Extend substs_to_args into a perfectly-sized Vec almost every time

Author: notriddle

src/librustdoc/clean/utils.rs

@@ -80,23 +80,23 @@ pub(crate) fn substs_to_args(
  substs: &[ty::subst::GenericArg<'_>],
  mut skip_first: bool,
 ) -> Vec<GenericArg> {
- 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()))
- }
- _ => lt.clean(cx).map(GenericArg::Lifetime),
- },
- GenericArgKind::Type(_) if skip_first => {
- skip_first = false;
- None
+ 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()))
  }
- GenericArgKind::Type(ty) => Some(GenericArg::Type(ty.clean(cx))),
- GenericArgKind::Const(ct) => Some(GenericArg::Const(Box::new(ct.clean(cx)))),
- })
- .collect()
+ _ => lt.clean(cx).map(GenericArg::Lifetime),
+ },
+ GenericArgKind::Type(_) if skip_first => {
+ skip_first = false;
+ None
+ }
+ GenericArgKind::Type(ty) => Some(GenericArg::Type(ty.clean(cx))),
+ GenericArgKind::Const(ct) => Some(GenericArg::Const(Box::new(ct.clean(cx)))),
+ }));
+ ret_val
 }
 
 fn external_generic_args(