Tweak diagnostics code

Author: Nadrieril

src/librustc_mir/hair/pattern/_match.rs

@@ -969,19 +969,21 @@ impl<'tcx> Constructor<'tcx> {
                 }
                 VarLen(prefix, _) => {
                     let mut prefix: Vec<_> = subpatterns.by_ref().take(prefix as usize).collect();
-                    let mut suffix: Vec<_> = subpatterns.collect();
                     if slice.array_len.is_some() {
                         // Improves diagnostics a bit: if the type is a known-size array, instead
                         // of reporting `[x, _, .., _, y]`, we prefer to report `[x, .., y]`.
                         // This is incorrect if the size is not known, since `[_, ..]` captures
                         // arrays of lengths `>= 1` whereas `[..]` captures any length.
-                        while !suffix.is_empty() && suffix.first().unwrap().is_wildcard() {
-                            suffix.remove(0);
-                        }
                         while !prefix.is_empty() && prefix.last().unwrap().is_wildcard() {
                             prefix.pop();
                         }
                     }
+                    let suffix: Vec<_> = if slice.array_len.is_some() {
+                        // Same as above.
+                        subpatterns.skip_while(Pat::is_wildcard).collect()
+                    } else {
+                        subpatterns.collect()
+                    };
                     let wild = Pat::wildcard_from_ty(ty);
                     PatKind::Slice { prefix, slice: Some(wild), suffix }
                 }