Lint unstable functions that are const stable

Author: jhpratt

compiler/rustc_lint/src/internal.rs

@@ -358,3 +358,47 @@ impl<'tcx> LateLintPass<'tcx> for ExistingDocKeyword {
         }
     }
 }
+
+declare_tool_lint! {
+    pub rustc::INCOMPATIBLE_STABILITY,
+    Deny,
+    "check that an unstable item is not const-stable"
+}
+
+declare_lint_pass!(IncompatibleStability => [INCOMPATIBLE_STABILITY]);
+
+impl LateLintPass<'_> for IncompatibleStability {
+    fn check_fn(
+        &mut self,
+        cx: &LateContext<'_>,
+        kind: rustc_hir::intravisit::FnKind<'_>,
+        _: &FnDecl<'_>,
+        _: &Body<'_>,
+        span: rustc_span::Span,
+        hir_id: HirId,
+    ) {
+        use rustc_attr::{ConstStability, Stability, StabilityLevel};
+
+        // Perform a cheap check up-front to avoid unnecessary querying.
+        if kind.constness() == rustc_hir::Constness::NotConst {
+            return;
+        }
+
+        if !matches!(
+            cx.tcx.lookup_stability(hir_id.owner),
+            Some(Stability { level: StabilityLevel::Unstable { .. }, .. })
+        ) {
+            return;
+        }
+        if !matches!(
+            cx.tcx.lookup_const_stability(hir_id.owner),
+            Some(ConstStability { level: StabilityLevel::Stable { .. }, .. })
+        ) {
+            return;
+        }
+
+        cx.struct_span_lint(INCOMPATIBLE_STABILITY, span, |lint| {
+            lint.build("functions cannot be const-stable if they are unstable").emit();
+        });
+    }
+}

compiler/rustc_lint/src/lib.rs

@@ -492,6 +492,8 @@ fn register_internals(store: &mut LintStore) {
     store.register_late_pass(|| Box::new(ExistingDocKeyword));
     store.register_lints(&TyTyKind::get_lints());
     store.register_late_pass(|| Box::new(TyTyKind));
+    store.register_lints(&IncompatibleStability::get_lints());
+    store.register_late_pass(|| Box::new(IncompatibleStability));
     store.register_group(
         false,
         "rustc::internal",
@@ -504,6 +506,7 @@ fn register_internals(store: &mut LintStore) {
             LintId::of(TY_PASS_BY_REFERENCE),
             LintId::of(USAGE_OF_QUALIFIED_TY),
             LintId::of(EXISTING_DOC_KEYWORD),
+            LintId::of(INCOMPATIBLE_STABILITY),
         ],
     );
 }

src/test/ui/lint/incompatible_stability.rs

@@ -0,0 +1,16 @@
+// compile-flags: -Zunstable-options
+#![stable(feature = "stable", since = "1.0.0")]
+#![feature(staged_api, rustc_attrs)]
+
+#[unstable(feature = "unstable", issue = "none")]
+#[rustc_const_stable(feature = "stable", since = "1.0.0")]
+const fn foo() {} //~ ERROR functions cannot be const-stable if they are unstable
+
+mod bar {
+    #![unstable(feature = "unstable", issue = "none")]
+
+    #[rustc_const_stable(feature = "stable", since = "1.0.0")]
+    const fn foo() {} //~ ERROR functions cannot be const-stable if they are unstable
+}
+
+fn main() {}

src/test/ui/lint/incompatible_stability.stderr

@@ -0,0 +1,16 @@
+error: functions cannot be const-stable if they are unstable
+  --> $DIR/incompatible_stability.rs:7:1
+   |
+LL | const fn foo() {}
+   | ^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[deny(rustc::incompatible_stability)]` on by default
+
+error: functions cannot be const-stable if they are unstable
+  --> $DIR/incompatible_stability.rs:13:5
+   |
+LL |     const fn foo() {}
+   |     ^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+