Auto merge of #49219 - eddyb:proc-macro-decouple, r=<try>

Decouple proc_macro from the rest of the compiler.

This PR removes all dependencies of `proc_macro` on compiler crates and allows multiple copies of `proc_macro`, built even by different compilers (but from the same source), to interoperate.

On the compiler (aka "frontend") side:
* `Registry` is implemented (as until now)
  * instead of function pointers, the `Expand{1,2}` wrapper types are received
* `FrontendInterface` is implemented to provide the concrete type and methods
  * the concrete types are completely separated from the `proc_macro` public API
  * the only use of the implementer is to be passed to `Expand{1,2}::run`

On the proc macro side (potentially using a different `proc_macro` instance):
* `&mut Registry` is passed to the registrar (as until now)
* `Expand{1,2}` wrappers are created to be passed to the `Registry`
  * they encapsulate the `proc_macro` instance used by the macro crate
  * their `run` method will set up the "current frontend" of that instance

`proc_macro` public APIs call into the "current frontend" via the internal `bridge`:
* only a currently running proc macro can interact with those APIs
  * the frontend itself might not be able to (if it uses a different `proc_macro` instance)
* the `bridge` uses C ABI to avoid Rust ABI instability and "generation tagging" for safety
* each invocation of a proc macro results in a different "generation"
  * specifically, opaque `proc_macro` types wrapping concrete frontend types are tagged
  * this prevents using values of those types across invocations (even if they can be kept)
* an ABI mismatch between two versions of `proc_macro` is only possible during stage1
  * specifically, rustc compiled by stage0 but proc macros compiled by stage1
  * can only affect running tests at stage1 or the compiler using proc macros
  * defficiencies in the `bridge` can be addressed without waiting for a release cycle

r? @alexcrichton cc @jseyfried @nikomatsakis @Zoxc @thepowersgang

src/bootstrap/builder.rs

@@ -306,7 +306,7 @@ impl<'a> Builder<'a> {
                 test::CompileFail, test::ParseFail, test::RunFail, test::RunPassValgrind,
                 test::MirOpt, test::Codegen, test::CodegenUnits, test::Incremental, test::Debuginfo,
                 test::UiFullDeps, test::RunPassFullDeps, test::RunFailFullDeps,
-                test::CompileFailFullDeps, test::IncrementalFullDeps, test::Rustdoc, test::Pretty,
+                test::CompileFailFullDeps, test::Rustdoc, test::Pretty,
                 test::RunPassPretty, test::RunFailPretty, test::RunPassValgrindPretty,
                 test::RunPassFullDepsPretty, test::RunFailFullDepsPretty, test::RunMake,
                 test::Crate, test::CrateLibrustc, test::CrateRustdoc, test::Linkcheck,

src/bootstrap/test.rs

@@ -704,12 +704,6 @@ host_test!(CompileFailFullDeps {
     suite: "compile-fail-fulldeps"
 });
 
-host_test!(IncrementalFullDeps {
-    path: "src/test/incremental-fulldeps",
-    mode: "incremental",
-    suite: "incremental-fulldeps"
-});
-
 host_test!(Rustdoc {
     path: "src/test/rustdoc",
     mode: "rustdoc",

src/libproc_macro/Cargo.toml

@@ -8,7 +8,4 @@ path = "lib.rs"
 crate-type = ["dylib"]
 
 [dependencies]
-syntax = { path = "../libsyntax" }
-syntax_pos = { path = "../libsyntax_pos" }
-rustc_errors = { path = "../librustc_errors" }
-rustc_data_structures = { path = "../librustc_data_structures" }
+lazy_static = "1.0.0"