Deduplicate types in the generator witness

src/librustc_typeck/check/generator_interior.rs

@@ -7,7 +7,7 @@ use super::FnCtxt;
 use rustc::hir::map::Map;
 use rustc::middle::region::{self, YieldData};
 use rustc::ty::{self, Ty};
-use rustc_data_structures::fx::FxHashMap;
+use rustc_data_structures::fx::{FxHashMap, FxHashSet};
 use rustc_hir as hir;
 use rustc_hir::def::{CtorKind, DefKind, Res};
 use rustc_hir::def_id::DefId;
@@ -160,33 +160,39 @@ pub fn resolve_interior<'a, 'tcx>(
 
     debug!("types in generator {:?}, span = {:?}", types, body.value.span);
 
-    // Replace all regions inside the generator interior with late bound regions
-    // Note that each region slot in the types gets a new fresh late bound region,
-    // which means that none of the regions inside relate to any other, even if
-    // typeck had previously found constraints that would cause them to be related.
     let mut counter = 0;
-    let fold_types: Vec<_> = types.iter().map(|(t, _)| t.ty).collect();
-    let folded_types = fcx.tcx.fold_regions(&fold_types, &mut false, |_, current_depth| {
-        counter += 1;
-        fcx.tcx.mk_region(ty::ReLateBound(current_depth, ty::BrAnon(counter)))
-    });
-
-    // Store the generator types and spans into the tables for this generator.
-    let types = types
+    let mut captured_tys = FxHashSet::default();
+    let type_causes: Vec<_> = types
         .into_iter()
-        .zip(&folded_types)
-        .map(|((mut interior_cause, _), ty)| {
-            interior_cause.ty = ty;
-            interior_cause
+        .filter_map(|(mut cause, _)| {
+            // Erase regions and canonicalize late-bound regions to deduplicate as many types as we
+            // can.
+            let erased = fcx.tcx.erase_regions(&cause.ty);
+            if captured_tys.insert(erased) {
+                // Replace all regions inside the generator interior with late bound regions.
+                // Note that each region slot in the types gets a new fresh late bound region,
+                // which means that none of the regions inside relate to any other, even if
+                // typeck had previously found constraints that would cause them to be related.
+                let folded = fcx.tcx.fold_regions(&erased, &mut false, |_, current_depth| {
+                    counter += 1;
+                    fcx.tcx.mk_region(ty::ReLateBound(current_depth, ty::BrAnon(counter)))
+                });
+
+                cause.ty = folded;
+                Some(cause)
+            } else {
+                None
+            }
         })
         .collect();
-    visitor.fcx.inh.tables.borrow_mut().generator_interior_types = types;
-
-    // Extract type components
-    let type_list = fcx.tcx.mk_type_list(folded_types.iter());
 
+    // Extract type components to build the witness type.
+    let type_list = fcx.tcx.mk_type_list(type_causes.iter().map(|cause| cause.ty));
     let witness = fcx.tcx.mk_generator_witness(ty::Binder::bind(type_list));
 
+    // Store the generator types and spans into the tables for this generator.
+    visitor.fcx.inh.tables.borrow_mut().generator_interior_types = type_causes;
+
     debug!(
         "types in generator after region replacement {:?}, span = {:?}",
         witness, body.value.span

src/test/ui/generator/not-send-sync.stderr

@@ -20,7 +20,7 @@ LL |     fn assert_sync<T: Sync>(_: T) {}
 LL |     assert_sync(|| {
    |     ^^^^^^^^^^^ future returned by `main` is not `Sync`
    |
-   = help: within `[generator@$DIR/not-send-sync.rs:9:17: 13:6 {std::cell::Cell<i32>, (), ()}]`, the trait `std::marker::Sync` is not implemented for `std::cell::Cell<i32>`
+   = help: within `[generator@$DIR/not-send-sync.rs:9:17: 13:6 {std::cell::Cell<i32>, ()}]`, the trait `std::marker::Sync` is not implemented for `std::cell::Cell<i32>`
 note: future is not `Sync` as this value is used across an yield
   --> $DIR/not-send-sync.rs:12:9
    |

src/test/ui/impl-trait/recursive-impl-trait-type-indirect.stderr

@@ -76,7 +76,7 @@ error[E0720]: opaque type expands to a recursive type
 LL | fn generator_capture() -> impl Sized {
    |                           ^^^^^^^^^^ expands to a recursive type
    |
-   = note: expanded type is `[generator@$DIR/recursive-impl-trait-type-indirect.rs:50:5: 50:26 x:impl Sized {(), ()}]`
+   = note: expanded type is `[generator@$DIR/recursive-impl-trait-type-indirect.rs:50:5: 50:26 x:impl Sized {()}]`
 
 error[E0720]: opaque type expands to a recursive type
   --> $DIR/recursive-impl-trait-type-indirect.rs:53:26
@@ -92,7 +92,7 @@ error[E0720]: opaque type expands to a recursive type
 LL | fn generator_hold() -> impl Sized {
    |                        ^^^^^^^^^^ expands to a recursive type
    |
-   = note: expanded type is `[generator@$DIR/recursive-impl-trait-type-indirect.rs:58:5: 62:6 {impl Sized, (), ()}]`
+   = note: expanded type is `[generator@$DIR/recursive-impl-trait-type-indirect.rs:58:5: 62:6 {impl Sized, ()}]`
 
 error[E0720]: opaque type expands to a recursive type
   --> $DIR/recursive-impl-trait-type-indirect.rs:69:26