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Diffstat (limited to '')
-rw-r--r-- | compiler/rustc_passes/src/check_const.rs | 236 |
1 files changed, 236 insertions, 0 deletions
diff --git a/compiler/rustc_passes/src/check_const.rs b/compiler/rustc_passes/src/check_const.rs new file mode 100644 index 000000000..70518284c --- /dev/null +++ b/compiler/rustc_passes/src/check_const.rs @@ -0,0 +1,236 @@ +//! This pass checks HIR bodies that may be evaluated at compile-time (e.g., `const`, `static`, +//! `const fn`) for structured control flow (e.g. `if`, `while`), which is forbidden in a const +//! context. +//! +//! By the time the MIR const-checker runs, these high-level constructs have been lowered to +//! control-flow primitives (e.g., `Goto`, `SwitchInt`), making it tough to properly attribute +//! errors. We still look for those primitives in the MIR const-checker to ensure nothing slips +//! through, but errors for structured control flow in a `const` should be emitted here. + +use rustc_attr as attr; +use rustc_errors::struct_span_err; +use rustc_hir as hir; +use rustc_hir::def_id::LocalDefId; +use rustc_hir::intravisit::{self, Visitor}; +use rustc_middle::hir::nested_filter; +use rustc_middle::ty::query::Providers; +use rustc_middle::ty::TyCtxt; +use rustc_session::parse::feature_err; +use rustc_span::{sym, Span, Symbol}; + +/// An expression that is not *always* legal in a const context. +#[derive(Clone, Copy)] +enum NonConstExpr { + Loop(hir::LoopSource), + Match(hir::MatchSource), +} + +impl NonConstExpr { + fn name(self) -> String { + match self { + Self::Loop(src) => format!("`{}`", src.name()), + Self::Match(src) => format!("`{}`", src.name()), + } + } + + fn required_feature_gates(self) -> Option<&'static [Symbol]> { + use hir::LoopSource::*; + use hir::MatchSource::*; + + let gates: &[_] = match self { + Self::Match(AwaitDesugar) => { + return None; + } + + Self::Loop(ForLoop) | Self::Match(ForLoopDesugar) => &[sym::const_for], + + Self::Match(TryDesugar) => &[sym::const_try], + + // All other expressions are allowed. + Self::Loop(Loop | While) | Self::Match(Normal) => &[], + }; + + Some(gates) + } +} + +fn check_mod_const_bodies(tcx: TyCtxt<'_>, module_def_id: LocalDefId) { + let mut vis = CheckConstVisitor::new(tcx); + tcx.hir().visit_item_likes_in_module(module_def_id, &mut vis); +} + +pub(crate) fn provide(providers: &mut Providers) { + *providers = Providers { check_mod_const_bodies, ..*providers }; +} + +#[derive(Copy, Clone)] +struct CheckConstVisitor<'tcx> { + tcx: TyCtxt<'tcx>, + const_kind: Option<hir::ConstContext>, + def_id: Option<LocalDefId>, +} + +impl<'tcx> CheckConstVisitor<'tcx> { + fn new(tcx: TyCtxt<'tcx>) -> Self { + CheckConstVisitor { tcx, const_kind: None, def_id: None } + } + + /// Emits an error when an unsupported expression is found in a const context. + fn const_check_violated(&self, expr: NonConstExpr, span: Span) { + let Self { tcx, def_id, const_kind } = *self; + + let features = tcx.features(); + let required_gates = expr.required_feature_gates(); + + let is_feature_allowed = |feature_gate| { + // All features require that the corresponding gate be enabled, + // even if the function has `#[rustc_allow_const_fn_unstable(the_gate)]`. + if !tcx.features().enabled(feature_gate) { + return false; + } + + // If `def_id` is `None`, we don't need to consider stability attributes. + let def_id = match def_id { + Some(x) => x, + None => return true, + }; + + // If the function belongs to a trait, then it must enable the const_trait_impl + // feature to use that trait function (with a const default body). + if tcx.trait_of_item(def_id.to_def_id()).is_some() { + return true; + } + + // If this crate is not using stability attributes, or this function is not claiming to be a + // stable `const fn`, that is all that is required. + if !tcx.features().staged_api + || tcx.has_attr(def_id.to_def_id(), sym::rustc_const_unstable) + { + return true; + } + + // However, we cannot allow stable `const fn`s to use unstable features without an explicit + // opt-in via `rustc_allow_const_fn_unstable`. + let attrs = tcx.hir().attrs(tcx.hir().local_def_id_to_hir_id(def_id)); + attr::rustc_allow_const_fn_unstable(&tcx.sess, attrs).any(|name| name == feature_gate) + }; + + match required_gates { + // Don't emit an error if the user has enabled the requisite feature gates. + Some(gates) if gates.iter().copied().all(is_feature_allowed) => return, + + // `-Zunleash-the-miri-inside-of-you` only works for expressions that don't have a + // corresponding feature gate. This encourages nightly users to use feature gates when + // possible. + None if tcx.sess.opts.unstable_opts.unleash_the_miri_inside_of_you => { + tcx.sess.span_warn(span, "skipping const checks"); + return; + } + + _ => {} + } + + let const_kind = + const_kind.expect("`const_check_violated` may only be called inside a const context"); + + let msg = format!("{} is not allowed in a `{}`", expr.name(), const_kind.keyword_name()); + + let required_gates = required_gates.unwrap_or(&[]); + let missing_gates: Vec<_> = + required_gates.iter().copied().filter(|&g| !features.enabled(g)).collect(); + + match missing_gates.as_slice() { + [] => { + struct_span_err!(tcx.sess, span, E0744, "{}", msg).emit(); + } + + [missing_primary, ref missing_secondary @ ..] => { + let mut err = feature_err(&tcx.sess.parse_sess, *missing_primary, span, &msg); + + // If multiple feature gates would be required to enable this expression, include + // them as help messages. Don't emit a separate error for each missing feature gate. + // + // FIXME(ecstaticmorse): Maybe this could be incorporated into `feature_err`? This + // is a pretty narrow case, however. + if tcx.sess.is_nightly_build() { + for gate in missing_secondary { + let note = format!( + "add `#![feature({})]` to the crate attributes to enable", + gate, + ); + err.help(¬e); + } + } + + err.emit(); + } + } + } + + /// Saves the parent `const_kind` before calling `f` and restores it afterwards. + fn recurse_into( + &mut self, + kind: Option<hir::ConstContext>, + def_id: Option<LocalDefId>, + f: impl FnOnce(&mut Self), + ) { + let parent_def_id = self.def_id; + let parent_kind = self.const_kind; + self.def_id = def_id; + self.const_kind = kind; + f(self); + self.def_id = parent_def_id; + self.const_kind = parent_kind; + } +} + +impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> { + type NestedFilter = nested_filter::OnlyBodies; + + fn nested_visit_map(&mut self) -> Self::Map { + self.tcx.hir() + } + + fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) { + intravisit::walk_item(self, item); + } + + fn visit_anon_const(&mut self, anon: &'tcx hir::AnonConst) { + let kind = Some(hir::ConstContext::Const); + self.recurse_into(kind, None, |this| intravisit::walk_anon_const(this, anon)); + } + + fn visit_body(&mut self, body: &'tcx hir::Body<'tcx>) { + let owner = self.tcx.hir().body_owner_def_id(body.id()); + let kind = self.tcx.hir().body_const_context(owner); + self.recurse_into(kind, Some(owner), |this| intravisit::walk_body(this, body)); + } + + fn visit_expr(&mut self, e: &'tcx hir::Expr<'tcx>) { + match &e.kind { + // Skip the following checks if we are not currently in a const context. + _ if self.const_kind.is_none() => {} + + hir::ExprKind::Loop(_, _, source, _) => { + self.const_check_violated(NonConstExpr::Loop(*source), e.span); + } + + hir::ExprKind::Match(_, _, source) => { + let non_const_expr = match source { + // These are handled by `ExprKind::Loop` above. + hir::MatchSource::ForLoopDesugar => None, + + _ => Some(NonConstExpr::Match(*source)), + }; + + if let Some(expr) = non_const_expr { + self.const_check_violated(expr, e.span); + } + } + + _ => {} + } + + intravisit::walk_expr(self, e); + } +} |