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(&note);
+                    }
+                }
+
+                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);
+    }
+}