From 909ec370286b193f67e738e7b9d6437cf32c014c Mon Sep 17 00:00:00 2001
From: Nadrieril <nadrieril+git@gmail.com>
Date: Sun, 3 Nov 2019 21:22:14 +0000
Subject: [PATCH] Simpler code path for subtracting from FixedLenSlice

---
 src/librustc_mir/hair/pattern/_match.rs | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/librustc_mir/hair/pattern/_match.rs b/src/librustc_mir/hair/pattern/_match.rs
index d0597817d32..31984daa5df 100644
--- a/src/librustc_mir/hair/pattern/_match.rs
+++ b/src/librustc_mir/hair/pattern/_match.rs
@@ -672,7 +672,15 @@ impl<'tcx> Constructor<'tcx> {
                     vec![self.clone()]
                 }
             }
-            FixedLenSlice(_) | VarLenSlice(_) => {
+            FixedLenSlice(self_len) => {
+                let overlaps = |c: &Constructor<'_>| match c {
+                    FixedLenSlice(other_len) => other_len == self_len,
+                    VarLenSlice(other_len) => other_len <= self_len,
+                    _ => false,
+                };
+                if other_ctors.iter().any(overlaps) { vec![] } else { vec![self.clone()] }
+            }
+            VarLenSlice(_) => {
                 let mut remaining_ctors = if let VarLenSlice(len) = self {
                     (*len..pcx.max_slice_length + 1).map(FixedLenSlice).collect()
                 } else {