner) }.await };
    (10, $inner:expr) => { async { async_recursive!(9, $inner) }.await };

    (9, $inner:expr) => { async { async_recursive!(8, $inner) }.await };
    (8, $inner:expr) => { async { async_recursive!(7, $inner) }.await };
    (7, $inner:expr) => { async { async_recursive!(6, $inner) }.await };
    (6, $inner:expr) => { async { async_recursive!(5, $inner) }.await };
    (5, $inner:expr) => { async { async_recursive!(4, $inner) }.await };
    (4, $inner:expr) => { async { async_recursive!(3, $inner) }.await };
    (3, $inner:expr) => { async { async_recursive!(2, $inner) }.await };
    (2, $inner:expr) => { async { async_recursive!(1, $inner) }.await };
    (1, $inner:expr) => { async { async_recursive!(0, $inner) }.await };
    (0, $inner:expr) => { async { h19(&String::from("owo"), &0).await; $inner }.await };
}

async fn f() {
    async_recursive!(14, println!("hello"));
}

fn main() {
    let _ = f();
}
```
r? `@eddyb` requires a perf run.

compiler/rustc_trait_selection/src/traits/query/normalize.rs

@@ -7,6 +7,7 @@ use crate::infer::canonical::OriginalQueryValues;
 use crate::infer::{InferCtxt, InferOk};
 use crate::traits::error_reporting::InferCtxtExt;
 use crate::traits::{Obligation, ObligationCause, PredicateObligation, Reveal};
+use rustc_data_structures::mini_map::MiniMap;
 use rustc_data_structures::stack::ensure_sufficient_stack;
 use rustc_infer::traits::Normalized;
 use rustc_middle::ty::fold::{TypeFoldable, TypeFolder};
@@ -57,6 +58,7 @@ impl<'cx, 'tcx> AtExt<'tcx> for At<'cx, 'tcx> {
             param_env: self.param_env,
             obligations: vec![],
             error: false,
+            cache: MiniMap::new(),
             anon_depth: 0,
         };
 
@@ -85,6 +87,7 @@ struct QueryNormalizer<'cx, 'tcx> {
     cause: &'cx ObligationCause<'tcx>,
     param_env: ty::ParamEnv<'tcx>,
     obligations: Vec<PredicateObligation<'tcx>>,
+    cache: MiniMap<Ty<'tcx>, Ty<'tcx>>,
     error: bool,
     anon_depth: usize,
 }
@@ -99,8 +102,12 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
             return ty;
         }
 
+        if let Some(ty) = self.cache.get(&ty) {
+            return ty;
+        }
+
         let ty = ty.super_fold_with(self);
-        match *ty.kind() {
+        let res = (|| match *ty.kind() {
             ty::Opaque(def_id, substs) => {
                 // Only normalize `impl Trait` after type-checking, usually in codegen.
                 match self.param_env.reveal() {
@@ -197,7 +204,9 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
             }
 
             _ => ty,
-        }
+        })();
+        self.cache.insert(ty, res);
+        res
     }
 
     fn fold_const(&mut self, constant: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {