Extend substs_to_args into a perfectly-sized Vec almost every time

src/librustdoc/clean/utils.rs

@@ -80,9 +80,9 @@ pub(crate) fn substs_to_args(
     substs: &[ty::subst::GenericArg<'_>],
     mut skip_first: bool,
 ) -> Vec<GenericArg> {
-    substs
-        .iter()
-        .filter_map(|kind| match kind.unpack() {
+    let mut ret_val =
+        Vec::with_capacity(substs.len().saturating_sub(if skip_first { 1 } else { 0 }));
+    ret_val.extend(substs.iter().filter_map(|kind| match kind.unpack() {
         GenericArgKind::Lifetime(lt) => match *lt {
             ty::ReLateBound(_, ty::BoundRegion { kind: ty::BrAnon(_), .. }) => {
                 Some(GenericArg::Lifetime(Lifetime::elided()))
@@ -95,8 +95,8 @@ pub(crate) fn substs_to_args(
         }
         GenericArgKind::Type(ty) => Some(GenericArg::Type(ty.clean(cx))),
         GenericArgKind::Const(ct) => Some(GenericArg::Const(Box::new(ct.clean(cx)))),
-        })
-        .collect()
+    }));
+    ret_val
 }
 
 fn external_generic_args(