Create stable metric to measure long computation in Const Eval

This patch adds a `MirPass` that tracks the number of back-edges and
function calls in the CFG, adds a new MIR instruction to increment a
counter every time they are encountered during Const Eval, and emit a
warning if a configured limit is breached.

compiler/rustc_const_eval/src/interpret/eval_context.rs

@@ -46,6 +46,9 @@ pub struct InterpCx<'mir, 'tcx, M: Machine<'mir, 'tcx>> {
 
     /// The recursion limit (cached from `tcx.recursion_limit(())`)
     pub recursion_limit: Limit,
+
+    pub const_eval_limit: u32,
+    pub const_eval_counter: u32,
 }
 
 // The Phantomdata exists to prevent this type from being `Send`. If it were sent across a thread
@@ -408,6 +411,8 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             param_env,
             memory: Memory::new(),
             recursion_limit: tcx.recursion_limit(),
+            const_eval_limit: 20,
+            const_eval_counter: 0,
         }
     }
 

compiler/rustc_const_eval/src/interpret/place.rs

@@ -293,6 +293,17 @@ where
     Prov: Provenance + 'static,
     M: Machine<'mir, 'tcx, Provenance = Prov>,
 {
+    pub fn increment_const_eval_counter(&mut self) {
+        self.const_eval_counter = self.const_eval_counter + 1;
+        if self.const_eval_counter == self.const_eval_limit {
+            let mut warn = self.tcx.sess.struct_warn(format!(
+                "Const eval counter limit ({}) has been crossed",
+                self.const_eval_limit
+            ));
+            warn.emit();
+        }
+    }
+
     /// Take a value, which represents a (thin or wide) reference, and make it a place.
     /// Alignment is just based on the type. This is the inverse of `MemPlace::to_ref()`.
     ///

compiler/rustc_const_eval/src/interpret/step.rs

@@ -129,6 +129,11 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             // FIXME(#73156): Handle source code coverage in const eval
             Coverage(..) => {}
 
+            // FIXME(bryangarza): Update this to do some logic!!!
+            ConstEvalCounter => {
+                self.increment_const_eval_counter();
+            }
+
             // Defined to do nothing. These are added by optimization passes, to avoid changing the
             // size of MIR constantly.
             Nop => {}