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Promote uninhabited_static lint to a hard error #149518

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18 changes: 7 additions & 11 deletions compiler/rustc_hir_analysis/src/check/check.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -23,7 +23,6 @@ use rustc_middle::ty::{
AdtDef, BottomUpFolder, FnSig, GenericArgKind, RegionKind, TypeFoldable, TypeSuperVisitable,
TypeVisitable, TypeVisitableExt, fold_regions,
};
use rustc_session::lint::builtin::UNINHABITED_STATIC;
use rustc_target::spec::{AbiMap, AbiMapping};
use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
use rustc_trait_selection::error_reporting::traits::on_unimplemented::OnUnimplementedDirective;
Expand Down Expand Up @@ -197,16 +196,13 @@ fn check_static_inhabited(tcx: TyCtxt<'_>, def_id: LocalDefId) {
}
};
if layout.is_uninhabited() {
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@fmease fmease Dec 1, 2025 •
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This obviously doesn't account for visibility meaning if upstream makes a public type with private fields uninhabited it could break downstream.

Is that already considered to be a breaking change due to other features? I see a bunch of uses of is_uninhabited in the compiler, so maybe?

Counterexamples: Exhaustiveness checking accounts for visibility at module granularity even, #[repr(transparent)] no longer permits external types with private fields (well, it's still a future-incompat deny-by-default lint, repr_transparent_non_zst_fields).

Edit: This concern probably doesn't really matter actually, it's super theoretical. It's more important that we reject the bad statics.

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@WaffleLapkin WaffleLapkin Dec 2, 2025

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I thought about it a bit and I think for this lint specifically this is fine.

a static implies the existence of the value, so there is either

  • Some library that provides a value of such type (extern static)
  • Some (accessible) function that returns a value of such type (normal static)

In both cases making the value uncostructable would already be a breaking change.

Another way to look at this: having a static of an uninhabited type is unsound and leads to UB on basically any use of such static. We shouldn't compile such programs, period.

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@fmease fmease Dec 2, 2025 •
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We shouldn't compile such programs, period.

I do agree.

having a static of an uninhabited type is unsound and leads to UB on basically any use of such static

Re. supposed unsoundness tho, is that really the case? Merely defining such a static isn't insta-UB or is it? And since extern statics are unsafe to reference, no language invariant is broken in safe code, so it's not unsound; sure it is UB but that would be the user's fault.

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Fair enough, it slipped from my mind that those accesses are unsafe.

tcx.node_span_lint(
UNINHABITED_STATIC,
tcx.local_def_id_to_hir_id(def_id),
span,
|lint| {
lint.primary_message("static of uninhabited type");
lint
.note("uninhabited statics cannot be initialized, and any access would be an immediate error");
},
);
tcx.dcx()
.struct_span_err(span, "static of uninhabited type")
.with_note(
"uninhabited statics cannot be initialized, \
and any access would be an immediate error",
)
.emit();
}
}

Expand Down
5 changes: 5 additions & 0 deletions compiler/rustc_lint/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -638,6 +638,11 @@ fn register_builtins(store: &mut LintStore) {
see <https://github.com/rust-lang/rust/issues/40107> for more information",
);
store.register_removed("wasm_c_abi", "the wasm C ABI has been fixed");
store.register_removed(
"uninhabited_statics",
"converted into a hard error, \
see <https://github.com/rust-lang/rust/issues/74840> for more information",
)
}

fn register_internals(store: &mut LintStore) {
Expand Down
30 changes: 0 additions & 30 deletions compiler/rustc_lint_defs/src/builtin.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -115,7 +115,6 @@ declare_lint_pass! {
UNCOVERED_PARAM_IN_PROJECTION,
UNEXPECTED_CFGS,
UNFULFILLED_LINT_EXPECTATIONS,
UNINHABITED_STATIC,
UNKNOWN_CRATE_TYPES,
UNKNOWN_DIAGNOSTIC_ATTRIBUTES,
UNKNOWN_LINTS,
Expand Down Expand Up @@ -2675,35 +2674,6 @@ declare_lint! {
"suggest casting to a function pointer when attempting to take references to function items",
}

