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;
12 use rustc_hir::def_id::LocalDefId;
13 use rustc_hir::intravisit::{self, Visitor};
14 use rustc_middle::hir::nested_filter;
15 use rustc_middle::ty::query::Providers;
16 use rustc_middle::ty::TyCtxt;
17 use rustc_session::parse::feature_err;
18 use rustc_span::{sym, Span, Symbol};
20 use crate::errors::ExprNotAllowedInContext;
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);
63 pub(crate) fn provide(providers: &mut Providers) {
64 *providers = Providers { check_mod_const_bodies, ..*providers };
67 #[derive(Copy, Clone)]
68 struct CheckConstVisitor<'tcx> {
70 const_kind: Option<hir::ConstContext>,
71 def_id: Option<LocalDefId>,
74 impl<'tcx> CheckConstVisitor<'tcx> {
75 fn new(tcx: TyCtxt<'tcx>) -> Self {
76 CheckConstVisitor { tcx, const_kind: None, def_id: None }
79 /// Emits an error when an unsupported expression is found in a const context.
80 fn const_check_violated(&self, expr: NonConstExpr, span: Span) {
81 let Self { tcx, def_id, const_kind } = *self;
83 let features = tcx.features();
84 let required_gates = expr.required_feature_gates();
86 let is_feature_allowed = |feature_gate| {
87 // All features require that the corresponding gate be enabled,
88 // even if the function has `#[rustc_allow_const_fn_unstable(the_gate)]`.
89 if !tcx.features().enabled(feature_gate) {
93 // If `def_id` is `None`, we don't need to consider stability attributes.
94 let def_id = match def_id {
99 // If the function belongs to a trait, then it must enable the const_trait_impl
100 // feature to use that trait function (with a const default body).
101 if tcx.trait_of_item(def_id.to_def_id()).is_some() {
105 // If this crate is not using stability attributes, or this function is not claiming to be a
106 // stable `const fn`, that is all that is required.
107 if !tcx.features().staged_api
108 || tcx.has_attr(def_id.to_def_id(), sym::rustc_const_unstable)
113 // However, we cannot allow stable `const fn`s to use unstable features without an explicit
114 // opt-in via `rustc_allow_const_fn_unstable`.
115 let attrs = tcx.hir().attrs(tcx.hir().local_def_id_to_hir_id(def_id));
116 attr::rustc_allow_const_fn_unstable(&tcx.sess, attrs).any(|name| name == feature_gate)
119 match required_gates {
120 // Don't emit an error if the user has enabled the requisite feature gates.
121 Some(gates) if gates.iter().copied().all(is_feature_allowed) => return,
123 // `-Zunleash-the-miri-inside-of-you` only works for expressions that don't have a
124 // corresponding feature gate. This encourages nightly users to use feature gates when
126 None if tcx.sess.opts.unstable_opts.unleash_the_miri_inside_of_you => {
127 tcx.sess.span_warn(span, "skipping const checks");
135 const_kind.expect("`const_check_violated` may only be called inside a const context");
137 let required_gates = required_gates.unwrap_or(&[]);
138 let missing_gates: Vec<_> =
139 required_gates.iter().copied().filter(|&g| !features.enabled(g)).collect();
141 match missing_gates.as_slice() {
143 tcx.sess.emit_err(ExprNotAllowedInContext {
146 context: const_kind.keyword_name(),
150 [missing_primary, ref missing_secondary @ ..] => {
152 format!("{} is not allowed in a `{}`", expr.name(), const_kind.keyword_name());
153 let mut err = feature_err(&tcx.sess.parse_sess, *missing_primary, span, &msg);
155 // If multiple feature gates would be required to enable this expression, include
156 // them as help messages. Don't emit a separate error for each missing feature gate.
158 // FIXME(ecstaticmorse): Maybe this could be incorporated into `feature_err`? This
159 // is a pretty narrow case, however.
160 if tcx.sess.is_nightly_build() {
161 for gate in missing_secondary {
163 "add `#![feature({})]` to the crate attributes to enable",
175 /// Saves the parent `const_kind` before calling `f` and restores it afterwards.
178 kind: Option<hir::ConstContext>,
179 def_id: Option<LocalDefId>,
180 f: impl FnOnce(&mut Self),
182 let parent_def_id = self.def_id;
183 let parent_kind = self.const_kind;
184 self.def_id = def_id;
185 self.const_kind = kind;
187 self.def_id = parent_def_id;
188 self.const_kind = parent_kind;
192 impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
193 type NestedFilter = nested_filter::OnlyBodies;
195 fn nested_visit_map(&mut self) -> Self::Map {
199 fn visit_anon_const(&mut self, anon: &'tcx hir::AnonConst) {
200 let kind = Some(hir::ConstContext::Const);
201 self.recurse_into(kind, None, |this| intravisit::walk_anon_const(this, anon));
204 fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
205 let owner = self.tcx.hir().body_owner_def_id(body.id());
206 let kind = self.tcx.hir().body_const_context(owner);
207 self.recurse_into(kind, Some(owner), |this| intravisit::walk_body(this, body));
210 fn visit_expr(&mut self, e: &'tcx hir::Expr<'tcx>) {
212 // Skip the following checks if we are not currently in a const context.
213 _ if self.const_kind.is_none() => {}
215 hir::ExprKind::Loop(_, _, source, _) => {
216 self.const_check_violated(NonConstExpr::Loop(*source), e.span);
219 hir::ExprKind::Match(_, _, source) => {
220 let non_const_expr = match source {
221 // These are handled by `ExprKind::Loop` above.
222 hir::MatchSource::ForLoopDesugar => None,
224 _ => Some(NonConstExpr::Match(*source)),
227 if let Some(expr) = non_const_expr {
228 self.const_check_violated(expr, e.span);
235 intravisit::walk_expr(self, e);