[perf] Cache structural resolution in typeck

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -1439,14 +1439,20 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         if self.next_trait_solver()
             && let ty::Alias(..) = ty.kind()
         {
+            if let Some(&normalized_ty) = self.normalization_cache.borrow().get(&ty) {
+                return normalized_ty;
+            }
             // We need to use a separate variable here as otherwise the temporary for
             // `self.fulfillment_cx.borrow_mut()` is alive in the `Err` branch, resulting
             // in a reentrant borrow, causing an ICE.
             let result = self
                 .at(&self.misc(sp), self.param_env)
                 .structurally_normalize_ty(ty, &mut **self.fulfillment_cx.borrow_mut());
             match result {
-                Ok(normalized_ty) => normalized_ty,
+                Ok(normalized_ty) => {
+                    self.normalization_cache.borrow_mut().insert(ty, normalized_ty);
+                    self.resolve_vars_with_obligations(normalized_ty)
+                }
                 Err(errors) => {
                     let guar = self.err_ctxt().report_fulfillment_errors(errors);
                     return Ty::new_error(self.tcx, guar);

compiler/rustc_hir_typeck/src/fn_ctxt/mod.rs

@@ -9,6 +9,7 @@ use std::cell::{Cell, RefCell};
 use std::ops::Deref;
 
 use hir::def_id::CRATE_DEF_ID;
+use rustc_data_structures::fx::FxHashMap;
 use rustc_errors::DiagCtxtHandle;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::{self as hir, HirId, ItemLocalMap};
@@ -122,6 +123,8 @@ pub(crate) struct FnCtxt<'a, 'tcx> {
     /// These are stored here so we may collect them when canonicalizing user
     /// type ascriptions later.
     pub(super) trait_ascriptions: RefCell<ItemLocalMap<Vec<ty::Clause<'tcx>>>>,
+
+    pub(super) normalization_cache: RefCell<FxHashMap<Ty<'tcx>, Ty<'tcx>>>,
 }
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
@@ -150,6 +153,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             diverging_fallback_behavior,
             diverging_block_behavior,
             trait_ascriptions: Default::default(),
+            normalization_cache: Default::default(),
         }
     }