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.

7 files changed, 83 insertions, 68 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 9dc6ef4608a..68288916e6d 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3467,6 +3467,7 @@ dependencies = [
 name = "rustc_data_structures"
 version = "0.0.0"
 dependencies = [
+ "arrayvec",
  "bitflags",
  "cfg-if",
  "crossbeam-utils 0.7.2",
diff --git a/compiler/rustc_data_structures/Cargo.toml b/compiler/rustc_data_structures/Cargo.toml
index bf75fc96f1f..caaf7c0c3c2 100644
--- a/compiler/rustc_data_structures/Cargo.toml
+++ b/compiler/rustc_data_structures/Cargo.toml
@@ -8,6 +8,7 @@ edition = "2018"
 doctest = false
 
 [dependencies]
+arrayvec = { version = "0.5.1", default-features = false }
 ena = "0.14"
 indexmap = "1.5.1"
 tracing = "0.1"
diff --git a/compiler/rustc_data_structures/src/lib.rs b/compiler/rustc_data_structures/src/lib.rs
index 8dafcdf3bc6..1f977805f5e 100644
--- a/compiler/rustc_data_structures/src/lib.rs
+++ b/compiler/rustc_data_structures/src/lib.rs
@@ -88,25 +88,27 @@ pub mod sorted_map;
 pub mod stable_set;
 #[macro_use]
 pub mod stable_hasher;
+mod atomic_ref;
+pub mod fingerprint;
+pub mod profiling;
 pub mod sharded;
 pub mod stack;
 pub mod sync;
 pub mod thin_vec;
 pub mod tiny_list;
 pub mod transitive_relation;
-pub use ena::undo_log;
-pub use ena::unify;
-mod atomic_ref;
-pub mod fingerprint;
-pub mod profiling;
 pub mod vec_linked_list;
 pub mod work_queue;
 pub use atomic_ref::AtomicRef;
 pub mod frozen;
+pub mod mini_map;
 pub mod tagged_ptr;
 pub mod temp_dir;
 pub mod unhash;
 
+pub use ena::undo_log;
+pub use ena::unify;
+
 pub struct OnDrop<F: Fn()>(pub F);
 
 impl<F: Fn()> OnDrop<F> {
diff --git a/compiler/rustc_data_structures/src/mini_map.rs b/compiler/rustc_data_structures/src/mini_map.rs
new file mode 100644
index 00000000000..cd3e949d383
--- /dev/null
+++ b/compiler/rustc_data_structures/src/mini_map.rs
@@ -0,0 +1,61 @@
+use crate::fx::FxHashMap;
+use arrayvec::ArrayVec;
+
+use std::hash::Hash;
+
+/// Small-storage-optimized implementation of a map
+/// made specifically for caching results.
+///
+/// Stores elements in a small array up to a certain length
+/// and switches to `HashMap` when that length is exceeded.
+pub enum MiniMap<K, V> {
+    Array(ArrayVec<[(K, V); 8]>),
+    Map(FxHashMap<K, V>),
+}
+
+impl<K: Eq + Hash, V> MiniMap<K, V> {
+    /// Creates an empty `MiniMap`.
+    pub fn new() -> Self {
+        MiniMap::Array(ArrayVec::new())
+    }
+
+    /// Inserts or updates value in the map.
+    pub fn insert(&mut self, key: K, value: V) {
+        match self {
+            MiniMap::Array(array) => {
+                for pair in array.iter_mut() {
+                    if pair.0 == key {
+                        pair.1 = value;
+                        return;
+                    }
+                }
+                if let Err(error) = array.try_push((key, value)) {
+                    let mut map: FxHashMap<K, V> = array.drain(..).collect();
+                    let (key, value) = error.element();
+                    map.insert(key, value);
+                    *self = MiniMap::Map(map);
+                }
+            }
+            MiniMap::Map(map) => {
+                map.insert(key, value);
+            }
+        }
+    }
+
+    /// Return value by key if any.
+    pub fn get(&self, key: &K) -> Option<&V> {
+        match self {
+            MiniMap::Array(array) => {
+                for pair in array {
+                    if pair.0 == *key {
+                        return Some(&pair.1);
+                    }
+                }
+                return None;
+            }
+            MiniMap::Map(map) => {
+                return map.get(key);
+            }
+        }
+    }
+}
diff --git a/compiler/rustc_index/Cargo.toml b/compiler/rustc_index/Cargo.toml
index 6ac7c06ee83..6e1471df195 100644
--- a/compiler/rustc_index/Cargo.toml
+++ b/compiler/rustc_index/Cargo.toml
@@ -8,6 +8,6 @@ edition = "2018"
 doctest = false
 