declare_lint! {
/// The `uninhabited_static` lint detects uninhabited statics.
///
/// ### Example
///
/// ```rust
/// enum Void {}
/// unsafe extern {
/// static EXTERN: Void;
/// }
/// ```
///
/// {{produces}}
///
/// ### Explanation
///
/// Statics with an uninhabited type can never be initialized, so they are impossible to define.
/// However, this can be side-stepped with an `extern static`, leading to problems later in the
/// compiler which assumes that there are no initialized uninhabited places (such as locals or
/// statics). This was accidentally allowed, but is being phased out.
pub UNINHABITED_STATIC,
Warn,
"uninhabited static",
@future_incompatible = FutureIncompatibleInfo {
reason: FutureIncompatibilityReason::FutureReleaseError,
reference: "issue #74840 <https://github.com/rust-lang/rust/issues/74840>",
};
}

declare_lint! {
/// The `unnameable_test_items` lint detects [`#[test]`][test] functions
/// that are not able to be run by the test harness because they are in a
Expand Down
11 changes: 4 additions & 7 deletions tests/ui/statics/uninhabited-static.rs
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@@ -1,19 +1,16 @@
#![feature(never_type)]
#![deny(uninhabited_static)]

enum Void {}
extern "C" {
static VOID: Void; //~ ERROR static of uninhabited type
//~| WARN: previously accepted
static NEVER: !; //~ ERROR static of uninhabited type
//~| WARN: previously accepted
}

static VOID2: Void = unsafe { std::mem::transmute(()) }; //~ ERROR static of uninhabited type
//~| WARN: previously accepted
static VOID2: Void = unsafe { std::mem::transmute(()) };
//~^ ERROR static of uninhabited type
//~| ERROR value of uninhabited type `Void`
static NEVER2: Void = unsafe { std::mem::transmute(()) }; //~ ERROR static of uninhabited type
//~| WARN: previously accepted
static NEVER2: Void = unsafe { std::mem::transmute(()) };
//~^ ERROR static of uninhabited type
//~| ERROR value of uninhabited type `Void`

fn main() {}
25 changes: 6 additions & 19 deletions tests/ui/statics/uninhabited-static.stderr
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Original file line number Diff line number Diff line change
@@ -1,56 +1,43 @@
error: static of uninhabited type
--> $DIR/uninhabited-static.rs:12:1
--> $DIR/uninhabited-static.rs:9:1
|
LL | static VOID2: Void = unsafe { std::mem::transmute(()) };
| ^^^^^^^^^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #74840 <https://github.com/rust-lang/rust/issues/74840>
= note: uninhabited statics cannot be initialized, and any access would be an immediate error
note: the lint level is defined here
--> $DIR/uninhabited-static.rs:2:9
|
LL | #![deny(uninhabited_static)]
| ^^^^^^^^^^^^^^^^^^

error: static of uninhabited type
--> $DIR/uninhabited-static.rs:15:1
--> $DIR/uninhabited-static.rs:12:1
|
LL | static NEVER2: Void = unsafe { std::mem::transmute(()) };
| ^^^^^^^^^^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #74840 <https://github.com/rust-lang/rust/issues/74840>
= note: uninhabited statics cannot be initialized, and any access would be an immediate error

error: static of uninhabited type
--> $DIR/uninhabited-static.rs:6:5
--> $DIR/uninhabited-static.rs:5:5
|
LL | static VOID: Void;
| ^^^^^^^^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #74840 <https://github.com/rust-lang/rust/issues/74840>
= note: uninhabited statics cannot be initialized, and any access would be an immediate error

error: static of uninhabited type
--> $DIR/uninhabited-static.rs:8:5
--> $DIR/uninhabited-static.rs:6:5
|
LL | static NEVER: !;
| ^^^^^^^^^^^^^^^
|
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #74840 <https://github.com/rust-lang/rust/issues/74840>
= note: uninhabited statics cannot be initialized, and any access would be an immediate error

error[E0080]: constructing invalid value: encountered a value of uninhabited type `Void`
--> $DIR/uninhabited-static.rs:12:31
--> $DIR/uninhabited-static.rs:9:31
|
LL | static VOID2: Void = unsafe { std::mem::transmute(()) };
| ^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `VOID2` failed here

error[E0080]: constructing invalid value: encountered a value of uninhabited type `Void`
--> $DIR/uninhabited-static.rs:15:32
--> $DIR/uninhabited-static.rs:12:32
|
LL | static NEVER2: Void = unsafe { std::mem::transmute(()) };
| ^^^^^^^^^^^^^^^^^^^^^^^ evaluation of `NEVER2` failed here
Expand Down
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