explain PassMode::Cast

compiler/rustc_target/src/abi/call/mod.rs

@@ -45,9 +45,8 @@ pub enum PassMode {
     ///
     /// The argument has a layout abi of `ScalarPair`.
     Pair(ArgAttributes, ArgAttributes),
-    /// Pass the argument after casting it, to either a single uniform or a
-    /// pair of registers. The bool indicates if a `Reg::i32()` dummy argument
-    /// is emitted before the real argument.
+    /// Pass the argument after casting it. See the `CastTarget` docs for details. The bool
+    /// indicates if a `Reg::i32()` dummy argument is emitted before the real argument.
     Cast(Box<CastTarget>, bool),
     /// Pass the argument indirectly via a hidden pointer.
     /// The `extra_attrs` value, if any, is for the extra data (vtable or length)
@@ -255,6 +254,13 @@ impl Uniform {
     }
 }
 
+/// Describes the type used for `PassMode::Cast`.
+///
+/// Passing arguments in this mode works as follows: the registers in the `prefix` (the ones that
+/// are `Some`) get laid out one after the other (using `repr(C)` layout rules). Then the
+/// `rest.unit` register type gets repeated often enough to cover `rest.size`. This describes the
+/// actual type used for the call; the Rust type of the argument is then transmuted to this ABI type
+/// (and all data in the padding between the registers is dropped).
 #[derive(Clone, PartialEq, Eq, Hash, Debug, HashStable_Generic)]
 pub struct CastTarget {
     pub prefix: [Option<Reg>; 8],