compiler/rustc_passes/src/check_const.rs

   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
   3 //! context.
   4 //!
   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.
   9
  10 use rustc_attr as attr;
  11 use rustc_errors::struct_span_err;
  12 use rustc_hir as hir;
  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;
  17 use rustc_middle::ty::query::Providers;
  18 use rustc_middle::ty::TyCtxt;
  19 use rustc_session::parse::feature_err;
  20 use rustc_span::{sym, Span, Symbol};
  21
  22 /// An expression that is not *always* legal in a const context.
  23 #[derive(Clone, Copy)]
  24 enum NonConstExpr {
  25     Loop(hir::LoopSource),
  26     Match(hir::MatchSource),
  27 }
  28
  29 impl NonConstExpr {
  30     fn name(self) -> String {
  31         match self {
  32             Self::Loop(src) => format!("`{}`", src.name()),
  33             Self::Match(src) => format!("`{}`", src.name()),
  34         }
  35     }
  36
  37     fn required_feature_gates(self) -> Option<&'static [Symbol]> {
  38         use hir::LoopSource::*;
  39         use hir::MatchSource::*;
  40
  41         let gates: &[_] = match self {
  42             Self::Match(AwaitDesugar) => {
  43                 return None;
  44             }
  45
  46             Self::Loop(ForLoop) | Self::Match(ForLoopDesugar) => &[sym::const_for],
  47
  48             Self::Match(TryDesugar) => &[sym::const_try],
  49
  50             // All other expressions are allowed.
  51             Self::Loop(Loop | While) | Self::Match(Normal) => &[],
  52         };
  53
  54         Some(gates)
  55     }
  56 }
  57
  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.as_deep_visitor());
  61     tcx.hir().visit_item_likes_in_module(module_def_id, &mut CheckConstTraitVisitor::new(tcx));
  62 }
  63
  64 pub(crate) fn provide(providers: &mut Providers) {
  65     *providers = Providers { check_mod_const_bodies, ..*providers };
  66 }
  67
  68 struct CheckConstTraitVisitor<'tcx> {
  69     tcx: TyCtxt<'tcx>,
  70 }
  71
  72 impl<'tcx> CheckConstTraitVisitor<'tcx> {
  73     fn new(tcx: TyCtxt<'tcx>) -> Self {
  74         CheckConstTraitVisitor { tcx }
  75     }
  76 }
  77
  78 impl<'tcx> hir::itemlikevisit::ItemLikeVisitor<'tcx> for CheckConstTraitVisitor<'tcx> {
  79     /// check for const trait impls, and errors if the impl uses provided/default functions
  80     /// of the trait being implemented; as those provided functions can be non-const.
  81     fn visit_item<'hir>(&mut self, item: &'hir hir::Item<'hir>) {
  82         let _: Option<_> = try {
  83             if let hir::ItemKind::Impl(ref imp) = item.kind && let hir::Constness::Const = imp.constness {
  84                     let trait_def_id = imp.of_trait.as_ref()?.trait_def_id()?;
  85                     let ancestors = self
  86                         .tcx
  87                         .trait_def(trait_def_id)
  88                         .ancestors(self.tcx, item.def_id.to_def_id())
  89                         .ok()?;
  90                     let mut to_implement = Vec::new();
  91
  92                     for trait_item in self.tcx.associated_items(trait_def_id).in_definition_order()
  93                     {
  94                         if let ty::AssocItem {
  95                             kind: ty::AssocKind::Fn,
  96                             defaultness,
  97                             def_id: trait_item_id,
  98                             ..
  99                         } = *trait_item
 100                         {
 101                             // we can ignore functions that do not have default bodies:
 102                             // if those are unimplemented it will be caught by typeck.
