src/librustc_passes/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_ast::ast::Mutability;
  11 use rustc_errors::struct_span_err;
  12 use rustc_hir as hir;
  13 use rustc_hir::def_id::DefId;
  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::config::nightly_options;
  19 use rustc_session::parse::feature_err;
  20 use rustc_span::{sym, Span, Symbol};
  21
  22 use std::fmt;
  23
  24 /// An expression that is not *always* legal in a const context.
  25 #[derive(Clone, Copy)]
  26 enum NonConstExpr {
  27     Loop(hir::LoopSource),
  28     Match(hir::MatchSource),
  29     OrPattern,
  30 }
  31
  32 impl NonConstExpr {
  33     fn name(self) -> String {
  34         match self {
  35             Self::Loop(src) => format!("`{}`", src.name()),
  36             Self::Match(src) => format!("`{}`", src.name()),
  37             Self::OrPattern => "or-pattern".to_string(),
  38         }
  39     }
  40
  41     fn required_feature_gates(self) -> Option<&'static [Symbol]> {
  42         use hir::LoopSource::*;
  43         use hir::MatchSource::*;
  44
  45         let gates: &[_] = match self {
  46             Self::Match(Normal)
  47             | Self::Match(IfDesugar { .. })
  48             | Self::Match(IfLetDesugar { .. })
  49             | Self::OrPattern => &[sym::const_if_match],
  50
  51             Self::Loop(Loop) => &[sym::const_loop],
  52
  53             Self::Loop(While)
  54             | Self::Loop(WhileLet)
  55             | Self::Match(WhileDesugar | WhileLetDesugar) => {
  56                 &[sym::const_loop, sym::const_if_match]
  57             }
  58
  59             // A `for` loop's desugaring contains a call to `IntoIterator::into_iter`,
  60             // so they are not yet allowed with `#![feature(const_loop)]`.
  61             _ => return None,
  62         };
  63
  64         Some(gates)
  65     }
  66 }
  67
  68 #[derive(Copy, Clone)]
  69 enum ConstKind {
  70     Static,
  71     StaticMut,
  72     ConstFn,
  73     Const,
  74     AnonConst,
  75 }
  76
  77 impl ConstKind {
  78     fn for_body(body: &hir::Body<'_>, tcx: TyCtxt<'_>) -> Option<Self> {
  79         let owner = tcx.hir().body_owner(body.id());
  80         let const_kind = match tcx.hir().body_owner_kind(owner) {
  81             hir::BodyOwnerKind::Const => Self::Const,
  82             hir::BodyOwnerKind::Static(Mutability::Mut) => Self::StaticMut,
  83             hir::BodyOwnerKind::Static(Mutability::Not) => Self::Static,
  84
  85             hir::BodyOwnerKind::Fn if tcx.is_const_fn_raw(tcx.hir().local_def_id(owner)) => {
  86                 Self::ConstFn
  87             }
  88             hir::BodyOwnerKind::Fn | hir::BodyOwnerKind::Closure => return None,
  89         };
  90
  91         Some(const_kind)
  92     }
  93 }
  94
  95 impl fmt::Display for ConstKind {
  96     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
  97         let s = match self {
  98             Self::Static => "static",
  99             Self::StaticMut => "static mut",
 100             Self::Const | Self::AnonConst => "const",
 101             Self::ConstFn => "const fn",
 102         };
 103
 104         write!(f, "{}", s)
 105     }
 106 }
 107
 108 fn check_mod_const_bodies(tcx: TyCtxt<'_>, module_def_id: DefId) {
 109     let mut vis = CheckConstVisitor::new(tcx);
 110     tcx.hir().visit_item_likes_in_module(module_def_id, &mut vis.as_deep_visitor());
 111 }
 112
 113 pub(crate) fn provide(providers: &mut Providers<'_>) {
 114     *providers = Providers { check_mod_const_bodies, ..*providers };
 115 }
 116
 117 #[derive(Copy, Clone)]
 118 struct CheckConstVisitor<'tcx> {
 119     tcx: TyCtxt<'tcx>,
 120     const_kind: Option<ConstKind>,
 121 }
 122
 123 impl<'tcx> CheckConstVisitor<'tcx> {
 124     fn new(tcx: TyCtxt<'tcx>) -> Self {
 125         CheckConstVisitor { tcx, const_kind: None }
 126     }
 127
 128     /// Emits an error when an unsupported expression is found in a const context.
 129     fn const_check_violated(&self, expr: NonConstExpr, span: Span) {
 130         let features = self.tcx.features();
 131         let required_gates = expr.required_feature_gates();
 132         match required_gates {
 133             // Don't emit an error if the user has enabled the requisite feature gates.
