Auto merge of #36551 - eddyb:meta-games, r=nikomatsakis

Refactor away RBML from rustc_metadata.

RBML and `ty{en,de}code` have had their long-overdue purge. Summary of changes:
* Metadata is now a tree encoded in post-order and with relative backward references pointing to children nodes. With auto-deriving and type safety, this makes maintenance and adding new information to metadata painless and bug-free by default. It's also more compact and cache-friendly (cache misses should be proportional to the depth of the node being accessed, not the number of siblings as in EBML/RBML).
* Metadata sizes have been reduced, for `libcore` it went down 16% (`8.38MB` -> `7.05MB`) and for `libstd` 14% (`3.53MB` -> `3.03MB`), while encoding more or less the same information
* Specialization is used in the bundled `libserialize` (crates.io `rustc_serialize` remains unaffected) to customize the encoding (and more importantly, decoding) of various types, most notably those interned in the `TyCtxt`. Some of this abuses a soundness hole pending a fix (cc @aturon), but when that fix arrives, we'll move to macros 1.1 `#[derive]` and custom `TyCtxt`-aware serialization traits.
* Enumerating children of modules from other crates is now orthogonal to describing those items via `Def` - this is a step towards bridging crate-local HIR and cross-crate metadata
* `CrateNum` has been moved to `rustc` and both it and `NodeId` are now newtypes instead of `u32` aliases, for specializing their decoding. This is `[syntax-breaking]` (cc @Manishearth ).

cc @rust-lang/compiler

mk/crates.mk

@@ -61,7 +61,7 @@ RUSTC_CRATES := rustc rustc_typeck rustc_mir rustc_borrowck rustc_resolve rustc_
                 rustc_plugin rustc_metadata rustc_passes rustc_save_analysis \
                 rustc_const_eval rustc_const_math rustc_incremental rustc_macro
 HOST_CRATES := syntax syntax_ext proc_macro syntax_pos $(RUSTC_CRATES) rustdoc fmt_macros \
-		flate arena graphviz rbml log serialize
+		flate arena graphviz log serialize
 TOOLS := compiletest rustdoc rustc rustbook error_index_generator
 
 DEPS_core :=
@@ -96,7 +96,6 @@ DEPS_getopts := std
 DEPS_graphviz := std
 DEPS_log := std
 DEPS_num := std
-DEPS_rbml := std log serialize
 DEPS_serialize := std log
 DEPS_term := std
 DEPS_test := std getopts term native:rust_test_helpers
@@ -110,7 +109,7 @@ DEPS_rustc_const_math := std syntax log serialize
 DEPS_rustc_const_eval := rustc_const_math rustc syntax log serialize \
 			     rustc_back graphviz syntax_pos
 
-DEPS_rustc := syntax fmt_macros flate arena serialize getopts rbml \
+DEPS_rustc := syntax fmt_macros flate arena serialize getopts \
               log graphviz rustc_llvm rustc_back rustc_data_structures\
 	      rustc_const_math syntax_pos rustc_errors
 DEPS_rustc_back := std syntax flate log libc
@@ -126,7 +125,7 @@ DEPS_rustc_errors := log libc serialize syntax_pos
 DEPS_rustc_lint := rustc log syntax syntax_pos rustc_const_eval
 DEPS_rustc_llvm := native:rustllvm libc std rustc_bitflags
 DEPS_rustc_macro := std syntax
-DEPS_rustc_metadata := rustc syntax syntax_pos rustc_errors rbml rustc_const_math \
+DEPS_rustc_metadata := rustc syntax syntax_pos rustc_errors rustc_const_math \
 			rustc_macro syntax_ext
 DEPS_rustc_passes := syntax syntax_pos rustc core rustc_const_eval rustc_errors
 DEPS_rustc_mir := rustc syntax syntax_pos rustc_const_math rustc_const_eval rustc_bitflags
@@ -137,7 +136,7 @@ DEPS_rustc_privacy := rustc log syntax syntax_pos
 DEPS_rustc_trans := arena flate getopts graphviz libc rustc rustc_back \
                     log syntax serialize rustc_llvm rustc_platform_intrinsics \
                     rustc_const_math rustc_const_eval rustc_incremental rustc_errors syntax_pos
-DEPS_rustc_incremental := rbml rustc syntax_pos serialize rustc_data_structures
+DEPS_rustc_incremental := rustc syntax_pos serialize rustc_data_structures
 DEPS_rustc_save_analysis := rustc log syntax syntax_pos serialize
 DEPS_rustc_typeck := rustc syntax syntax_pos rustc_platform_intrinsics rustc_const_math \
                      rustc_const_eval rustc_errors

mk/tests.mk

@@ -27,7 +27,7 @@ TEST_TARGET_CRATES = $(filter-out core rustc_unicode alloc_system libc \
 				  panic_abort,$(TARGET_CRATES)) \
 			collectionstest coretest
 TEST_DOC_CRATES = $(DOC_CRATES) arena flate fmt_macros getopts graphviz \
-                log rand rbml serialize syntax term test
+                log rand serialize syntax term test
 TEST_HOST_CRATES = $(filter-out rustc_typeck rustc_borrowck rustc_resolve \
 		   		rustc_trans rustc_lint,\
                      $(HOST_CRATES))