1 //! This pass checks HIR bodies that may be evaluated at compile-time (e.g., `const`, `static`,
2 //! `const fn`) for structured control flow (e.g. `if`, `while`), which is forbidden in a const
5 //! By the time the MIR const-checker runs, these high-level constructs have been lowered to
6 //! control-flow primitives (e.g., `Goto`, `SwitchInt`), making it tough to properly attribute
7 //! errors. We still look for those primitives in the MIR const-checker to ensure nothing slips
8 //! through, but errors for structured control flow in a `const` should be emitted here.
10 use rustc_attr as attr;
11 use rustc_data_structures::stable_set::FxHashSet;
12 use rustc_errors::struct_span_err;
14 use rustc_hir::def_id::LocalDefId;
15 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
16 use rustc_middle::hir::map::Map;
18 use rustc_middle::ty::query::Providers;
19 use rustc_middle::ty::TyCtxt;
20 use rustc_session::parse::feature_err;
21 use rustc_span::{sym, Span, Symbol};
23 /// An expression that is not *always* legal in a const context.
24 #[derive(Clone, Copy)]
26 Loop(hir::LoopSource),
27 Match(hir::MatchSource),
31 fn name(self) -> String {
33 Self::Loop(src) => format!("`{}`", src.name()),
34 Self::Match(src) => format!("`{}`", src.name()),
38 fn required_feature_gates(self) -> Option<&'static [Symbol]> {
39 use hir::LoopSource::*;
40 use hir::MatchSource::*;
42 let gates: &[_] = match self {
43 Self::Match(AwaitDesugar) => {
47 Self::Loop(ForLoop) | Self::Match(ForLoopDesugar) => &[sym::const_for],
49 Self::Match(TryDesugar) => &[sym::const_try],
51 // All other expressions are allowed.
52 Self::Loop(Loop | While) | Self::Match(Normal) => &[],
59 fn check_mod_const_bodies(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
60 let mut vis = CheckConstVisitor::new(tcx);
61 tcx.hir().visit_item_likes_in_module(module_def_id, &mut vis.as_deep_visitor());
62 tcx.hir().visit_item_likes_in_module(module_def_id, &mut CheckConstTraitVisitor::new(tcx));
65 pub(crate) fn provide(providers: &mut Providers) {
66 *providers = Providers { check_mod_const_bodies, ..*providers };
69 struct CheckConstTraitVisitor<'tcx> {
73 impl<'tcx> CheckConstTraitVisitor<'tcx> {
74 fn new(tcx: TyCtxt<'tcx>) -> Self {
75 CheckConstTraitVisitor { tcx }
79 impl<'tcx> hir::itemlikevisit::ItemLikeVisitor<'tcx> for CheckConstTraitVisitor<'tcx> {
80 /// check for const trait impls, and errors if the impl uses provided/default functions
81 /// of the trait being implemented; as those provided functions can be non-const.
82 fn visit_item(&mut self, item: &'hir hir::Item<'hir>) {
83 let _: Option<_> = try {
84 if let hir::ItemKind::Impl(ref imp) = item.kind {
85 if let hir::Constness::Const = imp.constness {
86 let did = imp.of_trait.as_ref()?.trait_def_id()?;
87 let mut to_implement = FxHashSet::default();
89 for did in self.tcx.associated_item_def_ids(did) {
90 if let ty::AssocItem {
91 kind: ty::AssocKind::Fn, ident, defaultness, ..
92 } = self.tcx.associated_item(*did)
94 // we can ignore functions that do not have default bodies:
95 // if those are unimplemented it will be catched by typeck.
96 if defaultness.has_value()
97 && !self.tcx.has_attr(*did, sym::default_method_body_is_const)
99 to_implement.insert(ident);
107 .filter(|it| matches!(it.kind, hir::AssocItemKind::Fn { .. }))
109 to_implement.remove(&it.ident);
112 // all nonconst trait functions (not marked with #[default_method_body_is_const])
113 // must be implemented
114 if !to_implement.is_empty() {
119 "const trait implementations may not use non-const default functions",
121 .note(&format!("`{}` not implemented", to_implement.into_iter().map(|id| id.to_string()).collect::<Vec<_>>().join("`, `")))
129 fn visit_trait_item(&mut self, _: &'hir hir::TraitItem<'hir>) {}
131 fn visit_impl_item(&mut self, _: &'hir hir::ImplItem<'hir>) {}
133 fn visit_foreign_item(&mut self, _: &'hir hir::ForeignItem<'hir>) {}
136 #[derive(Copy, Clone)]
137 struct CheckConstVisitor<'tcx> {
139 const_kind: Option<hir::ConstContext>,
140 def_id: Option<LocalDefId>,
143 impl<'tcx> CheckConstVisitor<'tcx> {
144 fn new(tcx: TyCtxt<'tcx>) -> Self {
145 CheckConstVisitor { tcx, const_kind: None, def_id: None }
148 /// Emits an error when an unsupported expression is found in a const context.
