MIR operators: clarify Shl/Shr handling of negative offsets

Author: RalfJung

compiler/rustc_middle/src/mir/syntax.rs

@@ -1480,13 +1480,15 @@ pub enum BinOp {
     BitOr,
     /// The `<<` operator (shift left)
     ///
-    /// The offset is truncated to the size of the first operand and made unsigned before shifting.
+    /// The offset (RHS) is re-interpreted as an unsigned value of the same size and then truncated
+    /// to the size of the first operand before shifting.
     Shl,
     /// Like `Shl`, but is UB if the RHS >= LHS::BITS or RHS < 0
     ShlUnchecked,
     /// The `>>` operator (shift right)
     ///
-    /// The offset is truncated to the size of the first operand and made unsigned before shifting.
+    /// The offset (RHS) is re-interpreted as an unsigned value of the same size and then truncated
+    /// to the size of the first operand before shifting.
     ///
     /// This is an arithmetic shift if the LHS is signed
     /// and a logical shift if the LHS is unsigned.