Fix ICE with `impl Trait` in type bounds

src/librustdoc/clean/mod.rs

@@ -1725,7 +1725,7 @@ fn clean(&self, cx: &DocContext<'_>) -> Generics {
         let mut impl_trait_proj =
             FxHashMap::<u32, Vec<(DefId, String, Ty<'tcx>)>>::default();
 
-        let mut where_predicates = preds.predicates.iter()
+        let where_predicates = preds.predicates.iter()
             .flat_map(|(p, _)| {
                 let mut projection = None;
                 let param_idx = (|| {
@@ -1747,10 +1747,10 @@ fn clean(&self, cx: &DocContext<'_>) -> Generics {
                     None
                 })();
 
-                let p = p.clean(cx)?;
-
                 if let Some(param_idx) = param_idx {
                     if let Some(b) = impl_trait.get_mut(&param_idx.into()) {
+                        let p = p.clean(cx)?;
+
                         b.extend(
                             p.get_bounds()
                                 .into_iter()
@@ -1805,6 +1805,13 @@ fn clean(&self, cx: &DocContext<'_>) -> Generics {
             cx.impl_trait_bounds.borrow_mut().insert(param, bounds);
         }
 
+        // Now that `cx.impl_trait_bounds` is populated, we can process
+        // remaining predicates which could contain `impl Trait`.
+        let mut where_predicates = where_predicates
+            .into_iter()
+            .flat_map(|p| p.clean(cx))
+            .collect::<Vec<_>>();
+
         // Type parameters and have a Sized bound by default unless removed with
         // ?Sized. Scan through the predicates and mark any type parameter with
         // a Sized bound, removing the bounds as we find them.

src/test/rustdoc/inline_cross/auxiliary/impl_trait_aux.rs

@@ -9,6 +9,8 @@ pub fn func2<T>(
 
 pub fn func3(_x: impl Iterator<Item = impl Iterator<Item = u8>> + Clone) {}
 
+pub fn func4<T: Iterator<Item = impl Clone>>(_x: T) {}
+
 pub struct Foo;
 
 impl Foo {

src/test/rustdoc/inline_cross/impl_trait.rs

@@ -20,6 +20,12 @@
 // @!has - '//pre[@class="rust fn"]' 'where'
 pub use impl_trait_aux::func3;
 
+
+// @has impl_trait/fn.func4.html
+// @has - '//pre[@class="rust fn"]' "func4<T>("
+// @has - '//pre[@class="rust fn"]' "T: Iterator<Item = impl Clone>,"
+pub use impl_trait_aux::func4;
+
 // @has impl_trait/struct.Foo.html
 // @has - '//code[@id="method.v"]' "pub fn method<'a>(_x: impl Clone + Into<Vec<u8>> + 'a)"
 // @!has - '//code[@id="method.v"]' 'where'