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