Add miri infinite loop detection

Use the approach suggested by @oli-obk, a table holding `EvalState`
hashes and a table holding full `EvalState` objects. When a hash
collision is observed, the state is cloned and put into the full
table. If the collision was not spurious, it will be detected during the
next iteration of the infinite loop.

src/librustc_mir/interpret/eval_context.rs

@@ -10,6 +10,7 @@
 use rustc::ty::subst::{Subst, Substs};
 use rustc::ty::{self, Ty, TyCtxt, TypeAndMut};
 use rustc::ty::query::TyCtxtAt;
+use rustc_data_structures::fx::{FxHashSet, FxHasher};
 use rustc_data_structures::indexed_vec::{IndexVec, Idx};
 use rustc::mir::interpret::{
     FrameInfo, GlobalId, Value, Scalar,
@@ -34,15 +35,16 @@ pub struct EvalContext<'a, 'mir, 'tcx: 'a + 'mir, M: Machine<'mir, 'tcx>> {
     pub param_env: ty::ParamEnv<'tcx>,
 
     /// Virtual memory and call stack.
-    state: EvalState<'a, 'mir, 'tcx, M>,
+    pub(crate) state: EvalState<'a, 'mir, 'tcx, M>,
 
     /// The maximum number of stack frames allowed
     pub(crate) stack_limit: usize,
 
-    /// The maximum number of terminators that may be evaluated.
-    /// This prevents infinite loops and huge computations from freezing up const eval.
-    /// Remove once halting problem is solved.
-    pub(crate) terminators_remaining: usize,
+    /// The number of terminators to be evaluated before enabling the infinite
+    /// loop detector.
+    pub(crate) steps_until_detector_enabled: usize,
+
+    pub(crate) loop_detector: InfiniteLoopDetector<'a, 'mir, 'tcx, M>,
 }
 
 pub(crate) struct EvalState<'a, 'mir, 'tcx: 'a + 'mir, M: Machine<'mir, 'tcx>> {
@@ -178,6 +180,56 @@ fn hash<H: Hasher>(&self, state: &mut H) {
     }
 }
 
+pub(crate) struct InfiniteLoopDetector<'a, 'mir, 'tcx: 'a + 'mir, M: Machine<'mir, 'tcx>> {
+    /// The set of all `EvalState` *hashes* observed by this detector.
+    ///
+    /// Not a proper bloom filter.
+    bloom: FxHashSet<u64>,
+
+    /// The set of all `EvalState`s observed by this detector.
+    ///
+    /// An `EvalState` will only be fully cloned once it has caused a collision
+    /// in `bloom`. As a result, the detector must observe *two* full cycles of
+    /// an infinite loop before it triggers.
+    snapshots: FxHashSet<EvalState<'a, 'mir, 'tcx, M>>,
+}
+
+impl<'a, 'mir, 'tcx, M> Default for InfiniteLoopDetector<'a, 'mir, 'tcx, M>
+    where M: Machine<'mir, 'tcx>,
+          'tcx: 'a + 'mir,
+{
+    fn default() -> Self {
+        InfiniteLoopDetector {
+            bloom: FxHashSet::default(),
+            snapshots: FxHashSet::default(),
+        }
+    }
+}
+
+impl<'a, 'mir, 'tcx, M> InfiniteLoopDetector<'a, 'mir, 'tcx, M>
+    where M: Machine<'mir, 'tcx>,
+          'tcx: 'a + 'mir,
+{
+    pub fn observe(&mut self, snapshot: &EvalState<'a, 'mir, 'tcx, M>) -> Result<(), (/*TODO*/)> {
+        let mut fx = FxHasher::default();
+        snapshot.hash(&mut fx);
+        let hash = fx.finish();
+
+        if self.bloom.insert(hash) {
+            // No collision
+            return Ok(())
+        }
+
+        if self.snapshots.insert(snapshot.clone()) {
+            // Spurious collision or first cycle
+            return Ok(())
+        }
+
+        // Second cycle,
+        Err(())
+    }
+}
+
 #[derive(Clone, Debug, Eq, PartialEq, Hash)]
 pub enum StackPopCleanup {
     /// The stackframe existed to compute the initial value of a static/constant, make sure it
@@ -280,16 +332,17 @@ pub fn new(
                 stack: Vec::new(),
             },
             stack_limit: tcx.sess.const_eval_stack_frame_limit,
-            terminators_remaining: MAX_TERMINATORS,
+            loop_detector: Default::default(),
+            steps_until_detector_enabled: MAX_TERMINATORS,
         }
     }
 
     pub(crate) fn with_fresh_body<F: FnOnce(&mut Self) -> R, R>(&mut self, f: F) -> R {
         let stack = mem::replace(self.stack_mut(), Vec::new());
-        let terminators_remaining = mem::replace(&mut self.terminators_remaining, MAX_TERMINATORS);
+        let steps = mem::replace(&mut self.steps_until_detector_enabled, MAX_TERMINATORS);
         let r = f(self);
         *self.stack_mut() = stack;
-        self.terminators_remaining = terminators_remaining;
+        self.steps_until_detector_enabled = steps;
         r
     }
 
@@ -634,7 +687,7 @@ pub(super) fn eval_rvalue_into_place(
             }
 
             Aggregate(ref kind, ref operands) => {
-                self.inc_step_counter_and_check_limit(operands.len());
+                self.inc_step_counter_and_detect_loops(operands.len());
 
                 let (dest, active_field_index) = match **kind {
                     mir::AggregateKind::Adt(adt_def, variant_index, _, active_field_index) => {

src/librustc_mir/interpret/step.rs

@@ -8,12 +8,16 @@
 use super::{EvalContext, Machine};
 
 impl<'a, 'mir, 'tcx, M: Machine<'mir, 'tcx>> EvalContext<'a, 'mir, 'tcx, M> {
-    pub fn inc_step_counter_and_check_limit(&mut self, n: usize) {
-        self.terminators_remaining = self.terminators_remaining.saturating_sub(n);
-        if self.terminators_remaining == 0 {
+    pub fn inc_step_counter_and_detect_loops(&mut self, n: usize) {
+        self.steps_until_detector_enabled
+            = self.steps_until_detector_enabled.saturating_sub(n);
+
+        if self.steps_until_detector_enabled == 0 {
+            let _ = self.loop_detector.observe(&self.state); // TODO: Handle error
+
             // FIXME(#49980): make this warning a lint
             self.tcx.sess.span_warn(self.frame().span, "Constant evaluating a complex constant, this might take some time");
-            self.terminators_remaining = 1_000_000;
+            self.steps_until_detector_enabled = 1_000_000;
         }
     }
 
@@ -36,7 +40,7 @@ pub fn step(&mut self) -> EvalResult<'tcx, bool> {
             return Ok(true);
         }
 
-        self.inc_step_counter_and_check_limit(1);
+        self.inc_step_counter_and_detect_loops(1);
 
         let terminator = basic_block.terminator();
         assert_eq!(old_frames, self.cur_frame());