Disallow reference to static mut for expressions

Add `E0796` error code.
Add `static_mut_ref` lint.

This is the idea for the 2024 edition.

compiler/rustc_hir_analysis/src/check/errs.rs

@@ -0,0 +1,78 @@
+use rustc_hir as hir;
+use rustc_hir_pretty::qpath_to_string;
+use rustc_lint_defs::builtin::STATIC_MUT_REF;
+use rustc_middle::ty::TyCtxt;
+use rustc_span::Span;
+use rustc_type_ir::Mutability;
+
+use crate::errors;
+
+/// Check for shared or mutable references of `static mut` inside expression
+pub fn maybe_expr_static_mut(tcx: TyCtxt<'_>, expr: hir::Expr<'_>) {
+    let span = expr.span;
+    let hir_id = expr.hir_id;
+    if let hir::ExprKind::AddrOf(borrow_kind, m, expr) = expr.kind
+        && matches!(borrow_kind, hir::BorrowKind::Ref)
+        && let Some(var) = is_path_static_mut(*expr)
+    {
+        handle_static_mut_ref(
+            tcx,
+            span,
+            var,
+            span.edition().at_least_rust_2024(),
+            matches!(m, Mutability::Mut),
+            hir_id,
+        );
+    }
+}
+
+fn is_path_static_mut(expr: hir::Expr<'_>) -> Option<String> {
+    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(mt) = def_kind
+        && matches!(mt, Mutability::Mut)
+    {
+        return Some(qpath_to_string(&qpath));
+    }
+    None
+}
+
+fn handle_static_mut_ref(
+    tcx: TyCtxt<'_>,
+    span: Span,
+    var: String,
+    e2024: bool,
+    mutable: bool,
+    hir_id: hir::HirId,
+) {
+    if e2024 {
+        let sugg = if mutable {
+            errors::StaticMutRefSugg::Mut { span, var }
+        } else {
+            errors::StaticMutRefSugg::Shared { span, var }
+        };
+        tcx.sess.parse_sess.dcx.emit_err(errors::StaticMutRef { span, sugg });
+        return;
+    }
+
+    let (label, sugg, shared) = if mutable {
+        (
+            errors::RefOfMutStaticLabel::Mut { span },
+            errors::RefOfMutStaticSugg::Mut { span, var },
+            "mutable ",
+        )
+    } else {
+        (
+            errors::RefOfMutStaticLabel::Shared { span },
+            errors::RefOfMutStaticSugg::Shared { span, var },
+            "shared ",
+        )
+    };
+    tcx.emit_spanned_lint(
+        STATIC_MUT_REF,
+        hir_id,
+        span,
+        errors::RefOfMutStatic { shared, why_note: (), label, sugg },
+    );
+}

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -66,6 +66,7 @@ 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/errors.rs

@@ -1410,3 +1410,94 @@ pub struct OnlyCurrentTraitsPointerSugg<'a> {
     pub mut_key: &'a str,
     pub ptr_ty: Ty<'a>,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_static_mut_ref, code = "E0796")]
+#[note]
+pub struct StaticMutRef {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    #[subdiagnostic]
+    pub sugg: StaticMutRefSugg,
+}
+
+#[derive(Subdiagnostic)]
+pub enum StaticMutRefSugg {
+    #[suggestion(
+        hir_analysis_suggestion,
+        style = "verbose",
+        code = "addr_of!({var})",
+        applicability = "maybe-incorrect"
+    )]
+    Shared {
+        #[primary_span]
+        span: Span,
+        var: String,
+    },
+    #[suggestion(
+        hir_analysis_suggestion_mut,
+        style = "verbose",
+        code = "addr_of_mut!({var})",
+        applicability = "maybe-incorrect"
+    )]
+    Mut {
+        #[primary_span]
+        span: Span,
+        var: String,
+    },
+}
+
+// STATIC_MUT_REF lint
+#[derive(LintDiagnostic)]
+#[diag(hir_analysis_static_mut_ref_lint)]
+#[note]
+pub struct RefOfMutStatic<'a> {
+    pub shared: &'a str,
+    #[note(hir_analysis_why_note)]
+    pub why_note: (),
+    #[subdiagnostic]
+    pub label: RefOfMutStaticLabel,
+    #[subdiagnostic]
+    pub sugg: RefOfMutStaticSugg,
+}
+
+#[derive(Subdiagnostic)]
+pub enum RefOfMutStaticLabel {
+    #[label(hir_analysis_label)]
+    Shared {
+        #[primary_span]
+        span: Span,
+    },
+    #[label(hir_analysis_label_mut)]
+    Mut {
+        #[primary_span]
+        span: Span,
+    },
+}
+
+#[derive(Subdiagnostic)]
+pub enum RefOfMutStaticSugg {
+    #[suggestion(
+        hir_analysis_suggestion,
+        style = "verbose",
+        code = "addr_of!({var})",
+        applicability = "maybe-incorrect"
+    )]
+    Shared {
+        #[primary_span]
+        span: Span,
+        var: String,
+    },
+    #[suggestion(
+        hir_analysis_suggestion_mut,
+        style = "verbose",
+        code = "addr_of_mut!({var})",
+        applicability = "maybe-incorrect"
+    )]
+    Mut {
+        #[primary_span]
+        span: Span,
+        var: String,
+    },
+}