| use rustc_errors::{DiagnosticBuilder, DiagnosticMessage}; |
| use rustc_middle::mir::visit::Visitor; |
| use rustc_middle::mir::*; |
| use rustc_middle::ty::TyCtxt; |
| use rustc_session::lint::builtin::CONST_ITEM_MUTATION; |
| use rustc_span::def_id::DefId; |
| |
| use crate::MirLint; |
| |
| pub struct CheckConstItemMutation; |
| |
| impl<'tcx> MirLint<'tcx> for CheckConstItemMutation { |
| fn run_lint(&self, tcx: TyCtxt<'tcx>, body: &Body<'tcx>) { |
| let mut checker = ConstMutationChecker { body, tcx, target_local: None }; |
| checker.visit_body(&body); |
| } |
| } |
| |
| struct ConstMutationChecker<'a, 'tcx> { |
| body: &'a Body<'tcx>, |
| tcx: TyCtxt<'tcx>, |
| target_local: Option<Local>, |
| } |
| |
| impl<'tcx> ConstMutationChecker<'_, 'tcx> { |
| fn is_const_item(&self, local: Local) -> Option<DefId> { |
| if let Some(box LocalInfo::ConstRef { def_id }) = self.body.local_decls[local].local_info { |
| Some(def_id) |
| } else { |
| None |
| } |
| } |
| |
| fn is_const_item_without_destructor(&self, local: Local) -> Option<DefId> { |
| let def_id = self.is_const_item(local)?; |
| |
| // We avoid linting mutation of a const item if the const's type has a |
| // Drop impl. The Drop logic observes the mutation which was performed. |
| // |
| // pub struct Log { msg: &'static str } |
| // pub const LOG: Log = Log { msg: "" }; |
| // impl Drop for Log { |
| // fn drop(&mut self) { println!("{}", self.msg); } |
| // } |
| // |
| // LOG.msg = "wow"; // prints "wow" |
| // |
| // FIXME(https://github.com/rust-lang/rust/issues/77425): |
| // Drop this exception once there is a stable attribute to suppress the |
| // const item mutation lint for a single specific const only. Something |
| // equivalent to: |
| // |
| // #[const_mutation_allowed] |
| // pub const LOG: Log = Log { msg: "" }; |
| match self.tcx.calculate_dtor(def_id, |_, _| Ok(())) { |
| Some(_) => None, |
| None => Some(def_id), |
| } |
| } |
| |
| fn lint_const_item_usage( |
| &self, |
| place: &Place<'tcx>, |
| const_item: DefId, |
| location: Location, |
| msg: impl Into<DiagnosticMessage>, |
| decorate: impl for<'a, 'b> FnOnce( |
| &'a mut DiagnosticBuilder<'b, ()>, |
| ) -> &'a mut DiagnosticBuilder<'b, ()>, |
| ) { |
| // Don't lint on borrowing/assigning when a dereference is involved. |
| // If we 'leave' the temporary via a dereference, we must |
| // be modifying something else |
| // |
| // `unsafe { *FOO = 0; *BAR.field = 1; }` |
| // `unsafe { &mut *FOO }` |
| // `unsafe { (*ARRAY)[0] = val; } |
| if !place.projection.iter().any(|p| matches!(p, PlaceElem::Deref)) { |
| let source_info = self.body.source_info(location); |
| let lint_root = self.body.source_scopes[source_info.scope] |
| .local_data |
| .as_ref() |
| .assert_crate_local() |
| .lint_root; |
| |
| self.tcx.struct_span_lint_hir( |
| CONST_ITEM_MUTATION, |
| lint_root, |
| source_info.span, |
| msg, |
| |lint| { |
| decorate(lint) |
| .span_note(self.tcx.def_span(const_item), "`const` item defined here") |
| }, |
| ); |
| } |
| } |
| } |
| |
| impl<'tcx> Visitor<'tcx> for ConstMutationChecker<'_, 'tcx> { |
| fn visit_statement(&mut self, stmt: &Statement<'tcx>, loc: Location) { |
| if let StatementKind::Assign(box (lhs, _)) = &stmt.kind { |
| // Check for assignment to fields of a constant |
| // Assigning directly to a constant (e.g. `FOO = true;`) is a hard error, |
| // so emitting a lint would be redundant. |
| if !lhs.projection.is_empty() { |
| if let Some(def_id) = self.is_const_item_without_destructor(lhs.local) { |
| self.lint_const_item_usage(&lhs, def_id, loc, "attempting to modify a `const` item",|lint| { |
| lint.note("each usage of a `const` item creates a new temporary; the original `const` item will not be modified") |
| }) |
| } |
| } |
| // We are looking for MIR of the form: |
| // |
| // ``` |
| // _1 = const FOO; |
| // _2 = &mut _1; |
| // method_call(_2, ..) |
| // ``` |
| // |
| // Record our current LHS, so that we can detect this |
| // pattern in `visit_rvalue` |
| self.target_local = lhs.as_local(); |
| } |
| self.super_statement(stmt, loc); |
| self.target_local = None; |
| } |
| fn visit_rvalue(&mut self, rvalue: &Rvalue<'tcx>, loc: Location) { |
| if let Rvalue::Ref(_, BorrowKind::Mut { .. }, place) = rvalue { |
| let local = place.local; |
| if let Some(def_id) = self.is_const_item(local) { |
| // If this Rvalue is being used as the right-hand side of a |
| // `StatementKind::Assign`, see if it ends up getting used as |
| // the `self` parameter of a method call (as the terminator of our current |
| // BasicBlock). If so, we emit a more specific lint. |
| let method_did = self.target_local.and_then(|target_local| { |
| crate::util::find_self_call(self.tcx, &self.body, target_local, loc.block) |
| }); |
| let lint_loc = |
| if method_did.is_some() { self.body.terminator_loc(loc.block) } else { loc }; |
| self.lint_const_item_usage(place, def_id, lint_loc, "taking a mutable reference to a `const` item", |lint| { |
| lint |
| .note("each usage of a `const` item creates a new temporary") |
| .note("the mutable reference will refer to this temporary, not the original `const` item"); |
| |
| if let Some((method_did, _substs)) = method_did { |
| lint.span_note(self.tcx.def_span(method_did), "mutable reference created due to call to this method"); |
| } |
| |
| lint |
| }); |
| } |
| } |
| self.super_rvalue(rvalue, loc); |
| } |
| } |
