clarify comment

Co-authored-by: scottmcm <scottmcm@users.noreply.github.com>

Author: RalfJung

compiler/rustc_const_eval/src/interpret/operator.rs

@@ -324,7 +324,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         }
     }
 
-    /// Returns the result of the specified operation, and whether it overflowed.
+    /// Returns the result of the specified operation.
+    ///
+    /// Whether this produces a scalar or a pair depends on the specific `bin_op`.
     pub fn binary_op(
         &self,
         bin_op: mir::BinOp,

compiler/rustc_middle/src/mir/tcx.rs

@@ -297,6 +297,8 @@ impl BorrowKind {
 impl BinOp {
     pub fn to_hir_binop(self) -> hir::BinOpKind {
         match self {
+            // HIR `+`/`-`/`*` can map to either of these MIR BinOp, depending
+            // on whether overflow checks are enabled or not.
             BinOp::Add | BinOp::AddWithOverflow => hir::BinOpKind::Add,
             BinOp::Sub | BinOp::SubWithOverflow => hir::BinOpKind::Sub,
             BinOp::Mul | BinOp::MulWithOverflow => hir::BinOpKind::Mul,
@@ -313,6 +315,7 @@ impl BinOp {
             BinOp::Gt => hir::BinOpKind::Gt,
             BinOp::Le => hir::BinOpKind::Le,
             BinOp::Ge => hir::BinOpKind::Ge,
+            // We don't have HIR syntax for these.
             BinOp::Cmp
             | BinOp::AddUnchecked
             | BinOp::SubUnchecked