Simplify lifetimes by allocating patterns on the arena

We want the lifetimes of the patterns contained in the matrix and the
candidate `PatStack` to be the same so that they can be mixed together.
A lot of this would not be necessary if `SmallVec` was covariant in its
type argument (see servo/rust-smallvec#146).

src/librustc_mir/hair/pattern/_match.rs

@@ -425,16 +425,12 @@ impl<'p, 'tcx> PatStack<'p, 'tcx> {
     }
 
     /// This computes `S(constructor, self)`. See top of the file for explanations.
-    fn specialize_constructor<'a, 'q>(
+    fn specialize_constructor(
         &self,
-        cx: &mut MatchCheckCtxt<'a, 'tcx>,
+        cx: &mut MatchCheckCtxt<'p, 'tcx>,
         constructor: &Constructor<'tcx>,
-        ctor_wild_subpatterns: &[&'q Pat<'tcx>],
-    ) -> Option<PatStack<'q, 'tcx>>
-    where
-        'a: 'q,
-        'p: 'q,
-    {
+        ctor_wild_subpatterns: &'p [Pat<'tcx>],
+    ) -> Option<PatStack<'p, 'tcx>> {
         let new_heads = specialize_one_pattern(cx, self.head(), constructor, ctor_wild_subpatterns);
         new_heads.map(|mut new_head| {
             new_head.0.extend_from_slice(&self.0[1..]);
@@ -486,16 +482,12 @@ impl<'p, 'tcx> Matrix<'p, 'tcx> {
     }
 
     /// This computes `S(constructor, self)`. See top of the file for explanations.
-    fn specialize_constructor<'a, 'q>(
+    fn specialize_constructor(
         &self,
-        cx: &mut MatchCheckCtxt<'a, 'tcx>,
+        cx: &mut MatchCheckCtxt<'p, 'tcx>,
         constructor: &Constructor<'tcx>,
-        ctor_wild_subpatterns: &[&'q Pat<'tcx>],
-    ) -> Matrix<'q, 'tcx>
-    where
-        'a: 'q,
-        'p: 'q,
-    {
+        ctor_wild_subpatterns: &'p [Pat<'tcx>],
+    ) -> Matrix<'p, 'tcx> {
         self.0
             .iter()
             .filter_map(|r| r.specialize_constructor(cx, constructor, ctor_wild_subpatterns))
@@ -1604,10 +1596,10 @@ impl<'tcx> fmt::Debug for MissingConstructors<'tcx> {
 /// relation to preceding patterns, it is not reachable) and exhaustiveness
 /// checking (if a wildcard pattern is useful in relation to a matrix, the
 /// matrix isn't exhaustive).
-pub fn is_useful<'p, 'a, 'tcx>(
-    cx: &mut MatchCheckCtxt<'a, 'tcx>,
+pub fn is_useful<'p, 'tcx>(
+    cx: &mut MatchCheckCtxt<'p, 'tcx>,
     matrix: &Matrix<'p, 'tcx>,
-    v: &PatStack<'_, 'tcx>,
+    v: &PatStack<'p, 'tcx>,
     witness_preference: WitnessPreference,
     hir_id: HirId,
 ) -> Usefulness<'tcx> {
@@ -1768,21 +1760,21 @@ pub fn is_useful<'p, 'a, 'tcx>(
 
 /// A shorthand for the `U(S(c, P), S(c, q))` operation from the paper. I.e., `is_useful` applied
 /// to the specialised version of both the pattern matrix `P` and the new pattern `q`.
-fn is_useful_specialized<'p, 'a, 'tcx>(
-    cx: &mut MatchCheckCtxt<'a, 'tcx>,
+fn is_useful_specialized<'p, 'tcx>(
+    cx: &mut MatchCheckCtxt<'p, 'tcx>,
     matrix: &Matrix<'p, 'tcx>,
-    v: &PatStack<'_, 'tcx>,
+    v: &PatStack<'p, 'tcx>,
     ctor: Constructor<'tcx>,
     lty: Ty<'tcx>,
     witness_preference: WitnessPreference,
     hir_id: HirId,
 ) -> Usefulness<'tcx> {
     debug!("is_useful_specialized({:#?}, {:#?}, {:?})", v, ctor, lty);
 
-    let ctor_wild_subpatterns_owned: Vec<_> = ctor.wildcard_subpatterns(cx, lty);
-    let ctor_wild_subpatterns: Vec<_> = ctor_wild_subpatterns_owned.iter().collect();
-    let matrix = matrix.specialize_constructor(cx, &ctor, &ctor_wild_subpatterns);
-    v.specialize_constructor(cx, &ctor, &ctor_wild_subpatterns)
+    let ctor_wild_subpatterns =
+        cx.pattern_arena.alloc_from_iter(ctor.wildcard_subpatterns(cx, lty));
+    let matrix = matrix.specialize_constructor(cx, &ctor, ctor_wild_subpatterns);
+    v.specialize_constructor(cx, &ctor, ctor_wild_subpatterns)
         .map(|v| is_useful(cx, &matrix, &v, witness_preference, hir_id))
         .map(|u| u.apply_constructor(cx, &ctor, lty))
         .unwrap_or(NotUseful)
@@ -2250,13 +2242,13 @@ fn constructor_covered_by_range<'tcx>(
     if intersects { Some(()) } else { None }
 }
 
