Add comments and remove unnecessary code

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -561,11 +561,6 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
         throw_unsup_format!("pointer arithmetic or comparison is not supported at compile-time");
     }
 
-    // Not used here, but used by Miri (see `src/tools/miri/src/machine.rs`).
-    fn before_terminator(_ecx: &mut InterpCx<'mir, 'tcx, Self>) -> InterpResult<'tcx> {
-        Ok(())
-    }
-
     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 {

compiler/rustc_middle/src/mir/syntax.rs

@@ -286,10 +286,7 @@ pub enum StatementKind<'tcx> {
     /// This is permitted for both generators and ADTs. This does not necessarily write to the
     /// entire place; instead, it writes to the minimum set of bytes as required by the layout for
     /// the type.
-    SetDiscriminant {
+    SetDiscriminant { place: Box<Place<'tcx>>, variant_index: VariantIdx },
-        place: Box<Place<'tcx>>,
-        variant_index: VariantIdx,
-    },
 
     /// Deinitializes the place.
     ///
@@ -358,6 +355,10 @@ pub enum StatementKind<'tcx> {
     /// This avoids adding a new block and a terminator for simple intrinsics.
     Intrinsic(Box<NonDivergingIntrinsic<'tcx>>),
 
+    /// Instructs the const eval interpreter to increment a counter; this counter is used to track
+    /// how many steps the interpreter has taken. It is used to prevent the user from writing const
+    /// code that runs for too long or infinitely. Other than in the const eval interpreter, this
+    /// is a no-op.
     ConstEvalCounter,
 
     /// No-op. Useful for deleting instructions without affecting statement indices.

compiler/rustc_mir_transform/src/ctfe_limit.rs

@@ -1,3 +1,5 @@
+//! A pass that inserts the `ConstEvalCounter` instruction into any blocks that have a back edge
+//! (thus indicating there is a loop in the CFG), or whose terminator is a function call.
 use crate::MirPass;
 
 use rustc_middle::mir::{