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::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 // A `for` loop's desugaring contains a call to `IntoIterator::into_iter`,
42 // so they are not yet allowed.
43 // Likewise, `?` desugars to a call to `Try::into_result`.
44 Self::Loop(ForLoop) | Self::Match(ForLoopDesugar | TryDesugar | AwaitDesugar) => {
48 Self::Match(IfLetGuardDesugar) => bug!("if-let guard outside a `match` expression"),
50 // All other expressions are allowed.
51 Self::Loop(Loop | While | WhileLet)
53 WhileDesugar | WhileLetDesugar | Normal | IfDesugar { .. } | IfLetDesugar { .. },
61 fn check_mod_const_bodies(tcx: TyCtxt<'_>, module_def_id: LocalDefId) {
62 let mut vis = CheckConstVisitor::new(tcx);
63 tcx.hir().visit_item_likes_in_module(module_def_id, &mut vis.as_deep_visitor());
66 pub(crate) fn provide(providers: &mut Providers) {
67 *providers = Providers { check_mod_const_bodies, ..*providers };
70 #[derive(Copy, Clone)]
71 struct CheckConstVisitor<'tcx> {
73 const_kind: Option<hir::ConstContext>,
74 def_id: Option<LocalDefId>,
77 impl<'tcx> CheckConstVisitor<'tcx> {
78 fn new(tcx: TyCtxt<'tcx>) -> Self {
79 CheckConstVisitor { tcx, const_kind: None, def_id: None }
82 /// Emits an error when an unsupported expression is found in a const context.
83 fn const_check_violated(&self, expr: NonConstExpr, span: Span) {
84 let Self { tcx, def_id, const_kind } = *self;
86 let features = tcx.features();
87 let required_gates = expr.required_feature_gates();
89 let is_feature_allowed = |feature_gate| {
90 // All features require that the corresponding gate be enabled,
91 // even if the function has `#[rustc_allow_const_fn_unstable(the_gate)]`.
92 if !tcx.features().enabled(feature_gate) {
96 // If `def_id` is `None`, we don't need to consider stability attributes.
97 let def_id = match def_id {
98 Some(x) => x.to_def_id(),
102 // If this crate is not using stability attributes, or this function is not claiming to be a
103 // stable `const fn`, that is all that is required.
104 if !tcx.features().staged_api || tcx.has_attr(def_id, sym::rustc_const_unstable) {
108 // However, we cannot allow stable `const fn`s to use unstable features without an explicit
109 // opt-in via `rustc_allow_const_fn_unstable`.
110 attr::rustc_allow_const_fn_unstable(&tcx.sess, &tcx.get_attrs(def_id))
111 .map_or(false, |mut features| features.any(|name| name == feature_gate))
114 match required_gates {
115 // Don't emit an error if the user has enabled the requisite feature gates.
116 Some(gates) if gates.iter().copied().all(is_feature_allowed) => return,
118 // `-Zunleash-the-miri-inside-of-you` only works for expressions that don't have a
119 // corresponding feature gate. This encourages nightly users to use feature gates when
121 None if tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you => {
122 tcx.sess.span_warn(span, "skipping const checks");
130 const_kind.expect("`const_check_violated` may only be called inside a const context");
132 let msg = format!("{} is not allowed in a `{}`", expr.name(), const_kind.keyword_name());
134 let required_gates = required_gates.unwrap_or(&[]);
135 let missing_gates: Vec<_> =
136 required_gates.iter().copied().filter(|&g| !features.enabled(g)).collect();
138 match missing_gates.as_slice() {
139 &[] => struct_span_err!(tcx.sess, span, E0744, "{}", msg).emit(),
141 &[missing_primary, ref missing_secondary @ ..] => {
142 let mut err = feature_err(&tcx.sess.parse_sess, missing_primary, span, &msg);
144 // If multiple feature gates would be required to enable this expression, include
145 // them as help messages. Don't emit a separate error for each missing feature gate.
147 // FIXME(ecstaticmorse): Maybe this could be incorporated into `feature_err`? This
148 // is a pretty narrow case, however.
149 if tcx.sess.is_nightly_build() {
150 for gate in missing_secondary {
152 "add `#![feature({})]` to the crate attributes to enable",
164 /// Saves the parent `const_kind` before calling `f` and restores it afterwards.
167 kind: Option<hir::ConstContext>,
168 def_id: Option<LocalDefId>,
169 f: impl FnOnce(&mut Self),
171 let parent_def_id = self.def_id;
172 let parent_kind = self.const_kind;
173 self.def_id = def_id;
174 self.const_kind = kind;
176 self.def_id = parent_def_id;
177 self.const_kind = parent_kind;
181 impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
182 type Map = Map<'tcx>;
184 fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<Self::Map> {
185 NestedVisitorMap::OnlyBodies(self.tcx.hir())
188 fn visit_anon_const(&mut self, anon: &'tcx hir::AnonConst) {
189 let kind = Some(hir::ConstContext::Const);
190 self.recurse_into(kind, None, |this| intravisit::walk_anon_const(this, anon));
193 fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
194 let owner = self.tcx.hir().body_owner_def_id(body.id());
195 let kind = self.tcx.hir().body_const_context(owner);
196 self.recurse_into(kind, Some(owner), |this| intravisit::walk_body(this, body));
199 fn visit_expr(&mut self, e: &'tcx hir::Expr<'tcx>) {
201 // Skip the following checks if we are not currently in a const context.
202 _ if self.const_kind.is_none() => {}
204 hir::ExprKind::Loop(_, _, source) => {
205 self.const_check_violated(NonConstExpr::Loop(*source), e.span);
208 hir::ExprKind::Match(_, _, source) => {
209 let non_const_expr = match source {
210 // These are handled by `ExprKind::Loop` above.
211 hir::MatchSource::WhileDesugar
212 | hir::MatchSource::WhileLetDesugar
213 | hir::MatchSource::ForLoopDesugar => None,
215 _ => Some(NonConstExpr::Match(*source)),
218 if let Some(expr) = non_const_expr {
219 self.const_check_violated(expr, e.span);
226 intravisit::walk_expr(self, e);