Disallow hidden references to mutable static

compiler/rustc_driver_impl/src/signal_handler.rs

@@ -35,6 +35,8 @@ macro raw_errln($tokens:tt) {
 }
 
 /// Signal handler installed for SIGSEGV
+// FIXME(static_mut_refs): Do not allow `static_mut_refs` lint
+#[allow(static_mut_refs)]
 extern "C" fn print_stack_trace(_: libc::c_int) {
     const MAX_FRAMES: usize = 256;
     // Reserve data segment so we don't have to malloc in a signal handler, which might fail

compiler/rustc_error_codes/src/error_codes/E0796.md

@@ -1,14 +1,14 @@
+#### Note: this error code is no longer emitted by the compiler.
+
 You have created a reference to a mutable static.
 
 Erroneous code example:
 
-```compile_fail,edition2024,E0796
+```
 static mut X: i32 = 23;
-
 fn work() {
   let _val = unsafe { X };
 }
-
 let x_ref = unsafe { &mut X };
 work();
 // The next line has Undefined Behavior!

compiler/rustc_error_codes/src/lib.rs

@@ -679,3 +679,4 @@ E0798: 0798,
 //  E0723, // unstable feature in `const` context
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
 //  E0744, // merged into E0728
+//  E0796, // unused error code. We use `static_mut_refs` lint instead.

compiler/rustc_hir_analysis/messages.ftl

@@ -467,25 +467,6 @@ hir_analysis_start_not_target_feature = `#[start]` function is not allowed to ha
 hir_analysis_start_not_track_caller = `#[start]` function is not allowed to be `#[track_caller]`
     .label = `#[start]` function is not allowed to be `#[track_caller]`
 
-hir_analysis_static_mut_ref = creating a {$shared} reference to a mutable static
-    .label = {$shared} reference to mutable static
-    .note = {$shared ->
-        [shared] this shared reference has lifetime `'static`, but if the static ever gets mutated, or a mutable reference is created, then any further use of this shared reference is Undefined Behavior
-        *[mutable] this mutable reference has lifetime `'static`, but if the static gets accessed (read or written) by any other means, or any other reference is created, then any further use of this mutable reference is Undefined Behavior
-    }
-    .suggestion = use `addr_of!` instead to create a raw pointer
-    .suggestion_mut = use `addr_of_mut!` instead to create a raw pointer
-
-hir_analysis_static_mut_refs_lint = creating a {$shared} reference to mutable static is discouraged
-    .label = {$shared} reference to mutable static
-    .suggestion = use `addr_of!` instead to create a raw pointer
-    .suggestion_mut = use `addr_of_mut!` instead to create a raw pointer
-    .note = this will be a hard error in the 2024 edition
-    .why_note = {$shared ->
-        [shared] this shared reference has lifetime `'static`, but if the static ever gets mutated, or a mutable reference is created, then any further use of this shared reference is Undefined Behavior
-        *[mutable] this mutable reference has lifetime `'static`, but if the static gets accessed (read or written) by any other means, or any other reference is created, then any further use of this mutable reference is Undefined Behavior
-    }
-
 hir_analysis_static_specialize = cannot specialize on `'static` lifetime
 
 hir_analysis_tait_forward_compat = item constrains opaque type that is not in its signature

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -66,7 +66,6 @@ mod check;
 mod compare_impl_item;
 pub mod dropck;
 mod entry;
-mod errs;
 pub mod intrinsic;
 pub mod intrinsicck;
 mod region;

compiler/rustc_hir_analysis/src/check/region.rs

@@ -20,8 +20,6 @@ use rustc_middle::ty::TyCtxt;
 use rustc_span::source_map;
 use tracing::debug;
 
-use super::errs::{maybe_expr_static_mut, maybe_stmt_static_mut};
-
 #[derive(Debug, Copy, Clone)]
 struct Context {
     /// The scope that contains any new variables declared, plus its depth in
@@ -229,8 +227,6 @@ fn resolve_stmt<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, stmt: &'tcx h
     let stmt_id = stmt.hir_id.local_id;
     debug!("resolve_stmt(stmt.id={:?})", stmt_id);
 