 134             Some(gates) if gates.iter().all(|&g| features.enabled(g)) => return,
 135
 136             // `-Zunleash-the-miri-inside-of-you` only works for expressions that don't have a
 137             // corresponding feature gate. This encourages nightly users to use feature gates when
 138             // possible.
 139             None if self.tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you => {
 140                 self.tcx.sess.span_warn(span, "skipping const checks");
 141                 return;
 142             }
 143
 144             _ => {}
 145         }
 146
 147         let const_kind = self
 148             .const_kind
 149             .expect("`const_check_violated` may only be called inside a const context");
 150         let msg = format!("{} is not allowed in a `{}`", expr.name(), const_kind);
 151
 152         let required_gates = required_gates.unwrap_or(&[]);
 153         let missing_gates: Vec<_> =
 154             required_gates.iter().copied().filter(|&g| !features.enabled(g)).collect();
 155
 156         match missing_gates.as_slice() {
 157             &[] => struct_span_err!(self.tcx.sess, span, E0744, "{}", msg).emit(),
 158
 159             // If the user enabled `#![feature(const_loop)]` but not `#![feature(const_if_match)]`,
 160             // explain why their `while` loop is being rejected.
 161             &[gate @ sym::const_if_match] if required_gates.contains(&sym::const_loop) => {
 162                 feature_err(&self.tcx.sess.parse_sess, gate, span, &msg)
 163                     .note(
 164                         "`#![feature(const_loop)]` alone is not sufficient, \
 165                            since this loop expression contains an implicit conditional",
 166                     )
 167                     .emit();
 168             }
 169
 170             &[missing_primary, ref missing_secondary @ ..] => {
 171                 let mut err = feature_err(&self.tcx.sess.parse_sess, missing_primary, span, &msg);
 172
 173                 // If multiple feature gates would be required to enable this expression, include
 174                 // them as help messages. Don't emit a separate error for each missing feature gate.
 175                 //
 176                 // FIXME(ecstaticmorse): Maybe this could be incorporated into `feature_err`? This
 177                 // is a pretty narrow case, however.
 178                 if nightly_options::is_nightly_build() {
 179                     for gate in missing_secondary {
 180                         let note = format!(
 181                             "add `#![feature({})]` to the crate attributes to enable",
 182                             gate,
 183                         );
 184                         err.help(&note);
 185                     }
 186                 }
 187
 188                 err.emit();
 189             }
 190         }
 191     }
 192
 193     /// Saves the parent `const_kind` before calling `f` and restores it afterwards.
 194     fn recurse_into(&mut self, kind: Option<ConstKind>, f: impl FnOnce(&mut Self)) {
 195         let parent_kind = self.const_kind;
 196         self.const_kind = kind;
 197         f(self);
 198         self.const_kind = parent_kind;
 199     }
 200 }
 201
 202 impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
 203     type Map = Map<'tcx>;
 204
 205     fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<Self::Map> {
 206         NestedVisitorMap::OnlyBodies(self.tcx.hir())
 207     }
 208
 209     fn visit_anon_const(&mut self, anon: &'tcx hir::AnonConst) {
 210         let kind = Some(ConstKind::AnonConst);
 211         self.recurse_into(kind, |this| intravisit::walk_anon_const(this, anon));
 212     }
 213
 214     fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) {
 215         let kind = ConstKind::for_body(body, self.tcx);
 216         self.recurse_into(kind, |this| intravisit::walk_body(this, body));
 217     }
 218
 219     fn visit_pat(&mut self, p: &'tcx hir::Pat<'tcx>) {
 220         if self.const_kind.is_some() {
 221             if let hir::PatKind::Or { .. } = p.kind {
 222                 self.const_check_violated(NonConstExpr::OrPattern, p.span);
 223             }
 224         }
 225         intravisit::walk_pat(self, p)
 226     }
 227
 228     fn visit_expr(&mut self, e: &'tcx hir::Expr<'tcx>) {
 229         match &e.kind {
 230             // Skip the following checks if we are not currently in a const context.
 231             _ if self.const_kind.is_none() => {}
 232
 233             hir::ExprKind::Loop(_, _, source) => {
 234                 self.const_check_violated(NonConstExpr::Loop(*source), e.span);
 235             }
 236
 237             hir::ExprKind::Match(_, _, source) => {
 238                 let non_const_expr = match source {
 239                     // These are handled by `ExprKind::Loop` above.
 240                     hir::MatchSource::WhileDesugar
 241                     | hir::MatchSource::WhileLetDesugar
 242                     | hir::MatchSource::ForLoopDesugar => None,
 243
 244                     _ => Some(NonConstExpr::Match(*source)),
 245                 };
 246
 247                 if let Some(expr) = non_const_expr {
 248                     self.const_check_violated(expr, e.span);
 249                 }
 250             }
 251
 252             _ => {}
 253         }
 254
 255         intravisit::walk_expr(self, e);
 256     }
 257 }