 103                             if !defaultness.has_value()
 104                                 || self
 105                                     .tcx
 106                                     .has_attr(trait_item_id, sym::default_method_body_is_const)
 107                             {
 108                                 continue;
 109                             }
 110
 111                             let is_implemented = ancestors
 112                                 .leaf_def(self.tcx, trait_item_id)
 113                                 .map(|node_item| !node_item.defining_node.is_from_trait())
 114                                 .unwrap_or(false);
 115
 116                             if !is_implemented {
 117                                 to_implement.push(self.tcx.item_name(trait_item_id).to_string());
 118                             }
 119                         }
 120                     }
 121
 122                     // all nonconst trait functions (not marked with #[default_method_body_is_const])
 123                     // must be implemented
 124                     if !to_implement.is_empty() {
 125                         self.tcx
 126                             .sess
 127                             .struct_span_err(
 128                                 item.span,
 129                                 "const trait implementations may not use non-const default functions",
 130                             )
 131                             .note(&format!("`{}` not implemented", to_implement.join("`, `")))
 132                             .emit();
 133                     }
 134             }
 135         };
 136     }
 137
 138     fn visit_trait_item<'hir>(&mut self, _: &'hir hir::TraitItem<'hir>) {}
 139
 140     fn visit_impl_item<'hir>(&mut self, _: &'hir hir::ImplItem<'hir>) {}
 141
 142     fn visit_foreign_item<'hir>(&mut self, _: &'hir hir::ForeignItem<'hir>) {}
 143 }
 144
 145 #[derive(Copy, Clone)]
 146 struct CheckConstVisitor<'tcx> {
 147     tcx: TyCtxt<'tcx>,
 148     const_kind: Option<hir::ConstContext>,
 149     def_id: Option<LocalDefId>,
 150 }
 151
 152 impl<'tcx> CheckConstVisitor<'tcx> {
 153     fn new(tcx: TyCtxt<'tcx>) -> Self {
 154         CheckConstVisitor { tcx, const_kind: None, def_id: None }
 155     }
 156
 157     /// Emits an error when an unsupported expression is found in a const context.
 158     fn const_check_violated(&self, expr: NonConstExpr, span: Span) {
 159         let Self { tcx, def_id, const_kind } = *self;
 160
 161         let features = tcx.features();
 162         let required_gates = expr.required_feature_gates();
 163
 164         let is_feature_allowed = |feature_gate| {
 165             // All features require that the corresponding gate be enabled,
 166             // even if the function has `#[rustc_allow_const_fn_unstable(the_gate)]`.
 167             if !tcx.features().enabled(feature_gate) {
 168                 return false;
 169             }
 170
 171             // If `def_id` is `None`, we don't need to consider stability attributes.
 172             let def_id = match def_id {
 173                 Some(x) => x,
 174                 None => return true,
 175             };
 176
 177             // If the function belongs to a trait, then it must enable the const_trait_impl
 178             // feature to use that trait function (with a const default body).
 179             if tcx.trait_of_item(def_id).is_some() {
 180                 return true;
 181             }
 182
 183             // If this crate is not using stability attributes, or this function is not claiming to be a
 184             // stable `const fn`, that is all that is required.
 185             if !tcx.features().staged_api
 186                 || tcx.has_attr(def_id.to_def_id(), sym::rustc_const_unstable)
 187             {
 188                 return true;
 189             }
 190
 191             // However, we cannot allow stable `const fn`s to use unstable features without an explicit
 192             // opt-in via `rustc_allow_const_fn_unstable`.
 193             let attrs = tcx.hir().attrs(tcx.hir().local_def_id_to_hir_id(def_id));
 194             attr::rustc_allow_const_fn_unstable(&tcx.sess, attrs).any(|name| name == feature_gate)
 195         };
 196
 197         match required_gates {
 198             // Don't emit an error if the user has enabled the requisite feature gates.
 199             Some(gates) if gates.iter().copied().all(is_feature_allowed) => return,
 200
 201             // `-Zunleash-the-miri-inside-of-you` only works for expressions that don't have a
 202             // corresponding feature gate. This encourages nightly users to use feature gates when
 203             // possible.
