Remove the branch merging optimisations for slice patterns

They were only correct in the simplest case. Some of the optimisations
are certainly possible but should be introduced carefully and only
when the whole pattern codegen infrastructure is in a better shape.

Fixes #16648.

src/librustc/middle/check_match.rs

@@ -28,8 +28,7 @@ use syntax::codemap::{Span, Spanned, DUMMY_SP};
 use syntax::fold::{Folder, noop_fold_pat};
 use syntax::print::pprust::pat_to_string;
 use syntax::parse::token;
-use syntax::visit;
-use syntax::visit::{Visitor, FnKind};
+use syntax::visit::{mod, Visitor, FnKind};
 use util::ppaux::ty_to_string;
 
 struct Matrix(Vec<Vec<Gc<Pat>>>);
@@ -103,7 +102,9 @@ pub enum Constructor {
     /// Ranges of literal values (2..5).
     ConstantRange(const_val, const_val),
     /// Array patterns of length n.
-    Slice(uint)
+    Slice(uint),
+    /// Array patterns with a subslice.
+    SliceWithSubslice(uint, uint)
 }
 
 #[deriving(Clone, PartialEq)]
@@ -270,13 +271,6 @@ fn check_arms(cx: &MatchCheckCtxt, arms: &[Arm]) {
     }
 }
 
-fn raw_pat(p: Gc<Pat>) -> Gc<Pat> {
-    match p.node {
-        PatIdent(_, _, Some(s)) => { raw_pat(s) }
-        _ => { p }
-    }
-}
-
 fn check_exhaustive(cx: &MatchCheckCtxt, sp: Span, matrix: &Matrix) {
     match is_useful(cx, matrix, [wild()], ConstructWitness) {
         UsefulWithWitness(pats) => {
@@ -821,6 +815,14 @@ pub fn specialize(cx: &MatchCheckCtxt, r: &[Gc<Pat>],
                     pats.push_all(after.as_slice());
                     Some(pats)
                 },
+                SliceWithSubslice(prefix, suffix)
+                    if before.len() == prefix
+                        && after.len() == suffix
+                        && slice.is_some() => {
+                    let mut pats = before.clone();
+                    pats.push_all(after.as_slice());
+                    Some(pats)
+                }
                 _ => None
             }
         }

src/librustc/middle/pat_util.rs

@@ -119,6 +119,13 @@ pub fn wild() -> Gc<Pat> {
     box (GC) Pat { id: 0, node: PatWild(PatWildSingle), span: DUMMY_SP }
 }
 
+pub fn raw_pat(p: Gc<Pat>) -> Gc<Pat> {
+    match p.node {
+        PatIdent(_, _, Some(s)) => { raw_pat(s) }
+        _ => { p }
+    }
+}
+
 pub fn def_to_path(tcx: &ty::ctxt, id: DefId) -> Path {
     ty::with_path(tcx, id, |mut path| Path {
         global: false,