codegen: tweak/extend shift comments

compiler/rustc_codegen_ssa/src/base.rs

@@ -293,12 +293,13 @@ pub fn coerce_unsized_into<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     }
 }
 
-/// Returns `rhs` sufficiently masked, truncated, and/or extended so that
-/// it can be used to shift `lhs`.
+/// Returns `rhs` sufficiently masked, truncated, and/or extended so that it can be used to shift
+/// `lhs`: it has the same size as `lhs`, and the value, when interpreted unsigned (no matter its
+/// type), will not exceed the size of `lhs`.
 ///
-/// Shifts in MIR are all allowed to have mismatched LHS & RHS types.
+/// Shifts in MIR are all allowed to have mismatched LHS & RHS types, and signed RHS.
 /// The shift methods in `BuilderMethods`, however, are fully homogeneous
-/// (both parameters and the return type are all the same type).
+/// (both parameters and the return type are all the same size) and assume an unsigned RHS.
 ///
 /// If `is_unchecked` is false, this masks the RHS to ensure it stays in-bounds,
 /// as the `BuilderMethods` shifts are UB for out-of-bounds shift amounts.

compiler/rustc_codegen_ssa/src/traits/builder.rs

@@ -110,8 +110,16 @@ pub trait BuilderMethods<'a, 'tcx>:
     fn frem(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn frem_fast(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn frem_algebraic(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
+    /// Generate a left-shift. Both operands must have the same size. The right operand must be
+    /// interpreted as unsigned and can be assumed to be less than the size of the left operand.
     fn shl(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
+    /// Generate a logical right-shift. Both operands must have the same size. The right operand
+    /// must be interpreted as unsigned and can be assumed to be less than the size of the left
+    /// operand.
     fn lshr(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
+    /// Generate an arithmetic right-shift. Both operands must have the same size. The right operand
+    /// must be interpreted as unsigned and can be assumed to be less than the size of the left
+    /// operand.
     fn ashr(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn unchecked_sadd(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn unchecked_uadd(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;