1 use rustc_errors::DiagnosticBuilder;
2 use rustc_middle::lint::LintDiagnosticBuilder;
3 use rustc_middle::mir::visit::Visitor;
4 use rustc_middle::mir::*;
5 use rustc_middle::ty::TyCtxt;
6 use rustc_session::lint::builtin::CONST_ITEM_MUTATION;
7 use rustc_span::def_id::DefId;
9 use crate::transform::MirPass;
11 pub struct CheckConstItemMutation;
13 impl<'tcx> MirPass<'tcx> for CheckConstItemMutation {
14 fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
15 let mut checker = ConstMutationChecker { body, tcx, target_local: None };
16 checker.visit_body(&body);
20 struct ConstMutationChecker<'a, 'tcx> {
23 target_local: Option<Local>,
26 impl<'a, 'tcx> ConstMutationChecker<'a, 'tcx> {
27 fn is_const_item(&self, local: Local) -> Option<DefId> {
28 if let Some(box LocalInfo::ConstRef { def_id }) = self.body.local_decls[local].local_info {
35 fn is_const_item_without_destructor(&self, local: Local) -> Option<DefId> {
36 let def_id = self.is_const_item(local)?;
38 // We avoid linting mutation of a const item if the const's type has a
39 // Drop impl. The Drop logic observes the mutation which was performed.
41 // pub struct Log { msg: &'static str }
42 // pub const LOG: Log = Log { msg: "" };
43 // impl Drop for Log {
44 // fn drop(&mut self) { println!("{}", self.msg); }
47 // LOG.msg = "wow"; // prints "wow"
49 // FIXME(https://github.com/rust-lang/rust/issues/77425):
50 // Drop this exception once there is a stable attribute to suppress the
51 // const item mutation lint for a single specific const only. Something
54 // #[const_mutation_allowed]
55 // pub const LOG: Log = Log { msg: "" };
56 match self.tcx.calculate_dtor(def_id, |_, _| Ok(())) {
62 fn lint_const_item_usage(
67 decorate: impl for<'b> FnOnce(LintDiagnosticBuilder<'b>) -> DiagnosticBuilder<'b>,
69 // Don't lint on borrowing/assigning to a dereference
72 // `unsafe { *FOO = 0; *BAR.field = 1; }`
73 // `unsafe { &mut *FOO }`
74 if !matches!(place.projection.last(), Some(PlaceElem::Deref)) {
75 let source_info = self.body.source_info(location);
76 let lint_root = self.body.source_scopes[source_info.scope]
82 self.tcx.struct_span_lint_hir(
88 .span_note(self.tcx.def_span(const_item), "`const` item defined here")
96 impl<'a, 'tcx> Visitor<'tcx> for ConstMutationChecker<'a, 'tcx> {
97 fn visit_statement(&mut self, stmt: &Statement<'tcx>, loc: Location) {
98 if let StatementKind::Assign(box (lhs, _)) = &stmt.kind {
99 // Check for assignment to fields of a constant
100 // Assigning directly to a constant (e.g. `FOO = true;`) is a hard error,
101 // so emitting a lint would be redundant.
102 if !lhs.projection.is_empty() {
103 if let Some(def_id) = self.is_const_item_without_destructor(lhs.local) {
104 self.lint_const_item_usage(&lhs, def_id, loc, |lint| {
105 let mut lint = lint.build("attempting to modify a `const` item");
106 lint.note("each usage of a `const` item creates a new temporary; the original `const` item will not be modified");
111 // We are looking for MIR of the form:
116 // method_call(_2, ..)
119 // Record our current LHS, so that we can detect this
120 // pattern in `visit_rvalue`
121 self.target_local = lhs.as_local();
123 self.super_statement(stmt, loc);
124 self.target_local = None;
126 fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, loc: Location) {
127 if let Rvalue::Ref(_, BorrowKind::Mut { .. }, place) = rvalue {
128 let local = place.local;
129 if let Some(def_id) = self.is_const_item(local) {
130 // If this Rvalue is being used as the right-hand side of a
131 // `StatementKind::Assign`, see if it ends up getting used as
132 // the `self` parameter of a method call (as the terminator of our current
133 // BasicBlock). If so, we emit a more specific lint.
134 let method_did = self.target_local.and_then(|target_local| {
135 crate::util::find_self_call(self.tcx, &self.body, target_local, loc.block)
138 if method_did.is_some() { self.body.terminator_loc(loc.block) } else { loc };
139 self.lint_const_item_usage(place, def_id, lint_loc, |lint| {
140 let mut lint = lint.build("taking a mutable reference to a `const` item");
142 .note("each usage of a `const` item creates a new temporary")
143 .note("the mutable reference will refer to this temporary, not the original `const` item");
145 if let Some((method_did, _substs)) = method_did {
146 lint.span_note(self.tcx.def_span(method_did), "mutable reference created due to call to this method");
153 self.super_rvalue(rvalue, loc);