Auto merge of #66070 - petrochenkov:regattr, r=matthewjasper

Support registering inert attributes and attribute tools using crate-level attributes

And remove `#[feature(custom_attribute)]`.
(`rustc_plugin::Registry::register_attribute` is not removed yet, I'll do it in a follow up PR.)

```rust
#![register_attr(my_attr)]
#![register_tool(my_tool)]

#[my_attr] // OK
#[my_tool::anything] // OK
fn main() {}
```

---
Some tools (`rustfmt` and `clippy`) used in tool attributes are hardcoded in the compiler.
We need some way to introduce them without hardcoding as well.

This PR introduces a way to do it with a crate level attribute.
The previous attempt to introduce them through command line (https://github.com/rust-lang/rust/pull/57921) met some resistance.

This probably needs to go through an RFC before stabilization.
However, I'd prefer to land *this* PR without an RFC to able to remove `#[feature(custom_attribute)]` and `Registry::register_attribute` while also providing a replacement.

---
`register_attr` is a direct replacement for `#![feature(custom_attribute)]` (https://github.com/rust-lang/rust/issues/29642), except it doesn't rely on implicit fallback from unresolved attributes to custom attributes (which was always hacky and is the primary reason for the removal of `custom_attribute`) and requires registering the attribute explicitly.
It's not clear whether it should go through stabilization or not.
It's quite possible that all the uses should migrate to `#![register_tool]` (https://github.com/rust-lang/rust/issues/66079) instead.

---

Details:
- The naming is `register_attr`/`register_tool` rather than some `register_attributes` (plural, no abbreviation) for consistency with already existing attributes like `cfg_attr`, or `feature`, etc.
---
Previous attempt: https://github.com/rust-lang/rust/pull/57921
cc https://github.com/rust-lang/rust/issues/44690
Tracking issues: #66079 (`register_tool`), #66080 (`register_attr`)
Closes https://github.com/rust-lang/rust/issues/29642

3 files changed, 17 insertions, 3 deletions

diff --git a/src/libsyntax/feature_gate/active.rs b/src/libsyntax/feature_gate/active.rs
index 1d198fca56b..d59d0f0e28e 100644
--- a/src/libsyntax/feature_gate/active.rs
+++ b/src/libsyntax/feature_gate/active.rs
@@ -265,9 +265,6 @@ declare_features! (
     /// Allows the use of SIMD types in functions declared in `extern` blocks.
     (active, simd_ffi, "1.0.0", Some(27731), None),
 
-    /// Allows using custom attributes (RFC 572).
-    (active, custom_attribute, "1.0.0", Some(29642), None),
-
     /// Allows using non lexical lifetimes (RFC 2094).
     (active, nll, "1.0.0", Some(43234), None),
 
@@ -526,6 +523,12 @@ declare_features! (
     /// Allows using the `efiapi` ABI.
     (active, abi_efiapi, "1.40.0", Some(65815), None),
 
+    /// Allows using the `#[register_attr]` attribute.
+    (active, register_attr, "1.41.0", Some(66080), None),
+
+    /// Allows using the `#[register_attr]` attribute.
+    (active, register_tool, "1.41.0", Some(66079), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------
diff --git a/src/libsyntax/feature_gate/builtin_attrs.rs b/src/libsyntax/feature_gate/builtin_attrs.rs
index b32a887c6b2..608cc2a09cb 100644
--- a/src/libsyntax/feature_gate/builtin_attrs.rs
+++ b/src/libsyntax/feature_gate/builtin_attrs.rs
@@ -329,6 +329,14 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
 
     gated!(ffi_returns_twice, Whitelisted, template!(Word), experimental!(ffi_returns_twice)),
     gated!(track_caller, Whitelisted, template!(Word), experimental!(track_caller)),
+    gated!(
+        register_attr, CrateLevel, template!(List: "attr1, attr2, ..."),
+        experimental!(register_attr),
+    ),
+    gated!(
+        register_tool, CrateLevel, template!(List: "tool1, tool2, ..."),
+        experimental!(register_tool),
+    ),
 
     // ==========================================================================
     // Internal attributes: Stability, deprecation, and unsafe:
diff --git a/src/libsyntax/feature_gate/removed.rs b/src/libsyntax/feature_gate/removed.rs
index c7b931a6f70..f0aa74c65df 100644
--- a/src/libsyntax/feature_gate/removed.rs
+++ b/src/libsyntax/feature_gate/removed.rs
@@ -67,6 +67,9 @@ declare_features! (
      Some("merged into `#![feature(slice_patterns)]`")),
     (removed, macro_reexport, "1.0.0", Some(29638), None,
      Some("subsumed by `pub use`")),
+    /// Allows using custom attributes (RFC 572).
+    (removed, custom_attribute, "1.0.0", Some(29642), None,
+     Some("removed in favor of `#![register_tool]` and `#![register_attr]`")),
     (removed, pushpop_unsafe, "1.2.0", None, None, None),
     (removed, needs_allocator, "1.4.0", Some(27389), None,
      Some("subsumed by `#![feature(allocator_internals)]`")),