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Add comments explaining how codegen works for dyn Trait methods

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This commit is contained in:
Michael Hewson 2018-11-01 18:28:52 -04:00
parent a468da9cfb
commit 192e7c4b51
2 changed files with 11 additions and 2 deletions
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5
src/librustc_codegen_llvm/abi.rs
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@ -315,6 +315,11 @@ impl<'tcx> FnTypeExt<'tcx> for FnType<'tcx, Ty<'tcx>> {
_ => bug!("receiver type has unsupported layout: {:?}", layout)
}
// In the case of Rc<Self>, we need to explicitly pass a *mut RcBox<Self>
// with a Scalar (not ScalarPair) ABI. This is a hack that is understood
// elsewhere in the compiler as a method on a `dyn Trait`.
// To get the type `*mut RcBox<Self>`, we just keep unwrapping newtypes until we
// get a built-in pointer type
let mut fat_pointer_layout = layout;
'descend_newtypes: while !fat_pointer_layout.ty.is_unsafe_ptr()
&& !fat_pointer_layout.ty.is_region_ptr()

8
src/librustc_codegen_llvm/mir/block.rs
View File

@ -647,8 +647,12 @@ impl FunctionCx<'a, 'll, 'tcx> {
if let (0, Some(ty::InstanceDef::Virtual(_, idx))) = (i, def) {
if let Pair(..) = op.val {
// descend through newtype wrappers until `op` is a builtin pointer to
// `dyn Trait`, e.g. `*const dyn Trait`, `&mut dyn Trait`
// In the case of Rc<Self>, we need to explicitly pass a
// *mut RcBox<Self> with a Scalar (not ScalarPair) ABI. This is a hack
// that is understood elsewhere in the compiler as a method on
// `dyn Trait`.
// To get a `*mut RcBox<Self>`, we just keep unwrapping newtypes until
// we get a value of a built-in pointer type
'descend_newtypes: while !op.layout.ty.is_unsafe_ptr()
&& !op.layout.ty.is_region_ptr()
{