Fix emission of niche-filling discriminant values

src/librustc_codegen_llvm/debuginfo/metadata.rs

@@ -1279,7 +1279,7 @@ impl EnumMemberDescriptionFactory<'ll, 'tcx> {
                 niche_start,
                 ref variants,
                 dataful_variant,
-                ..
+                ref niche,
             } => {
                 if fallback {
                     let variant = self.layout.for_variant(cx, dataful_variant);
@@ -1361,11 +1361,11 @@ impl EnumMemberDescriptionFactory<'ll, 'tcx> {
                         let niche_value = if i == dataful_variant {
                             None
                         } else {
-                            let niche = (i as u128)
+                            let value = (i as u128)
                                 .wrapping_sub(*niche_variants.start() as u128)
                                 .wrapping_add(niche_start);
-                            assert_eq!(niche as u64 as u128, niche);
-                            Some(niche as u64)
+                            let value = value & ((1u128 << niche.value.size(cx).bits()) - 1);
+                            Some(value as u64)
                         };
 
                         MemberDescription {

src/test/codegen/enum-debug-niche-2.rs

@@ -0,0 +1,63 @@
+// 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.
+
+// This test depends on a patch that was committed to upstream LLVM
+// before 7.0, then backported to the Rust LLVM fork.  It tests that
+// optimized enum debug info accurately reflects the enum layout.
+
+// ignore-tidy-linelength
+// ignore-windows
+// min-system-llvm-version 7.0
+
+// compile-flags: -g -C no-prepopulate-passes
+
+// CHECK: {{.*}}DICompositeType{{.*}}tag: DW_TAG_variant_part,{{.*}}size: 32,{{.*}}
+// CHECK: {{.*}}DIDerivedType{{.*}}tag: DW_TAG_member,{{.*}}name: "Placeholder",{{.*}}extraData: i64 4294967295{{[,)].*}}
+// CHECK: {{.*}}DIDerivedType{{.*}}tag: DW_TAG_member,{{.*}}name: "Error",{{.*}}extraData: i64 0{{[,)].*}}
+
+#![feature(never_type)]
+#![feature(nll)]
+
+#[derive(Copy, Clone)]
+pub struct Entity {
+    private: std::num::NonZeroU32,
+}
+
+#[derive(Copy, Clone, PartialEq, Eq)]
+pub struct Declaration;
+
+impl TypeFamily for Declaration {
+    type Base = Base;
+    type Placeholder = !;
+
+    fn intern_base_data(_: BaseKind<Self>) {}
+}
+
+#[derive(Copy, Clone)]
+pub struct Base;
+
+pub trait TypeFamily: Copy + 'static {
+    type Base: Copy;
+    type Placeholder: Copy;
+
+    fn intern_base_data(_: BaseKind<Self>);
+}
+
+#[derive(Copy, Clone)]
+pub enum BaseKind<F: TypeFamily> {
+    Named(Entity),
+    Placeholder(F::Placeholder),
+    Error,
+}
+
+pub fn main() {
+    let x = BaseKind::Error::<Declaration>;
+    let y = 7;
+}