diff options
Diffstat (limited to 'compiler/rustc_mir_transform/src/check_const_item_mutation.rs')
| -rw-r--r-- | compiler/rustc_mir_transform/src/check_const_item_mutation.rs | 156 |
1 files changed, 156 insertions, 0 deletions
diff --git a/compiler/rustc_mir_transform/src/check_const_item_mutation.rs b/compiler/rustc_mir_transform/src/check_const_item_mutation.rs new file mode 100644 index 000000000..8838b14c5 --- /dev/null +++ b/compiler/rustc_mir_transform/src/check_const_item_mutation.rs @@ -0,0 +1,156 @@ +use rustc_errors::{DiagnosticBuilder, LintDiagnosticBuilder}; +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, + decorate: impl for<'b> FnOnce(LintDiagnosticBuilder<'b, ()>) -> 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, + |lint| { + decorate(lint) + .span_note(self.tcx.def_span(const_item), "`const` item defined here") + .emit(); + }, + ); + } + } +} + +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, |lint| { + let mut lint = lint.build("attempting to modify a `const` item"); + lint.note("each usage of a `const` item creates a new temporary; the original `const` item will not be modified"); + lint + }) + } + } + // 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, |lint| { + let mut lint = lint.build("taking a mutable reference to a `const` item"); + 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); + } +} |