 [dependencies]
-arrayvec = "0.5.1"
+arrayvec = { version = "0.5.1", default-features = false }
 rustc_serialize = { path = "../rustc_serialize" }
 rustc_macros = { path = "../rustc_macros" }
diff --git a/compiler/rustc_infer/src/infer/combine.rs b/compiler/rustc_infer/src/infer/combine.rs
index 33d26317c71..a540face4f2 100644
--- a/compiler/rustc_infer/src/infer/combine.rs
+++ b/compiler/rustc_infer/src/infer/combine.rs
@@ -31,13 +31,11 @@ use super::unify_key::replace_if_possible;
 use super::unify_key::{ConstVarValue, ConstVariableValue};
 use super::unify_key::{ConstVariableOrigin, ConstVariableOriginKind};
 use super::{InferCtxt, MiscVariable, TypeTrace};
-use arrayvec::ArrayVec;
-use rustc_data_structures::fx::FxHashMap;
-use std::hash::Hash;
 
 use crate::traits::{Obligation, PredicateObligations};
 
 use rustc_ast as ast;
+use rustc_data_structures::mini_map::MiniMap;
 use rustc_hir::def_id::DefId;
 use rustc_middle::traits::ObligationCause;
 use rustc_middle::ty::error::TypeError;
@@ -47,63 +45,6 @@ use rustc_middle::ty::{self, InferConst, ToPredicate, Ty, TyCtxt, TypeFoldable};
 use rustc_middle::ty::{IntType, UintType};
 use rustc_span::{Span, DUMMY_SP};
 
-/// Small-storage-optimized implementation of a map
-/// made specifically for caching results.
-///
-/// Stores elements in a small array up to a certain length
-/// and switches to `HashMap` when that length is exceeded.
-enum MiniMap<K, V> {
-    Array(ArrayVec<[(K, V); 8]>),
-    Map(FxHashMap<K, V>),
-}
-
-impl<K: Eq + Hash, V> MiniMap<K, V> {
-    /// Creates an empty `MiniMap`.
-    pub fn new() -> Self {
-        MiniMap::Array(ArrayVec::new())
-    }
-
-    /// Inserts or updates value in the map.
-    pub fn insert(&mut self, key: K, value: V) {
-        match self {
-            MiniMap::Array(array) => {
-                for pair in array.iter_mut() {
-                    if pair.0 == key {
-                        pair.1 = value;
-                        return;
-                    }
-                }
-                if let Err(error) = array.try_push((key, value)) {
-                    let mut map: FxHashMap<K, V> = array.drain(..).collect();
-                    let (key, value) = error.element();
-                    map.insert(key, value);
-                    *self = MiniMap::Map(map);
-                }
-            }
-            MiniMap::Map(map) => {
-                map.insert(key, value);
-            }
-        }
-    }
-
-    /// Return value by key if any.
-    pub fn get(&self, key: &K) -> Option<&V> {
-        match self {
-            MiniMap::Array(array) => {
-                for pair in array {
-                    if pair.0 == *key {
-                        return Some(&pair.1);
-                    }
-                }
-                return None;
-            }
-            MiniMap::Map(map) => {
-                return map.get(key);
-            }
-        }
-    }
-}
-
 #[derive(Clone)]
 pub struct CombineFields<'infcx, 'tcx> {
     pub infcx: &'infcx InferCtxt<'infcx, 'tcx>,
diff --git a/compiler/rustc_trait_selection/src/traits/query/normalize.rs b/compiler/rustc_trait_selection/src/traits/query/normalize.rs
index 323063b9584..3dcebbcc244 100644
--- a/compiler/rustc_trait_selection/src/traits/query/normalize.rs
+++ b/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> {