add implementation details

src/rustc_design.md

@@ -1,9 +1,29 @@
 # rustc Design:
-This chapter is not relevant for an autodiff user, but might still be interesting for thos curious to include Enzyme into a new language. It is mostly tailored towards people already working on rustc.
+This chapter is not relevant for an autodiff user, but might still be interesting for those curious to include Enzyme into a new language. It is mostly tailored towards people already working on rustc. I would like to claim it is also there to help reviewers, but realistically I just write things down here because I can't remember what I coded yesterday. This is likely incomplete, so please create PR's to extend it! 
 
 The first step was to integrate Enzyme (core) itself. We have added it as a submodule to `src/tools/enzyme` and updated the bootstraping accordingly. 
-We had added our autodiff macro in `/library`, but this approach had to be abandonned for cross-compilation). The current macro is implemented as a `rustc_internal_macro`.
+We had added our autodiff macro in `/library`, but this approach had to be abandonned for cross-compilation. The current macro is implemented as a `rustc_internal_macro`.
+
+We had to alter the compilation pipeline in a few places when autodiff is used.
+The obvious one is that we have to prevent our source function from getting competely inlined. 
+
+
+`compiler/rustc_codegen_llvm/src/attributes.rs`: 
+In `from_fn_attrs` we query `cx.tcx.autodiff_attrs(instance.def_id());`
+and if we get an autodiff is active (that is a source or a placeholder). 
+This is to make sure that neither of the too gets inlined, for that we mark them as `InlineAttr::Never` and pray for the best.
+
+`compiler/rustc_codegen_ssa/src/codegen_attrs.rs`
+We added `autodiff_attrs`, in which we parse `rustc_autodiff` attributes applied to Functions, and create `AutoDiffAttrs` out of it, which we return.
+
+`compiler/rustc_monomorphize/src/partitioning.rs`
+In `place_mono_items` we check if the `characteristic_def_id` of a function exists, and if yes (and if it has an autodiff attrs) we block it from being inserted into `internalization_candidates` to prevent inlining.
+
+`compiler/rustc_monomorphize/src/partitioning.rs`
+In `collect_and_partition_mono_items` we update things.
+
+`compiler/rustc_codegen_ssa/src/back/write.rs`
+In `generate_lto_work` we pass the autodiff worklist to our backend autodiff function doing the actual work. We check there that our autodiff worklist is empty if we don't use fat-lto.
+
 
-There do have to alter the compilation pipeline in a few aspects when autodiff is used.
-The obvious one is that we have to prevent our source function from getting competely inlined. We do this in <> by marking it as <>