-fn patterns_for_variant<'p, 'a: 'p, 'tcx>(
-    cx: &mut MatchCheckCtxt<'a, 'tcx>,
+fn patterns_for_variant<'p, 'tcx>(
+    cx: &mut MatchCheckCtxt<'p, 'tcx>,
     subpatterns: &'p [FieldPat<'tcx>],
-    ctor_wild_subpatterns: &[&'p Pat<'tcx>],
+    ctor_wild_subpatterns: &'p [Pat<'tcx>],
     is_non_exhaustive: bool,
 ) -> PatStack<'p, 'tcx> {
-    let mut result = SmallVec::from_slice(ctor_wild_subpatterns);
+    let mut result: SmallVec<_> = ctor_wild_subpatterns.iter().collect();
 
     for subpat in subpatterns {
         if !is_non_exhaustive || !cx.is_uninhabited(subpat.pattern.ty) {
@@ -2280,11 +2272,11 @@ fn patterns_for_variant<'p, 'a: 'p, 'tcx>(
 /// different patterns.
 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
 /// fields filled with wild patterns.
-fn specialize_one_pattern<'p, 'a: 'p, 'q: 'p, 'tcx>(
-    cx: &mut MatchCheckCtxt<'a, 'tcx>,
-    pat: &'q Pat<'tcx>,
+fn specialize_one_pattern<'p, 'tcx>(
+    cx: &mut MatchCheckCtxt<'p, 'tcx>,
+    pat: &'p Pat<'tcx>,
     constructor: &Constructor<'tcx>,
-    ctor_wild_subpatterns: &[&'p Pat<'tcx>],
+    ctor_wild_subpatterns: &'p [Pat<'tcx>],
 ) -> Option<PatStack<'p, 'tcx>> {
     if let NonExhaustive = constructor {
         // Only a wildcard pattern can match the special extra constructor
@@ -2294,9 +2286,7 @@ fn specialize_one_pattern<'p, 'a: 'p, 'q: 'p, 'tcx>(
     let result = match *pat.kind {
         PatKind::AscribeUserType { .. } => bug!(), // Handled by `expand_pattern`
 
-        PatKind::Binding { .. } | PatKind::Wild => {
-            Some(PatStack::from_slice(ctor_wild_subpatterns))
-        }
+        PatKind::Binding { .. } | PatKind::Wild => Some(ctor_wild_subpatterns.iter().collect()),
 
         PatKind::Variant { adt_def, variant_index, ref subpatterns, .. } => {
             let ref variant = adt_def.variants[variant_index];
@@ -2406,7 +2396,6 @@ fn specialize_one_pattern<'p, 'a: 'p, 'q: 'p, 'tcx>(
                                 .chain(
                                     ctor_wild_subpatterns
                                         .iter()
-                                        .map(|p| *p)
                                         .skip(prefix.len())
                                         .take(slice_count)
                                         .chain(suffix.iter()),

src/librustc_mir/hair/pattern/check_match.rs

@@ -261,8 +261,8 @@ impl<'tcx> MatchVisitor<'_, 'tcx> {
             patcx.include_lint_checks();
             let pattern = patcx.lower_pattern(pat);
             let pattern_ty = pattern.ty;
-            let pattern = expand_pattern(cx, pattern);
-            let pats: Matrix<'_, '_> = vec![PatStack::from_pattern(&pattern)].into_iter().collect();
+            let pattern = cx.pattern_arena.alloc(expand_pattern(cx, pattern));
+            let pats: Matrix<'_, '_> = vec![PatStack::from_pattern(pattern)].into_iter().collect();
 
             let witnesses = match check_not_useful(cx, pattern_ty, &pats, pat.hir_id) {
                 Ok(_) => return,
@@ -398,7 +398,7 @@ fn pat_is_catchall(pat: &Pat) -> bool {
 
 // Check for unreachable patterns
 fn check_arms<'p, 'tcx>(
-    cx: &mut MatchCheckCtxt<'_, 'tcx>,
+    cx: &mut MatchCheckCtxt<'p, 'tcx>,
     arms: &[(Vec<(&'p super::Pat<'tcx>, &hir::Pat)>, Option<&hir::Expr>)],
     source: hir::MatchSource,
 ) -> Matrix<'p, 'tcx> {
@@ -482,14 +482,14 @@ fn check_arms<'p, 'tcx>(
     seen
 }
 
-fn check_not_useful(
-    cx: &mut MatchCheckCtxt<'_, 'tcx>,
+fn check_not_useful<'p, 'tcx>(
+    cx: &mut MatchCheckCtxt<'p, 'tcx>,
     ty: Ty<'tcx>,
-    matrix: &Matrix<'_, 'tcx>,
+    matrix: &Matrix<'p, 'tcx>,
     hir_id: HirId,
 ) -> Result<(), Vec<super::Pat<'tcx>>> {
-    let wild_pattern = super::Pat::wildcard_from_ty(ty);
-    match is_useful(cx, matrix, &PatStack::from_pattern(&wild_pattern), ConstructWitness, hir_id) {
+    let wild_pattern = cx.pattern_arena.alloc(super::Pat::wildcard_from_ty(ty));
+    match is_useful(cx, matrix, &PatStack::from_pattern(wild_pattern), ConstructWitness, hir_id) {
         NotUseful => Ok(()), // This is good, wildcard pattern isn't reachable.
         UsefulWithWitness(pats) => Err(if pats.is_empty() {
             bug!("Exhaustiveness check returned no witnesses")
@@ -500,11 +500,11 @@ fn check_not_useful(
     }
 }
 
-fn check_exhaustive<'tcx>(
-    cx: &mut MatchCheckCtxt<'_, 'tcx>,
+fn check_exhaustive<'p, 'tcx>(
+    cx: &mut MatchCheckCtxt<'p, 'tcx>,
     scrut_ty: Ty<'tcx>,
     sp: Span,
-    matrix: &Matrix<'_, 'tcx>,
+    matrix: &Matrix<'p, 'tcx>,
     hir_id: HirId,
 ) {
     let witnesses = match check_not_useful(cx, scrut_ty, matrix, hir_id) {