Auto merge of #106227 - bryangarza:ctfe-limit, r=oli-obk

Use stable metric for const eval limit instead of current terminator-based logic

This patch adds a `MirPass` that inserts a new MIR instruction `ConstEvalCounter` to any loops and function calls in the CFG. This instruction is used during Const Eval to count against the `const_eval_limit`, and emit the `StepLimitReached` error, replacing the current logic which uses Terminators only.

The new method of counting loops and function calls should be more stable across compiler versions (i.e., not cause crates that compiled successfully before, to no longer compile when changes to the MIR generation/optimization are made).

Also see: #103877

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -561,8 +561,8 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         throw_unsup_format!("pointer arithmetic or comparison is not supported at compile-time");
     }
 
-    fn before_terminator(ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
-        // The step limit has already been hit in a previous call to `before_terminator`.
+    fn increment_const_eval_counter(ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
+        // The step limit has already been hit in a previous call to `increment_const_eval_counter`.
         if ecx.machine.steps_remaining == 0 {
             return Ok(());
         }

compiler/rustc_const_eval/src/interpret/machine.rs

@@ -244,12 +244,18 @@ pub trait Machine<'mir, 'tcx>: Sized {
     }
 
     /// Called before a basic block terminator is executed.
-    /// You can use this to detect endlessly running programs.
     #[inline]
     fn before_terminator(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
         Ok(())
     }
 
+    /// Called when the interpreter encounters a `StatementKind::ConstEvalCounter` instruction.
+    /// You can use this to detect long or endlessly running programs.
+    #[inline]
+    fn increment_const_eval_counter(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
+        Ok(())
+    }
+
     /// Called before a global allocation is accessed.
     /// `def_id` is `Some` if this is the "lazy" allocation of a static.
     #[inline]

compiler/rustc_const_eval/src/interpret/step.rs

@@ -129,6 +129,10 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             // FIXME(#73156): Handle source code coverage in const eval
             Coverage(..) => {}
 
+            ConstEvalCounter => {
+                M::increment_const_eval_counter(self)?;
+            }
+
             // Defined to do nothing. These are added by optimization passes, to avoid changing the
             // size of MIR constantly.
             Nop => {}

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -693,6 +693,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
             | StatementKind::AscribeUserType(..)
             | StatementKind::Coverage(..)
             | StatementKind::Intrinsic(..)
+            | StatementKind::ConstEvalCounter
             | StatementKind::Nop => {}
         }
     }

compiler/rustc_const_eval/src/transform/validate.rs

@@ -761,6 +761,7 @@ impl<'a, 'tcx> Visitor<'tcx> for TypeChecker<'a, 'tcx> {
             StatementKind::StorageLive(..)
             | StatementKind::StorageDead(..)
             | StatementKind::Coverage(_)
+            | StatementKind::ConstEvalCounter
             | StatementKind::Nop => {}
         }