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::hir::def_id::DefId;
11 use rustc::hir::intravisit::{Visitor, NestedVisitorMap};
12 use rustc::hir::map::Map;
14 use rustc::ty::TyCtxt;
15 use rustc::ty::query::Providers;
16 use syntax::ast::Mutability;
19 use rustc_error_codes::*;
23 #[derive(Copy, Clone)]
33 fn for_body(body: &hir::Body, hir_map: &Map<'_>) -> Option<Self> {
34 let is_const_fn = |id| hir_map.fn_sig_by_hir_id(id).unwrap().header.is_const();
36 let owner = hir_map.body_owner(body.id());
37 let const_kind = match hir_map.body_owner_kind(owner) {
38 hir::BodyOwnerKind::Const => Self::Const,
39 hir::BodyOwnerKind::Static(Mutability::Mutable) => Self::StaticMut,
40 hir::BodyOwnerKind::Static(Mutability::Immutable) => Self::Static,
42 hir::BodyOwnerKind::Fn if is_const_fn(owner) => Self::ConstFn,
43 hir::BodyOwnerKind::Fn | hir::BodyOwnerKind::Closure => return None,
50 impl fmt::Display for ConstKind {
51 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
53 Self::Static => "static",
54 Self::StaticMut => "static mut",
55 Self::Const | Self::AnonConst => "const",
56 Self::ConstFn => "const fn",
63 fn check_mod_const_bodies(tcx: TyCtxt<'_>, module_def_id: DefId) {
64 let mut vis = CheckConstVisitor::new(tcx);
65 tcx.hir().visit_item_likes_in_module(module_def_id, &mut vis.as_deep_visitor());
68 pub(crate) fn provide(providers: &mut Providers<'_>) {
69 *providers = Providers {
70 check_mod_const_bodies,
75 #[derive(Copy, Clone)]
76 struct CheckConstVisitor<'tcx> {
78 const_kind: Option<ConstKind>,
81 impl<'tcx> CheckConstVisitor<'tcx> {
82 fn new(tcx: TyCtxt<'tcx>) -> Self {
89 /// Emits an error when an unsupported expression is found in a const context.
90 fn const_check_violated(&self, bad_op: &str, span: Span) {
91 if self.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
92 self.tcx.sess.span_warn(span, "skipping const checks");
96 let const_kind = self.const_kind
97 .expect("`const_check_violated` may only be called inside a const context");
99 span_err!(self.tcx.sess, span, E0744, "`{}` is not allowed in a `{}`", bad_op, const_kind);
102 /// Saves the parent `const_kind` before calling `f` and restores it afterwards.
103 fn recurse_into(&mut self, kind: Option<ConstKind>, f: impl FnOnce(&mut Self)) {
104 let parent_kind = self.const_kind;
105 self.const_kind = kind;
107 self.const_kind = parent_kind;
111 impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
112 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
113 NestedVisitorMap::OnlyBodies(&self.tcx.hir())
116 fn visit_anon_const(&mut self, anon: &'tcx hir::AnonConst) {
117 let kind = Some(ConstKind::AnonConst);
118 self.recurse_into(kind, |this| hir::intravisit::walk_anon_const(this, anon));
121 fn visit_body(&mut self, body: &'tcx hir::Body) {
122 let kind = ConstKind::for_body(body, self.tcx.hir());
123 self.recurse_into(kind, |this| hir::intravisit::walk_body(this, body));
126 fn visit_expr(&mut self, e: &'tcx hir::Expr) {
128 // Skip the following checks if we are not currently in a const context.
129 _ if self.const_kind.is_none() => {}
131 hir::ExprKind::Loop(_, _, source) => {
132 self.const_check_violated(source.name(), e.span);
135 hir::ExprKind::Match(_, _, source) if !self.tcx.features().const_if_match => {
136 use hir::MatchSource::*;
138 let op = match source {
139 Normal => Some("match"),
140 IfDesugar { .. } | IfLetDesugar { .. } => Some("if"),
141 TryDesugar => Some("?"),
142 AwaitDesugar => Some(".await"),
144 // These are handled by `ExprKind::Loop` above.
145 WhileDesugar | WhileLetDesugar | ForLoopDesugar => None,
148 if let Some(op) = op {
149 self.const_check_violated(op, e.span);
156 hir::intravisit::walk_expr(self, e);