Do not get proc_macro from the sysroot in rustc

With the stage0 refactor the proc_macro version found in the sysroot
will no longer always match the proc_macro version that proc-macros get
compiled with by the rustc executable that uses this proc_macro. This
will cause problems as soon as the ABI of the bridge gets changed to
implement new features or change the way existing features work.

To fix this, this commit changes rustc crates to depend directly on the
local version of proc_macro which will also be used in the sysroot that
rustc will build.

Author: bjorn3

proc_macro/Cargo.toml

@@ -10,3 +10,7 @@ std = { path = "../std" }
 # loaded from sysroot causing duplicate lang items and other similar errors.
 core = { path = "../core" }
 rustc-literal-escaper = { version = "0.0.2", features = ["rustc-dep-of-std"] }
+
+[features]
+default = ["rustc-dep-of-std"]
+rustc-dep-of-std = []

proc_macro/src/lib.rs

@@ -32,6 +32,7 @@
 #![recursion_limit = "256"]
 #![allow(internal_features)]
 #![deny(ffi_unwind_calls)]
+#![allow(rustc::internal)] // Can't use FxHashMap when compiled as part of the standard library
 #![warn(rustdoc::unescaped_backticks)]
 #![warn(unreachable_pub)]
 #![deny(unsafe_op_in_unsafe_fn)]
@@ -95,7 +96,7 @@ pub fn is_available() -> bool {
 ///
 /// This is both the input and output of `#[proc_macro]`, `#[proc_macro_attribute]`
 /// and `#[proc_macro_derive]` definitions.
-#[rustc_diagnostic_item = "TokenStream"]
+#[cfg_attr(feature = "rustc-dep-of-std", rustc_diagnostic_item = "TokenStream")]
 #[stable(feature = "proc_macro_lib", since = "1.15.0")]
 #[derive(Clone)]
 pub struct TokenStream(Option<bridge::client::TokenStream>);