-    maybe_stmt_static_mut(visitor.tcx, *stmt);
-
     // Every statement will clean up the temporaries created during
     // execution of that statement. Therefore each statement has an
     // associated destruction scope that represents the scope of the
@@ -249,8 +245,6 @@ fn resolve_stmt<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, stmt: &'tcx h
 fn resolve_expr<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, expr: &'tcx hir::Expr<'tcx>) {
     debug!("resolve_expr - pre-increment {} expr = {:?}", visitor.expr_and_pat_count, expr);
 
-    maybe_expr_static_mut(visitor.tcx, *expr);
-
     let prev_cx = visitor.cx;
     visitor.enter_node_scope_with_dtor(expr.hir_id.local_id);
 

compiler/rustc_hir_analysis/src/errors.rs

@@ -1522,57 +1522,6 @@ pub(crate) struct OnlyCurrentTraitsPointerSugg<'a> {
     pub ptr_ty: Ty<'a>,
 }
 
-#[derive(Diagnostic)]
-#[diag(hir_analysis_static_mut_ref, code = E0796)]
-#[note]
-pub(crate) struct StaticMutRef<'a> {
-    #[primary_span]
-    #[label]
-    pub span: Span,
-    #[subdiagnostic]
-    pub sugg: MutRefSugg,
-    pub shared: &'a str,
-}
-
-#[derive(Subdiagnostic)]
-pub(crate) enum MutRefSugg {
-    #[multipart_suggestion(
-        hir_analysis_suggestion,
-        style = "verbose",
-        applicability = "maybe-incorrect"
-    )]
-    Shared {
-        #[suggestion_part(code = "addr_of!(")]
-        lo: Span,
-        #[suggestion_part(code = ")")]
-        hi: Span,
-    },
-    #[multipart_suggestion(
-        hir_analysis_suggestion_mut,
-        style = "verbose",
-        applicability = "maybe-incorrect"
-    )]
-    Mut {
-        #[suggestion_part(code = "addr_of_mut!(")]
-        lo: Span,
-        #[suggestion_part(code = ")")]
-        hi: Span,
-    },
-}
-
-// STATIC_MUT_REF lint
-#[derive(LintDiagnostic)]
-#[diag(hir_analysis_static_mut_refs_lint)]
-#[note]
-#[note(hir_analysis_why_note)]
-pub(crate) struct RefOfMutStatic<'a> {
-    #[label]
-    pub span: Span,
-    #[subdiagnostic]
-    pub sugg: MutRefSugg,
-    pub shared: &'a str,
-}
-
 #[derive(Diagnostic)]
 #[diag(hir_analysis_not_supported_delegation)]
 pub(crate) struct UnsupportedDelegation<'a> {

compiler/rustc_lint/messages.ftl

@@ -769,6 +769,13 @@ lint_single_use_lifetime = lifetime parameter `{$ident}` only used once
 
 lint_span_use_eq_ctxt = use `.eq_ctxt()` instead of `.ctxt() == .ctxt()`
 
+lint_static_mut_refs_lint = creating a {$shared_label}reference to mutable static is discouraged
+    .label = {$shared_label}reference to mutable static
+    .suggestion = use `&raw const` instead to create a raw pointer
+    .suggestion_mut = use `&raw mut` instead to create a raw pointer
+    .shared_note = shared references to mutable statics are dangerous; it's undefined behavior if the static is mutated or if a mutable reference is created for it while the shared reference lives
+    .mut_note = mutable references to mutable statics are dangerous; it's undefined behavior if any other pointer to the static is used or if any other reference is created for the static while the mutable reference lives
+
 lint_supertrait_as_deref_target = this `Deref` implementation is covered by an implicit supertrait coercion
     .label = `{$self_ty}` implements `Deref<Target = dyn {$target_principal}>` which conflicts with supertrait `{$supertrait_principal}`
     .label2 = target type is a supertrait of `{$self_ty}`

