Use a SmallVec for Candidate::match_pairs.

This is a small win for `encoding`.

Author: nnethercote

src/librustc_mir/build/matches/mod.rs

@@ -17,6 +17,7 @@ use rustc::ty::{self, CanonicalUserTypeAnnotation, Ty};
 use rustc::ty::layout::VariantIdx;
 use rustc_index::bit_set::BitSet;
 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
+use smallvec::{SmallVec, smallvec};
 use syntax::ast::Name;
 use syntax_pos::Span;
 
@@ -166,7 +167,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         |(pattern, pre_binding_block)| {
                             Candidate {
                                 span: pattern.span,
-                                match_pairs: vec![
+                                match_pairs: smallvec![
                                     MatchPair::new(scrutinee_place.clone(), pattern),
                                 ],
                                 bindings: vec![],
@@ -421,7 +422,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         // create a dummy candidate
         let mut candidate = Candidate {
             span: irrefutable_pat.span,
-            match_pairs: vec![MatchPair::new(initializer.clone(), &irrefutable_pat)],
+            match_pairs: smallvec![MatchPair::new(initializer.clone(), &irrefutable_pat)],
             bindings: vec![],
             ascriptions: vec![],
 
@@ -671,7 +672,7 @@ pub struct Candidate<'pat, 'tcx> {
     span: Span,
 
     // all of these must be satisfied...
-    match_pairs: Vec<MatchPair<'pat, 'tcx>>,
+    match_pairs: SmallVec<[MatchPair<'pat, 'tcx>; 1]>,
 
     // ...these bindings established...
     bindings: Vec<Binding<'tcx>>,

src/librustc_mir/build/matches/util.rs

@@ -2,6 +2,7 @@ use crate::build::Builder;
 use crate::build::matches::MatchPair;
 use crate::hair::*;
 use rustc::mir::*;
+use smallvec::SmallVec;
 use std::u32;
 use std::convert::TryInto;
 
@@ -25,7 +26,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     }
 
     pub fn prefix_slice_suffix<'pat>(&mut self,
-                                     match_pairs: &mut Vec<MatchPair<'pat, 'tcx>>,
+                                     match_pairs: &mut SmallVec<[MatchPair<'pat, 'tcx>; 1]>,
                                      place: &Place<'tcx>,
                                      prefix: &'pat [Pat<'tcx>],
                                      opt_slice: Option<&'pat Pat<'tcx>>,