Add comments and remove unnecessary code

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 {
-        place: Box<Place<'tcx>>,
-        variant_index: VariantIdx,
-    },
+    SetDiscriminant { 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.