compiler/rustc_lint/src/lib.rs

@@ -81,6 +81,7 @@ mod ptr_nulls;
 mod redundant_semicolon;
 mod reference_casting;
 mod shadowed_into_iter;
+mod static_mut_refs;
 mod tail_expr_drop_order;
 mod traits;
 mod types;
@@ -120,6 +121,7 @@ use rustc_middle::query::Providers;
 use rustc_middle::ty::TyCtxt;
 use shadowed_into_iter::ShadowedIntoIter;
 pub use shadowed_into_iter::{ARRAY_INTO_ITER, BOXED_SLICE_INTO_ITER};
+use static_mut_refs::*;
 use tail_expr_drop_order::TailExprDropOrder;
 use traits::*;
 use types::*;
@@ -246,6 +248,7 @@ late_lint_methods!(
             ImplTraitOvercaptures: ImplTraitOvercaptures,
             TailExprDropOrder: TailExprDropOrder,
             IfLetRescope: IfLetRescope::default(),
+            StaticMutRefs: StaticMutRefs,
         ]
     ]
 );

compiler/rustc_lint/src/lints.rs

@@ -3060,3 +3060,35 @@ pub(crate) struct UnsafeAttrOutsideUnsafeSuggestion {
 pub(crate) struct OutOfScopeMacroCalls {
     pub path: String,
 }
+
+#[derive(LintDiagnostic)]
+#[diag(lint_static_mut_refs_lint)]
+pub(crate) struct RefOfMutStatic<'a> {
+    #[label]
+    pub span: Span,
+    #[subdiagnostic]
+    pub sugg: Option<MutRefSugg>,
+    pub shared_label: &'a str,
+    #[note(lint_shared_note)]
+    pub shared_note: bool,
+    #[note(lint_mut_note)]
+    pub mut_note: bool,
+}
+
+#[derive(Subdiagnostic)]
+pub(crate) enum MutRefSugg {
+    #[multipart_suggestion(lint_suggestion, style = "verbose", applicability = "maybe-incorrect")]
+    Shared {
+        #[suggestion_part(code = "&raw const ")]
+        span: Span,
+    },
+    #[multipart_suggestion(
+        lint_suggestion_mut,
+        style = "verbose",
+        applicability = "maybe-incorrect"
+    )]
+    Mut {
+        #[suggestion_part(code = "&raw mut ")]
+        span: Span,
+    },
+}