 204             None if tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you => {
 205                 tcx.sess.span_warn(span, "skipping const checks");
 206                 return;
 207             }
 208
 209             _ => {}
 210         }
 211
 212         let const_kind =
 213             const_kind.expect("`const_check_violated` may only be called inside a const context");
 214
 215         let msg = format!("{} is not allowed in a `{}`", expr.name(), const_kind.keyword_name());
 216
 217         let required_gates = required_gates.unwrap_or(&[]);
 218         let missing_gates: Vec<_> =
 219             required_gates.iter().copied().filter(|&g| !features.enabled(g)).collect();
 220
 221         match missing_gates.as_slice() {
 222             [] => {
 223                 struct_span_err!(tcx.sess, span, E0744, "{}", msg).emit();
 224             }
 225
 226             [missing_primary, ref missing_secondary @ ..] => {
 227                 let mut err = feature_err(&tcx.sess.parse_sess, *missing_primary, span, &msg);
 228
 229                 // If multiple feature gates would be required to enable this expression, include
 230                 // them as help messages. Don't emit a separate error for each missing feature gate.
 231                 //
 232                 // FIXME(ecstaticmorse): Maybe this could be incorporated into `feature_err`? This
 233                 // is a pretty narrow case, however.
 234                 if tcx.sess.is_nightly_build() {
 235                     for gate in missing_secondary {
 236                         let note = format!(
 237                             "add `#![feature({})]` to the crate attributes to enable",
 238                             gate,
 239                         );
 240                         err.help(&note);
 241                     }
 242                 }
 243
 244                 err.emit();
 245             }
 246         }
 247     }
 248
 249     /// Saves the parent `const_kind` before calling `f` and restores it afterwards.
 250     fn recurse_into(
 251         &mut self,
 252         kind: Option<hir::ConstContext>,
 253         def_id: Option<LocalDefId>,
 254         f: impl FnOnce(&mut Self),
 255     ) {
 256         let parent_def_id = self.def_id;
 257         let parent_kind = self.const_kind;
 258         self.def_id = def_id;
 259         self.const_kind = kind;
 260         f(self);
 261         self.def_id = parent_def_id;
 262         self.const_kind = parent_kind;
 263     }
 264 }
 265
 266 impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
 267     type NestedFilter = nested_filter::OnlyBodies;
 268
 269     fn nested_visit_map(&mut self) -> Self::Map {
 270         self.tcx.hir()
 271     }
 272
 273     fn visit_anon_const(&mut self, anon: &'tcx hir::AnonConst) {
 274         let kind = Some(hir::ConstContext::Const);
 275         self.recurse_into(kind, None, |this| intravisit::walk_anon_const(this, anon));
 276     }
 277
 278     fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
 279         let owner = self.tcx.hir().body_owner_def_id(body.id());
 280         let kind = self.tcx.hir().body_const_context(owner);
 281         self.recurse_into(kind, Some(owner), |this| intravisit::walk_body(this, body));
 282     }
 283
 284     fn visit_expr(&mut self, e: &'tcx hir::Expr<'tcx>) {
 285         match &e.kind {
 286             // Skip the following checks if we are not currently in a const context.
 287             _ if self.const_kind.is_none() => {}
 288
 289             hir::ExprKind::Loop(_, _, source, _) => {
 290                 self.const_check_violated(NonConstExpr::Loop(*source), e.span);
 291             }
 292
 293             hir::ExprKind::Match(_, _, source) => {
 294                 let non_const_expr = match source {
 295                     // These are handled by `ExprKind::Loop` above.
 296                     hir::MatchSource::ForLoopDesugar => None,
 297
 298                     _ => Some(NonConstExpr::Match(*source)),
 299                 };
 300
 301                 if let Some(expr) = non_const_expr {
 302                     self.const_check_violated(expr, e.span);
 303                 }
 304             }
 305
 306             _ => {}
 307         }
 308
 309         intravisit::walk_expr(self, e);
 310     }
 311 }