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_errors::struct_span_err;
13 use rustc_hir::def_id::LocalDefId;
14 use rustc_hir::intravisit::{self, Visitor};
15 use rustc_middle::hir::nested_filter;
17 use rustc_middle::ty::query::Providers;
18 use rustc_middle::ty::TyCtxt;
19 use rustc_session::parse::feature_err;
20 use rustc_span::{sym, Span, Symbol};
22 /// An expression that is not *always* legal in a const context.
23 #[derive(Clone, Copy)]
25 Loop(hir::LoopSource),
26 Match(hir::MatchSource),
30 fn name(self) -> String {
32 Self::Loop(src) => format!("`{}`", src.name()),
33 Self::Match(src) => format!("`{}`", src.name()),
37 fn required_feature_gates(self) -> Option<&'static [Symbol]> {
38 use hir::LoopSource::*;
39 use hir::MatchSource::*;
41 let gates: &[_] = match self {
42 Self::Match(AwaitDesugar) => {
46 Self::Loop(ForLoop) | Self::Match(ForLoopDesugar) => &[sym::const_for],
48 Self::Match(TryDesugar) => &[sym::const_try],
50 // All other expressions are allowed.
51 Self::Loop(Loop | While) | Self::Match(Normal) => &[],
58 fn check_mod_const_bodies(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
59 let mut vis = CheckConstVisitor::new(tcx);
60 tcx.hir().visit_item_likes_in_module(module_def_id, &mut vis.as_deep_visitor());
61 tcx.hir().visit_item_likes_in_module(module_def_id, &mut CheckConstTraitVisitor::new(tcx));
64 pub(crate) fn provide(providers: &mut Providers) {
65 *providers = Providers { check_mod_const_bodies, ..*providers };
68 struct CheckConstTraitVisitor<'tcx> {
72 impl<'tcx> CheckConstTraitVisitor<'tcx> {
73 fn new(tcx: TyCtxt<'tcx>) -> Self {
74 CheckConstTraitVisitor { tcx }
78 impl<'tcx> hir::itemlikevisit::ItemLikeVisitor<'tcx> for CheckConstTraitVisitor<'tcx> {
79 /// check for const trait impls, and errors if the impl uses provided/default functions
80 /// of the trait being implemented; as those provided functions can be non-const.
81 fn visit_item<'hir>(&mut self, item: &'hir hir::Item<'hir>) {
82 let _: Option<_> = try {
83 if let hir::ItemKind::Impl(ref imp) = item.kind && let hir::Constness::Const = imp.constness {
84 let trait_def_id = imp.of_trait.as_ref()?.trait_def_id()?;
87 .trait_def(trait_def_id)
88 .ancestors(self.tcx, item.def_id.to_def_id())
90 let mut to_implement = Vec::new();
92 for trait_item in self.tcx.associated_items(trait_def_id).in_definition_order()
94 if let ty::AssocItem {
95 kind: ty::AssocKind::Fn,
97 def_id: trait_item_id,
101 // we can ignore functions that do not have default bodies:
102 // if those are unimplemented it will be catched by typeck.
103 if !defaultness.has_value()
106 .has_attr(trait_item_id, sym::default_method_body_is_const)
111 let is_implemented = ancestors
112 .leaf_def(self.tcx, trait_item_id)
113 .map(|node_item| !node_item.defining_node.is_from_trait())
117 to_implement.push(self.tcx.item_name(trait_item_id).to_string());
122 // all nonconst trait functions (not marked with #[default_method_body_is_const])
123 // must be implemented
124 if !to_implement.is_empty() {
129 "const trait implementations may not use non-const default functions",
131 .note(&format!("`{}` not implemented", to_implement.join("`, `")))
138 fn visit_trait_item<'hir>(&mut self, _: &'hir hir::TraitItem<'hir>) {}
140 fn visit_impl_item<'hir>(&mut self, _: &'hir hir::ImplItem<'hir>) {}
142 fn visit_foreign_item<'hir>(&mut self, _: &'hir hir::ForeignItem<'hir>) {}
145 #[derive(Copy, Clone)]
146 struct CheckConstVisitor<'tcx> {
148 const_kind: Option<hir::ConstContext>,
149 def_id: Option<LocalDefId>,
152 impl<'tcx> CheckConstVisitor<'tcx> {
153 fn new(tcx: TyCtxt<'tcx>) -> Self {
154 CheckConstVisitor { tcx, const_kind: None, def_id: None }
157 /// Emits an error when an unsupported expression is found in a const context.
