Ignore CTFE errors while lowering patterns

Closes #43105.

src/librustc_const_eval/pattern.rs

@@ -280,7 +280,7 @@ impl<'a, 'tcx> Pattern<'tcx> {
         let mut pcx = PatternContext::new(tcx, param_env_and_substs, tables);
         let result = pcx.lower_pattern(pat);
         if !pcx.errors.is_empty() {
-            span_bug!(pat.span, "encountered errors lowering pattern: {:?}", pcx.errors)
+            debug!("encountered errors lowering pattern: {:?}", pcx.errors)
         }
         debug!("Pattern::from_hir({:?}) = {:?}", pat, result);
         result

src/test/compile-fail/issue-43105.rs

@@ -0,0 +1,21 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn xyz() -> u8 { 42 }
+
+const NUM: u8 = xyz();
+//~^ ERROR calls in constants are limited to constant functions, struct and enum constructors
+
+fn main() {
+    match 1 {
+        NUM => unimplemented!(),
+        _ => unimplemented!(),
+    }
+}