Auto merge of rust-lang#116195 - fmease:rustdoc-investigate-perf-regression, r=GuillaumeGomez

rustdoc: speed up processing of cross-crate fns to fix a perf regression

* The first commit doesn't affect perf but get's rid of a `.clone()` and a bunch of lines of code. I can drop it if you'd like me to
* The second commit, *“reduce the amount of `asyncness` query executions”*, addresses the perf regression introduced in rust-lang#116084

r? `@ghost`

Author: bors

src/librustdoc/clean/mod.rs

@@ -1183,7 +1183,13 @@ fn clean_fn_decl_from_did_and_sig<'tcx>(
     // but shouldn't change any code meaning.
     let mut output = clean_middle_ty(sig.output(), cx, None, None);
 
-    if let Some(did) = did && cx.tcx.asyncness(did).is_async() {
+    // If the return type isn't an `impl Trait`, we can safely assume that this
+    // function isn't async without needing to execute the query `asyncness` at
+    // all which gives us a noticeable performance boost.
+    if let Some(did) = did
+        && let Type::ImplTrait(_) = output
+        && cx.tcx.asyncness(did).is_async()
+    {
         output = output.sugared_async_return_type();
     }
 

src/librustdoc/clean/types.rs

@@ -1604,14 +1604,16 @@ impl Type {
     ///
     /// This function will panic if the return type does not match the expected sugaring for async
     /// functions.
-    pub(crate) fn sugared_async_return_type(&self) -> Type {
-        if let Type::ImplTrait(v) = self &&
-            let [GenericBound::TraitBound(PolyTrait { trait_, .. }, _ )] = &v[..]
+    pub(crate) fn sugared_async_return_type(self) -> Type {
+        if let Type::ImplTrait(mut v) = self
+            && let Some(GenericBound::TraitBound(PolyTrait { mut trait_, .. }, _ )) = v.pop()
+            && let Some(segment) = trait_.segments.pop()
+            && let GenericArgs::AngleBracketed { mut bindings, .. } = segment.args
+            && let Some(binding) = bindings.pop()
+            && let TypeBindingKind::Equality { term } = binding.kind
+            && let Term::Type(ty) = term
         {
-            let bindings = trait_.bindings().unwrap();
-            let ret_ty = bindings[0].term();
-            let ty = ret_ty.ty().expect("unexpected constant in async fn return term");
-            ty.clone()
+            ty
         } else {
             panic!("unexpected async fn return type")
         }
@@ -2189,16 +2191,6 @@ impl Path {
             }
         })
     }
-
-    pub(crate) fn bindings(&self) -> Option<&[TypeBinding]> {
-        self.segments.last().and_then(|seg| {
-            if let GenericArgs::AngleBracketed { ref bindings, .. } = seg.args {
-                Some(&**bindings)
-            } else {
-                None
-            }
-        })
-    }
 }
 
 #[derive(Clone, PartialEq, Eq, Debug, Hash)]
@@ -2478,15 +2470,6 @@ pub(crate) enum TypeBindingKind {
     Constraint { bounds: Vec<GenericBound> },
 }
 
-impl TypeBinding {
-    pub(crate) fn term(&self) -> &Term {
-        match self.kind {
-            TypeBindingKind::Equality { ref term } => term,
-            _ => panic!("expected equality type binding for parenthesized generic args"),
-        }
-    }
-}
-
 /// The type, lifetime, or constant that a private type alias's parameter should be
 /// replaced with when expanding a use of that type alias.
 ///