From 192e7c4b51ab35f4cfbbacde9eeffa8e12dba873 Mon Sep 17 00:00:00 2001
From: Michael Hewson <michael@michaelhewson.ca>
Date: Thu, 1 Nov 2018 18:28:52 -0400
Subject: [PATCH] Add comments explaining how codegen works for `dyn Trait`
 methods

---
 src/librustc_codegen_llvm/abi.rs       | 5 +++++
 src/librustc_codegen_llvm/mir/block.rs | 8 ++++++--
 2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/src/librustc_codegen_llvm/abi.rs b/src/librustc_codegen_llvm/abi.rs
index 1662b57b8b3..e50534a4e1d 100644
--- a/src/librustc_codegen_llvm/abi.rs
+++ b/src/librustc_codegen_llvm/abi.rs
@@ -315,6 +315,11 @@ fn new_vtable(cx: &CodegenCx<'ll, '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()
diff --git a/src/librustc_codegen_llvm/mir/block.rs b/src/librustc_codegen_llvm/mir/block.rs
index 0cb4963f97f..a7f4c48c89b 100644
--- a/src/librustc_codegen_llvm/mir/block.rs
+++ b/src/librustc_codegen_llvm/mir/block.rs
@@ -647,8 +647,12 @@ fn codegen_terminator(&mut self,
 
                     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()
                             {
-- 
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