Generalize get_nullable_type to allow types where null is all-ones.

compiler/rustc_feature/src/builtin_attrs.rs

@@ -606,17 +606,17 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     rustc_attr!(
         rustc_layout_scalar_valid_range_start, Normal, template!(List: "value"), ErrorFollowing,
         "the `#[rustc_layout_scalar_valid_range_start]` attribute is just used to enable \
-        niche optimizations in libcore and will never be stable",
+        niche optimizations in libcore and libstd and will never be stable",
     ),
     rustc_attr!(
         rustc_layout_scalar_valid_range_end, Normal, template!(List: "value"), ErrorFollowing,
         "the `#[rustc_layout_scalar_valid_range_end]` attribute is just used to enable \
-        niche optimizations in libcore and will never be stable",
+        niche optimizations in libcore and libstd and will never be stable",
     ),
     rustc_attr!(
         rustc_nonnull_optimization_guaranteed, Normal, template!(Word), WarnFollowing,
         "the `#[rustc_nonnull_optimization_guaranteed]` attribute is just used to enable \
-        niche optimizations in libcore and will never be stable",
+        niche optimizations in libcore and libstd and will never be stable",
     ),
 
     // ==========================================================================

compiler/rustc_lint/src/types.rs

@@ -795,7 +795,9 @@ crate fn repr_nullable_ptr<'tcx>(
         let field_ty_abi = &cx.layout_of(field_ty).unwrap().abi;
         if let Abi::Scalar(field_ty_scalar) = field_ty_abi {
             match (field_ty_scalar.valid_range.start, field_ty_scalar.valid_range.end) {
-                (0, _) => unreachable!("Non-null optimisation extended to a non-zero value."),
+                (0, x) if x == field_ty_scalar.value.size(&cx.tcx).unsigned_int_max() - 1 => {
+                    return Some(get_nullable_type(cx, field_ty).unwrap());
+                }
                 (1, _) => {
                     return Some(get_nullable_type(cx, field_ty).unwrap());
                 }

src/test/ui/feature-gates/feature-gate-rustc-attrs-1.rs

@@ -2,6 +2,6 @@
 
 #[rustc_variance] //~ ERROR the `#[rustc_variance]` attribute is just used for rustc unit tests and will never be stable
 #[rustc_error] //~ ERROR the `#[rustc_error]` attribute is just used for rustc unit tests and will never be stable
-#[rustc_nonnull_optimization_guaranteed] //~ ERROR the `#[rustc_nonnull_optimization_guaranteed]` attribute is just used to enable niche optimizations in libcore and will never be stable
+#[rustc_nonnull_optimization_guaranteed] //~ ERROR the `#[rustc_nonnull_optimization_guaranteed]` attribute is just used to enable niche optimizations in libcore and libstd and will never be stable
 
 fn main() {}

src/test/ui/feature-gates/feature-gate-rustc-attrs-1.stderr

@@ -14,7 +14,7 @@ LL | #[rustc_error]
    |
    = help: add `#![feature(rustc_attrs)]` to the crate attributes to enable
 
-error[E0658]: the `#[rustc_nonnull_optimization_guaranteed]` attribute is just used to enable niche optimizations in libcore and will never be stable
+error[E0658]: the `#[rustc_nonnull_optimization_guaranteed]` attribute is just used to enable niche optimizations in libcore and libstd and will never be stable
   --> $DIR/feature-gate-rustc-attrs-1.rs:5:1
    |
 LL | #[rustc_nonnull_optimization_guaranteed]