Add HIR pass to check for ifs and loops in a const

These high-level constructs get mapped to control-flow primitives by the
time the MIR const-checker runs, making it hard to get the span for the
erroneous expression.

src/librustc/query/mod.rs

@@ -329,6 +329,11 @@ rustc_queries! {
             desc { |tcx| "checking for unstable API usage in {}", key.describe_as_module(tcx) }
         }
 
+        /// Checks the const bodies in the module for illegal operations (e.g. `if` or `loop`).
+        query check_mod_const_bodies(key: DefId) -> () {
+            desc { |tcx| "checking consts in {}", key.describe_as_module(tcx) }
+        }
+
         /// Checks the loops in the module.
         query check_mod_loops(key: DefId) -> () {
             desc { |tcx| "checking loops in {}", key.describe_as_module(tcx) }

src/librustc_passes/check_const.rs

@@ -0,0 +1,157 @@
+//! This pass checks the HIR bodies in a const context (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::hir::def_id::DefId;
+use rustc::hir::intravisit::{Visitor, NestedVisitorMap};
+use rustc::hir::map::Map;
+use rustc::hir;
+use rustc::session::Session;
+use rustc::ty::TyCtxt;
+use rustc::ty::query::Providers;
+use syntax::span_err;
+use syntax_pos::Span;
+
+use std::fmt;
+
+#[derive(Copy, Clone)]
+enum ConstKind {
+    Static,
+    StaticMut,
+    ConstFn,
+    Const,
+    AnonConst,
+}
+
+impl ConstKind {
+    fn for_body(body: &hir::Body, hir_map: &Map<'_>) -> Option<Self> {
+        let is_const_fn = |id| hir_map.fn_sig_by_hir_id(id).unwrap().header.is_const();
+
+        let owner = hir_map.body_owner(body.id());
+        let const_kind = match hir_map.body_owner_kind(owner) {
+            hir::BodyOwnerKind::Const => Self::Const,
+            hir::BodyOwnerKind::Static(hir::Mutability::MutMutable) => Self::StaticMut,
+            hir::BodyOwnerKind::Static(hir::Mutability::MutImmutable) => Self::Static,
+
+            hir::BodyOwnerKind::Fn if is_const_fn(owner) => Self::ConstFn,
+            hir::BodyOwnerKind::Fn | hir::BodyOwnerKind::Closure => return None,
+        };
+
+        Some(const_kind)
+    }
+}
+
+impl fmt::Display for ConstKind {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let s = match self {
+            Self::Static => "static",
+            Self::StaticMut => "static mut",
+            Self::Const | Self::AnonConst => "const",
+            Self::ConstFn => "const fn",
+        };
+
+        write!(f, "{}", s)
+    }
+}
+
+fn check_mod_const_bodies(tcx: TyCtxt<'_>, module_def_id: DefId) {
+    if tcx.sess.opts.debugging_opts.unleash_the_miri_inside_of_you {
+        return;
+    }
+
+    let mut vis = CheckConstVisitor::new(tcx);
+    tcx.hir().visit_item_likes_in_module(module_def_id, &mut vis.as_deep_visitor());
+}
+
+pub(crate) fn provide(providers: &mut Providers<'_>) {
+    *providers = Providers {
+        check_mod_const_bodies,
+        ..*providers
+    };
+}
+
+#[derive(Copy, Clone)]
+struct CheckConstVisitor<'tcx> {
+    sess: &'tcx Session,
+    hir_map: &'tcx Map<'tcx>,
+    const_kind: Option<ConstKind>,
+}
+
+impl<'tcx> CheckConstVisitor<'tcx> {
+    fn new(tcx: TyCtxt<'tcx>) -> Self {
+        CheckConstVisitor {
+            sess: &tcx.sess,
+            hir_map: tcx.hir(),
+            const_kind: None,
+        }
+    }
+
+    /// Emits an error when an unsupported expression is found in a const context.
+    fn const_check_violated(&self, bad_op: &str, span: Span) {
+        let const_kind = self.const_kind
+            .expect("`const_check_violated` may only be called inside a const context");
+
+        span_err!(self.sess, span, E0744, "`{}` is not allowed in a `{}`", bad_op, const_kind);
+    }
+
+    /// Saves the parent `const_kind` before visiting a nested `Body` and restores it afterwards.
+    fn recurse_into(&mut self, kind: Option<ConstKind>, f: impl FnOnce(&mut Self)) {
+        let parent_kind = self.const_kind;
+        self.const_kind = kind;
+        f(self);
+        self.const_kind = parent_kind;
+    }
+}
+
+impl<'tcx> Visitor<'tcx> for CheckConstVisitor<'tcx> {
+    fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
+        NestedVisitorMap::OnlyBodies(&self.hir_map)
+    }
+
+    fn visit_anon_const(&mut self, anon: &'tcx hir::AnonConst) {
+        let kind = Some(ConstKind::AnonConst);
+        self.recurse_into(kind, |this| hir::intravisit::walk_anon_const(this, anon));
+    }
+
+    fn visit_body(&mut self, body: &'tcx hir::Body) {
+        let kind = ConstKind::for_body(body, self.hir_map);
+        self.recurse_into(kind, |this| hir::intravisit::walk_body(this, body));
+    }
+
+    fn visit_expr(&mut self, e: &'tcx hir::Expr) {
+        match &e.kind {
+            // Skip these checks if the current item is not const.
+            _ if self.const_kind.is_none() => {}
+
+            hir::ExprKind::Loop(_, _, source) => {
+                self.const_check_violated(source.name(), e.span);
+            }
+
+            hir::ExprKind::Match(_, _, source) => {
+                use hir::MatchSource::*;
+
+                let op = match source {
+                    Normal => Some("match"),
+                    IfDesugar { .. } | IfLetDesugar { .. } => Some("if"),
+                    TryDesugar => Some("?"),
+                    AwaitDesugar => Some(".await"),
+
+                    // These are handled by `ExprKind::Loop` above.
+                    WhileDesugar | WhileLetDesugar | ForLoopDesugar => None,
+                };
+
+                if let Some(op) = op {
+                    self.const_check_violated(op, e.span);
+                }
+            }
+
+            _ => {},
+        }
+
+        hir::intravisit::walk_expr(self, e);
+    }
+}

src/librustc_passes/lib.rs

@@ -23,6 +23,7 @@ use rustc::ty::query::Providers;
 pub mod error_codes;
 
 pub mod ast_validation;
+mod check_const;
 pub mod hir_stats;
 pub mod layout_test;
 pub mod loops;
@@ -32,6 +33,7 @@ mod liveness;
 mod intrinsicck;
 
 pub fn provide(providers: &mut Providers<'_>) {
+    check_const::provide(providers);
     entry::provide(providers);
     loops::provide(providers);
     liveness::provide(providers);