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;
16 use rustc_middle::ty::query::Providers;
17 use rustc_middle::ty::TyCtxt;
18 use rustc_session::parse::feature_err;
19 use rustc_span::{sym, Span, Symbol};
21 /// An expression that is not *always* legal in a const context.
22 #[derive(Clone, Copy)]
24 Loop(hir::LoopSource),
25 Match(hir::MatchSource),
29 fn name(self) -> String {
31 Self::Loop(src) => format!("`{}`", src.name()),
32 Self::Match(src) => format!("`{}`", src.name()),
36 fn required_feature_gates(self) -> Option<&'static [Symbol]> {
37 use hir::LoopSource::*;
38 use hir::MatchSource::*;
40 let gates: &[_] = match self {
41 Self::Match(AwaitDesugar) => {
45 Self::Loop(ForLoop) | Self::Match(ForLoopDesugar) => &[sym::const_for],
47 Self::Match(TryDesugar) => &[sym::const_try],
49 // All other expressions are allowed.
50 Self::Loop(Loop | While) | Self::Match(Normal) => &[],
57 fn check_mod_const_bodies(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
58 let mut vis = CheckConstVisitor::new(tcx);
59 tcx.hir().visit_item_likes_in_module(module_def_id, &mut vis);
62 pub(crate) fn provide(providers: &mut Providers) {
63 *providers = Providers { check_mod_const_bodies, ..*providers };
66 #[derive(Copy, Clone)]
67 struct CheckConstVisitor<'tcx> {
69 const_kind: Option<hir::ConstContext>,
70 def_id: Option<LocalDefId>,
73 impl<'tcx> CheckConstVisitor<'tcx> {
74 fn new(tcx: TyCtxt<'tcx>) -> Self {
75 CheckConstVisitor { tcx, const_kind: None, def_id: None }
78 /// Emits an error when an unsupported expression is found in a const context.
79 fn const_check_violated(&self, expr: NonConstExpr, span: Span) {
80 let Self { tcx, def_id, const_kind } = *self;
82 let features = tcx.features();
83 let required_gates = expr.required_feature_gates();
85 let is_feature_allowed = |feature_gate| {
86 // All features require that the corresponding gate be enabled,
87 // even if the function has `#[rustc_allow_const_fn_unstable(the_gate)]`.
88 if !tcx.features().enabled(feature_gate) {
92 // If `def_id` is `None`, we don't need to consider stability attributes.
93 let def_id = match def_id {
98 // If the function belongs to a trait, then it must enable the const_trait_impl
99 // feature to use that trait function (with a const default body).
100 if tcx.trait_of_item(def_id.to_def_id()).is_some() {
104 // If this crate is not using stability attributes, or this function is not claiming to be a
105 // stable `const fn`, that is all that is required.
106 if !tcx.features().staged_api
107 || tcx.has_attr(def_id.to_def_id(), sym::rustc_const_unstable)
112 // However, we cannot allow stable `const fn`s to use unstable features without an explicit
113 // opt-in via `rustc_allow_const_fn_unstable`.
114 let attrs = tcx.hir().attrs(tcx.hir().local_def_id_to_hir_id(def_id));
115 attr::rustc_allow_const_fn_unstable(&tcx.sess, attrs).any(|name| name == feature_gate)
118 match required_gates {
119 // Don't emit an error if the user has enabled the requisite feature gates.
120 Some(gates) if gates.iter().copied().all(is_feature_allowed) => return,
122 // `-Zunleash-the-miri-inside-of-you` only works for expressions that don't have a
123 // corresponding feature gate. This encourages nightly users to use feature gates when
125 None if tcx.sess.opts.unstable_opts.unleash_the_miri_inside_of_you => {
126 tcx.sess.span_warn(span, "skipping const checks");
134 const_kind.expect("`const_check_violated` may only be called inside a const context");
136 let msg = format!("{} is not allowed in a `{}`", expr.name(), const_kind.keyword_name());
138 let required_gates = required_gates.unwrap_or(&[]);
139 let missing_gates: Vec<_> =
140 required_gates.iter().copied().filter(|&g| !features.enabled(g)).collect();
142 match missing_gates.as_slice() {
144 struct_span_err!(tcx.sess, span, E0744, "{}", msg).emit();
147 [missing_primary, ref missing_secondary @ ..] => {
148 let mut err = feature_err(&tcx.sess.parse_sess, *missing_primary, span, &msg);
150 // If multiple feature gates would be required to enable this expression, include
151 // them as help messages. Don't emit a separate error for each missing feature gate.
153 // FIXME(ecstaticmorse): Maybe this could be incorporated into `feature_err`? This
154 // is a pretty narrow case, however.
155 if tcx.sess.is_nightly_build() {
156 for gate in missing_secondary {
158 "add `#![feature({})]` to the crate attributes to enable",
170 /// Saves the parent `const_kind` before calling `f` and restores it afterwards.
173 kind: Option<hir::ConstContext>,
174 def_id: Option<LocalDefId>,
175 f: impl FnOnce(&mut Self),
177 let parent_def_id = self.def_id;
178 let parent_kind = self.const_kind;
179 self.def_id = def_id;
180 self.const_kind = kind;
182 self.def_id = parent_def_id;
183 self.const_kind = parent_kind;
187 impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
188 type NestedFilter = nested_filter::OnlyBodies;
190 fn nested_visit_map(&mut self) -> Self::Map {
194 fn visit_anon_const(&mut self, anon: &'tcx hir::AnonConst) {
195 let kind = Some(hir::ConstContext::Const);
196 self.recurse_into(kind, None, |this| intravisit::walk_anon_const(this, anon));
199 fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
200 let owner = self.tcx.hir().body_owner_def_id(body.id());
201 let kind = self.tcx.hir().body_const_context(owner);
202 self.recurse_into(kind, Some(owner), |this| intravisit::walk_body(this, body));
205 fn visit_expr(&mut self, e: &'tcx hir::Expr<'tcx>) {
207 // Skip the following checks if we are not currently in a const context.
208 _ if self.const_kind.is_none() => {}
210 hir::ExprKind::Loop(_, _, source, _) => {
211 self.const_check_violated(NonConstExpr::Loop(*source), e.span);
214 hir::ExprKind::Match(_, _, source) => {
215 let non_const_expr = match source {
216 // These are handled by `ExprKind::Loop` above.
217 hir::MatchSource::ForLoopDesugar => None,
219 _ => Some(NonConstExpr::Match(*source)),
222 if let Some(expr) = non_const_expr {
223 self.const_check_violated(expr, e.span);
230 intravisit::walk_expr(self, e);