Fixed ICEs with pattern matching in const fn. Fixes #38199, fixes #31577, fixes #29093, and fixes #40012.

src/librustc_mir/transform/qualify_consts.rs

@@ -722,11 +722,9 @@ impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
             }
 
             Rvalue::Discriminant(..) => {
-                // FIXME discriminant
+                // FIXME implement discriminant const qualify
                 self.add(Qualif::NOT_CONST);
-                if self.mode != Mode::Fn {
-                    bug!("implement discriminant const qualify");
-                }
+                // Discriminants in consts will error elsewhere as an unimplemented expression type
             }
 
             Rvalue::Box(_) => {

src/librustc_typeck/check/dropck.rs

@@ -278,9 +278,12 @@ pub fn check_safety_of_destructor_if_necessary<'a, 'gcx, 'tcx>(
     debug!("check_safety_of_destructor_if_necessary typ: {:?} scope: {:?}",
            typ, scope);
 
-    let parent_scope = rcx.tcx.region_maps.opt_encl_scope(scope).unwrap_or_else(|| {
-        span_bug!(span, "no enclosing scope found for scope: {:?}", scope)
-    });
+
+    let parent_scope = match rcx.tcx.region_maps.opt_encl_scope(scope) {
+      Some(parent_scope) => parent_scope,
+      // If no enclosing scope, then it must be the root scope which cannot be outlived.
+      None => return
+    };
 
     let result = iterate_over_potentially_unsafe_regions_in_type(
         &mut DropckContext {

src/test/compile-fail/const-match-pattern-arm.rs

@@ -0,0 +1,25 @@
+// Copyright 2017 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.
+
+const x: bool = match Some(true) {
+    Some(value) => true,
+    //~^ ERROR: constant contains unimplemented expression type [E0019]
+    _ => false
+};
+
+const y: bool = {
+    match Some(true) {
+        Some(value) => true,
+        //~^ ERROR: constant contains unimplemented expression type [E0019]
+        _ => false
+    }
+};
+
+fn main() {}