149 fn const_check_violated(&self, expr: NonConstExpr, span: Span) {
150 let Self { tcx, def_id, const_kind } = *self;
152 let features = tcx.features();
153 let required_gates = expr.required_feature_gates();
155 let is_feature_allowed = |feature_gate| {
156 // All features require that the corresponding gate be enabled,
157 // even if the function has `#[rustc_allow_const_fn_unstable(the_gate)]`.
158 if !tcx.features().enabled(feature_gate) {
162 // If `def_id` is `None`, we don't need to consider stability attributes.
163 let def_id = match def_id {
164 Some(x) => x.to_def_id(),
168 // If this crate is not using stability attributes, or this function is not claiming to be a
169 // stable `const fn`, that is all that is required.
170 if !tcx.features().staged_api || tcx.has_attr(def_id, sym::rustc_const_unstable) {
174 // However, we cannot allow stable `const fn`s to use unstable features without an explicit
175 // opt-in via `rustc_allow_const_fn_unstable`.
176 attr::rustc_allow_const_fn_unstable(&tcx.sess, &tcx.get_attrs(def_id))
177 .any(|name| name == feature_gate)
180 match required_gates {
181 // Don't emit an error if the user has enabled the requisite feature gates.
182 Some(gates) if gates.iter().copied().all(is_feature_allowed) => return,
184 // `-Zunleash-the-miri-inside-of-you` only works for expressions that don't have a
185 // corresponding feature gate. This encourages nightly users to use feature gates when
187 None if tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you => {
188 tcx.sess.span_warn(span, "skipping const checks");
196 const_kind.expect("`const_check_violated` may only be called inside a const context");
198 let msg = format!("{} is not allowed in a `{}`", expr.name(), const_kind.keyword_name());
200 let required_gates = required_gates.unwrap_or(&[]);
201 let missing_gates: Vec<_> =
202 required_gates.iter().copied().filter(|&g| !features.enabled(g)).collect();
204 match missing_gates.as_slice() {
205 &[] => struct_span_err!(tcx.sess, span, E0744, "{}", msg).emit(),
207 &[missing_primary, ref missing_secondary @ ..] => {
208 let mut err = feature_err(&tcx.sess.parse_sess, missing_primary, span, &msg);
210 // If multiple feature gates would be required to enable this expression, include
211 // them as help messages. Don't emit a separate error for each missing feature gate.
213 // FIXME(ecstaticmorse): Maybe this could be incorporated into `feature_err`? This
214 // is a pretty narrow case, however.
215 if tcx.sess.is_nightly_build() {
216 for gate in missing_secondary {
218 "add `#![feature({})]` to the crate attributes to enable",
230 /// Saves the parent `const_kind` before calling `f` and restores it afterwards.
233 kind: Option<hir::ConstContext>,
234 def_id: Option<LocalDefId>,
235 f: impl FnOnce(&mut Self),
237 let parent_def_id = self.def_id;
238 let parent_kind = self.const_kind;
239 self.def_id = def_id;
240 self.const_kind = kind;
242 self.def_id = parent_def_id;
243 self.const_kind = parent_kind;
247 impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
248 type Map = Map<'tcx>;
250 fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<Self::Map> {
251 NestedVisitorMap::OnlyBodies(self.tcx.hir())
254 fn visit_anon_const(&mut self, anon: &'tcx hir::AnonConst) {
255 let kind = Some(hir::ConstContext::Const);
256 self.recurse_into(kind, None, |this| intravisit::walk_anon_const(this, anon));
259 fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
260 let owner = self.tcx.hir().body_owner_def_id(body.id());
261 let kind = self.tcx.hir().body_const_context(owner);
262 self.recurse_into(kind, Some(owner), |this| intravisit::walk_body(this, body));
265 fn visit_expr(&mut self, e: &'tcx hir::Expr<'tcx>) {
267 // Skip the following checks if we are not currently in a const context.
268 _ if self.const_kind.is_none() => {}
270 hir::ExprKind::Loop(_, _, source, _) => {
271 self.const_check_violated(NonConstExpr::Loop(*source), e.span);
274 hir::ExprKind::Match(_, _, source) => {
275 let non_const_expr = match source {
276 // These are handled by `ExprKind::Loop` above.
277 hir::MatchSource::ForLoopDesugar => None,
279 _ => Some(NonConstExpr::Match(*source)),
282 if let Some(expr) = non_const_expr {
283 self.const_check_violated(expr, e.span);
290 intravisit::walk_expr(self, e);