158 fn const_check_violated(&self, expr: NonConstExpr, span: Span) {
159 let Self { tcx, def_id, const_kind } = *self;
161 let features = tcx.features();
162 let required_gates = expr.required_feature_gates();
164 let is_feature_allowed = |feature_gate| {
165 // All features require that the corresponding gate be enabled,
166 // even if the function has `#[rustc_allow_const_fn_unstable(the_gate)]`.
167 if !tcx.features().enabled(feature_gate) {
171 // If `def_id` is `None`, we don't need to consider stability attributes.
172 let def_id = match def_id {
173 Some(x) => x.to_def_id(),
177 // If the function belongs to a trait, then it must enable the const_trait_impl
178 // feature to use that trait function (with a const default body).
179 if tcx.trait_of_item(def_id).is_some() {
183 // If this crate is not using stability attributes, or this function is not claiming to be a
184 // stable `const fn`, that is all that is required.
185 if !tcx.features().staged_api || tcx.has_attr(def_id, sym::rustc_const_unstable) {
189 // However, we cannot allow stable `const fn`s to use unstable features without an explicit
190 // opt-in via `rustc_allow_const_fn_unstable`.
191 attr::rustc_allow_const_fn_unstable(&tcx.sess, &tcx.get_attrs(def_id))
192 .any(|name| name == feature_gate)
195 match required_gates {
196 // Don't emit an error if the user has enabled the requisite feature gates.
197 Some(gates) if gates.iter().copied().all(is_feature_allowed) => return,
199 // `-Zunleash-the-miri-inside-of-you` only works for expressions that don't have a
200 // corresponding feature gate. This encourages nightly users to use feature gates when
202 None if tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you => {
203 tcx.sess.span_warn(span, "skipping const checks");
211 const_kind.expect("`const_check_violated` may only be called inside a const context");
213 let msg = format!("{} is not allowed in a `{}`", expr.name(), const_kind.keyword_name());
215 let required_gates = required_gates.unwrap_or(&[]);
216 let missing_gates: Vec<_> =
217 required_gates.iter().copied().filter(|&g| !features.enabled(g)).collect();
219 match missing_gates.as_slice() {
221 struct_span_err!(tcx.sess, span, E0744, "{}", msg).emit();
224 [missing_primary, ref missing_secondary @ ..] => {
225 let mut err = feature_err(&tcx.sess.parse_sess, *missing_primary, span, &msg);
227 // If multiple feature gates would be required to enable this expression, include
228 // them as help messages. Don't emit a separate error for each missing feature gate.
230 // FIXME(ecstaticmorse): Maybe this could be incorporated into `feature_err`? This
231 // is a pretty narrow case, however.
232 if tcx.sess.is_nightly_build() {
233 for gate in missing_secondary {
235 "add `#![feature({})]` to the crate attributes to enable",
247 /// Saves the parent `const_kind` before calling `f` and restores it afterwards.
250 kind: Option<hir::ConstContext>,
251 def_id: Option<LocalDefId>,
252 f: impl FnOnce(&mut Self),
254 let parent_def_id = self.def_id;
255 let parent_kind = self.const_kind;
256 self.def_id = def_id;
257 self.const_kind = kind;
259 self.def_id = parent_def_id;
260 self.const_kind = parent_kind;
264 impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
265 type NestedFilter = nested_filter::OnlyBodies;
267 fn nested_visit_map(&mut self) -> Self::Map {
271 fn visit_anon_const(&mut self, anon: &'tcx hir::AnonConst) {
272 let kind = Some(hir::ConstContext::Const);
273 self.recurse_into(kind, None, |this| intravisit::walk_anon_const(this, anon));
276 fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
277 let owner = self.tcx.hir().body_owner_def_id(body.id());
278 let kind = self.tcx.hir().body_const_context(owner);
279 self.recurse_into(kind, Some(owner), |this| intravisit::walk_body(this, body));
282 fn visit_expr(&mut self, e: &'tcx hir::Expr<'tcx>) {
284 // Skip the following checks if we are not currently in a const context.
285 _ if self.const_kind.is_none() => {}
287 hir::ExprKind::Loop(_, _, source, _) => {
288 self.const_check_violated(NonConstExpr::Loop(*source), e.span);
291 hir::ExprKind::Match(_, _, source) => {
292 let non_const_expr = match source {
293 // These are handled by `ExprKind::Loop` above.
294 hir::MatchSource::ForLoopDesugar => None,
296 _ => Some(NonConstExpr::Match(*source)),
299 if let Some(expr) = non_const_expr {
300 self.const_check_violated(expr, e.span);
307 intravisit::walk_expr(self, e);