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, NestedVisitorMap, Visitor};
15 use rustc_middle::hir::map::Map;
16 use rustc_middle::ty::query::Providers;
17 use rustc_middle::ty::TyCtxt;
18 use rustc_session::config::nightly_options;
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 // A `for` loop's desugaring contains a call to `IntoIterator::into_iter`,
43 // so they are not yet allowed.
44 // Likewise, `?` desugars to a call to `Try::into_result`.
45 Self::Loop(ForLoop) | Self::Match(ForLoopDesugar | TryDesugar | AwaitDesugar) => {
49 // All other expressions are allowed.
50 Self::Loop(Loop | While | WhileLet)
52 WhileDesugar | WhileLetDesugar | Normal | IfDesugar { .. } | IfLetDesugar { .. },
60 fn check_mod_const_bodies(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
61 let mut vis = CheckConstVisitor::new(tcx);
62 tcx.hir().visit_item_likes_in_module(module_def_id, &mut vis.as_deep_visitor());
65 pub(crate) fn provide(providers: &mut Providers<'_>) {
66 *providers = Providers { check_mod_const_bodies, ..*providers };
69 #[derive(Copy, Clone)]
70 struct CheckConstVisitor<'tcx> {
72 const_kind: Option<hir::ConstContext>,
73 def_id: Option<LocalDefId>,
76 impl<'tcx> CheckConstVisitor<'tcx> {
77 fn new(tcx: TyCtxt<'tcx>) -> Self {
78 CheckConstVisitor { tcx, const_kind: None, def_id: None }
81 /// Emits an error when an unsupported expression is found in a const context.
82 fn const_check_violated(&self, expr: NonConstExpr, span: Span) {
83 let Self { tcx, def_id, const_kind } = *self;
85 let features = tcx.features();
86 let required_gates = expr.required_feature_gates();
88 let is_feature_allowed = |feature_gate| {
89 // All features require that the corresponding gate be enabled,
90 // even if the function has `#[allow_internal_unstable(the_gate)]`.
91 if !tcx.features().enabled(feature_gate) {
95 // If `def_id` is `None`, we don't need to consider stability attributes.
96 let def_id = match def_id {
97 Some(x) => x.to_def_id(),
101 // If this crate is not using stability attributes, or this function is not claiming to be a
102 // stable `const fn`, that is all that is required.
103 if !tcx.features().staged_api || tcx.has_attr(def_id, sym::rustc_const_unstable) {
107 // However, we cannot allow stable `const fn`s to use unstable features without an explicit
108 // opt-in via `allow_internal_unstable`.
109 attr::allow_internal_unstable(&tcx.get_attrs(def_id), &tcx.sess.diagnostic())
110 .map_or(false, |mut features| features.any(|name| name == feature_gate))
113 match required_gates {
114 // Don't emit an error if the user has enabled the requisite feature gates.
115 Some(gates) if gates.iter().copied().all(is_feature_allowed) => return,
117 // `-Zunleash-the-miri-inside-of-you` only works for expressions that don't have a
118 // corresponding feature gate. This encourages nightly users to use feature gates when
120 None if tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you => {
121 tcx.sess.span_warn(span, "skipping const checks");
129 const_kind.expect("`const_check_violated` may only be called inside a const context");
131 let msg = format!("{} is not allowed in a `{}`", expr.name(), const_kind.keyword_name());
133 let required_gates = required_gates.unwrap_or(&[]);
134 let missing_gates: Vec<_> =
135 required_gates.iter().copied().filter(|&g| !features.enabled(g)).collect();
137 match missing_gates.as_slice() {
138 &[] => struct_span_err!(tcx.sess, span, E0744, "{}", msg).emit(),
140 &[missing_primary, ref missing_secondary @ ..] => {
141 let mut err = feature_err(&tcx.sess.parse_sess, missing_primary, span, &msg);
143 // If multiple feature gates would be required to enable this expression, include
144 // them as help messages. Don't emit a separate error for each missing feature gate.
146 // FIXME(ecstaticmorse): Maybe this could be incorporated into `feature_err`? This
147 // is a pretty narrow case, however.
148 if nightly_options::is_nightly_build() {
149 for gate in missing_secondary {
151 "add `#![feature({})]` to the crate attributes to enable",
163 /// Saves the parent `const_kind` before calling `f` and restores it afterwards.
166 kind: Option<hir::ConstContext>,
167 def_id: Option<LocalDefId>,
168 f: impl FnOnce(&mut Self),
170 let parent_def_id = self.def_id;
171 let parent_kind = self.const_kind;
172 self.def_id = def_id;
173 self.const_kind = kind;
175 self.def_id = parent_def_id;
176 self.const_kind = parent_kind;
180 impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
181 type Map = Map<'tcx>;
183 fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<Self::Map> {
184 NestedVisitorMap::OnlyBodies(self.tcx.hir())
187 fn visit_anon_const(&mut self, anon: &'tcx hir::AnonConst) {
188 let kind = Some(hir::ConstContext::Const);
189 self.recurse_into(kind, None, |this| intravisit::walk_anon_const(this, anon));
192 fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
193 let owner = self.tcx.hir().body_owner_def_id(body.id());
194 let kind = self.tcx.hir().body_const_context(owner);
195 self.recurse_into(kind, Some(owner), |this| intravisit::walk_body(this, body));
198 fn visit_expr(&mut self, e: &'tcx hir::Expr<'tcx>) {
200 // Skip the following checks if we are not currently in a const context.
201 _ if self.const_kind.is_none() => {}
203 hir::ExprKind::Loop(_, _, source) => {
204 self.const_check_violated(NonConstExpr::Loop(*source), e.span);
207 hir::ExprKind::Match(_, _, source) => {
208 let non_const_expr = match source {
209 // These are handled by `ExprKind::Loop` above.
210 hir::MatchSource::WhileDesugar
211 | hir::MatchSource::WhileLetDesugar
212 | hir::MatchSource::ForLoopDesugar => None,
214 _ => Some(NonConstExpr::Match(*source)),
217 if let Some(expr) = non_const_expr {
218 self.const_check_violated(expr, e.span);
225 intravisit::walk_expr(self, e);