compiler/rustc_lint/src/static_mut_refs.rs

@@ -0,0 +1,154 @@
+use rustc_hir as hir;
+use rustc_hir::{Expr, Stmt};
+use rustc_middle::ty::{Mutability, TyKind};
+use rustc_session::lint::FutureIncompatibilityReason;
+use rustc_session::{declare_lint, declare_lint_pass};
+use rustc_span::edition::Edition;
+use rustc_span::Span;
+
+use crate::lints::{MutRefSugg, RefOfMutStatic};
+use crate::{LateContext, LateLintPass, LintContext};
+
+declare_lint! {
+    /// The `static_mut_refs` lint checks for shared or mutable references
+    /// of mutable static inside `unsafe` blocks and `unsafe` functions.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,edition2021
+    /// fn main() {
+    ///     static mut X: i32 = 23;
+    ///     static mut Y: i32 = 24;
+    ///
+    ///     unsafe {
+    ///         let y = &X;
+    ///         let ref x = X;
+    ///         let (x, y) = (&X, &Y);
+    ///         foo(&X);
+    ///     }
+    /// }
+    ///
+    /// unsafe fn _foo() {
+    ///     static mut X: i32 = 23;
+    ///     static mut Y: i32 = 24;
+    ///
+    ///     let y = &X;
+    ///     let ref x = X;
+    ///     let (x, y) = (&X, &Y);
+    ///     foo(&X);
+    /// }
+    ///
+    /// fn foo<'a>(_x: &'a i32) {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Shared or mutable references of mutable static are almost always a mistake and
+    /// can lead to undefined behavior and various other problems in your code.
+    ///
+    /// This lint is "warn" by default on editions up to 2021, in 2024 is "deny".
+    pub STATIC_MUT_REFS,
+    Warn,
+    "shared references or mutable references of mutable static is discouraged",
+    @future_incompatible = FutureIncompatibleInfo {
+        reason: FutureIncompatibilityReason::EditionError(Edition::Edition2024),
+        reference: "<https://doc.rust-lang.org/nightly/edition-guide/rust-2024/static-mut-references.html>",
+        explain_reason: false,
+    };
+    @edition Edition2024 => Deny;
+}
+
+declare_lint_pass!(StaticMutRefs => [STATIC_MUT_REFS]);
+
+impl<'tcx> LateLintPass<'tcx> for StaticMutRefs {
+    #[allow(rustc::usage_of_ty_tykind)]
+    fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &Expr<'_>) {
+        let err_span = expr.span;
+        match expr.kind {
+            hir::ExprKind::AddrOf(borrow_kind, m, ex)
+                if matches!(borrow_kind, hir::BorrowKind::Ref)
+                    && let Some(err_span) = path_is_static_mut(ex, err_span) =>
+            {
+                emit_static_mut_refs(
+                    cx,
+                    err_span,
+                    err_span.with_hi(ex.span.lo()),
+                    m,
+                    !expr.span.from_expansion(),
+                );
+            }
+            hir::ExprKind::MethodCall(_, e, _, _)
+                if let Some(err_span) = path_is_static_mut(e, expr.span)
+                    && let typeck = cx.typeck_results()
+                    && let Some(method_def_id) = typeck.type_dependent_def_id(expr.hir_id)
+                    && let inputs =
+                        cx.tcx.fn_sig(method_def_id).skip_binder().inputs().skip_binder()
+                    && let Some(receiver) = inputs.get(0)
+                    && let TyKind::Ref(_, _, m) = receiver.kind() =>
+            {
+                emit_static_mut_refs(cx, err_span, err_span.shrink_to_lo(), *m, false);
+            }
+            _ => {}
+        }
+    }
+
+    fn check_stmt(&mut self, cx: &LateContext<'tcx>, stmt: &Stmt<'_>) {
+        if let hir::StmtKind::Let(loc) = stmt.kind
+            && let hir::PatKind::Binding(ba, _, _, _) = loc.pat.kind
+            && let hir::ByRef::Yes(m) = ba.0
+            && let Some(init) = loc.init
+            && let Some(err_span) = path_is_static_mut(init, init.span)
+        {
+            emit_static_mut_refs(cx, err_span, err_span.shrink_to_lo(), m, false);
+        }
+    }
+}
+
+fn path_is_static_mut(mut expr: &hir::Expr<'_>, mut err_span: Span) -> Option<Span> {
+    if err_span.from_expansion() {
+        err_span = expr.span;
+    }
+
+    while let hir::ExprKind::Field(e, _) = expr.kind {
+        expr = e;
+    }
+
+    if let hir::ExprKind::Path(qpath) = expr.kind
+        && let hir::QPath::Resolved(_, path) = qpath
+        && let hir::def::Res::Def(def_kind, _) = path.res
+        && let hir::def::DefKind::Static { safety: _, mutability: Mutability::Mut, nested: false } =
+            def_kind
+    {
+        return Some(err_span);
+    }
+    None
+}
+
+fn emit_static_mut_refs(
+    cx: &LateContext<'_>,
+    span: Span,
+    sugg_span: Span,
+    mutable: Mutability,
+    suggest_addr_of: bool,
+) {
+    let (shared_label, shared_note, mut_note, sugg) = match mutable {
+        Mutability::Mut => {
+            let sugg =
+                if suggest_addr_of { Some(MutRefSugg::Mut { span: sugg_span }) } else { None };
+            ("mutable ", false, true, sugg)
+        }
+        Mutability::Not => {
+            let sugg =
+                if suggest_addr_of { Some(MutRefSugg::Shared { span: sugg_span }) } else { None };
+            ("shared ", true, false, sugg)
+        }
+    };
+
+    cx.emit_span_lint(
+        STATIC_MUT_REFS,
+        span,
+        RefOfMutStatic { span, sugg, shared_label, shared_note, mut_note },
+    );
+}