Fix emission of niche-filling discriminant values

Bug #55606 points out a regression introduced by #54004; namely that
an assertion can erroneously fire when a niche-filling discriminant
value is emitted.

This fixes the bug by removing the assertion, and furthermore by
arranging for the discriminant value to be masked according to the
size of the niche.  This makes handling the discriminant a bit simpler
for debuggers.

The test case is from Jonathan Turner.

Closes #55606

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;
+}