Auto merge of #62086 - petrochenkov:builtout, r=eddyb

Define built-in macros through libcore

This PR defines built-in macros through libcore using a scheme similar to lang items (attribute `#[rustc_builtin_macro]`).
All the macro properties (stability, visibility, etc.) are taken from the source code in libcore, with exception of the expander function transforming input tokens/AST into output tokens/AST, which is still provided by the compiler.

The macros are made available to user code through the standard library prelude (`{core,std}::prelude::v1`), so they are still always in scope.
As a result **built-in macros now have stable absolute addresses in the library**, like `core::prelude::v1::line!()`, this is an insta-stable change.

Right now `prelude::v1` is the only publicly available absolute address for these macros, but eventually they can be moved into more appropriate locations with library team approval (e.g. `Clone` derive -> `core::clone::Clone`).

Now when built-in macros have canonical definitions they can be imported or reexported without issues (https://github.com/rust-lang/rust/issues/61687).

Other changes:
- You can now define a derive macro with a name matching one of the built-in derives (https://github.com/rust-lang/rust/issues/52269). This was an artificial restriction that could be worked around with import renaming anyway.

Known regressions:
- Empty library crate with a crate-level `#![test]` attribute no longer compiles without `--test`. Previously it didn't compile *with* `--test` or with the bin crate type.

Fixes https://github.com/rust-lang/rust/issues/61687
Fixes https://github.com/rust-lang/rust/issues/61804
r? @eddyb

1 files changed, 0 insertions, 6 deletions

diff --git a/src/libsyntax_ext/proc_macro_decls.rs b/src/libsyntax_ext/proc_macro_decls.rs
index 08c40dde56c..303d5f00deb 100644
--- a/src/libsyntax_ext/proc_macro_decls.rs
+++ b/src/libsyntax_ext/proc_macro_decls.rs
@@ -1,7 +1,5 @@
 use std::mem;
 
-use crate::deriving;
-
 use syntax::ast::{self, Ident};
 use syntax::attr;
 use syntax::source_map::{ExpnInfo, ExpnKind, respan};
@@ -136,10 +134,6 @@ impl<'a> CollectProcMacros<'a> {
             self.handler.span_err(trait_attr.span,
                                   &format!("`{}` cannot be a name of derive macro", trait_ident));
         }
-        if deriving::is_builtin_trait(trait_ident.name) {
-            self.handler.span_err(trait_attr.span,
-                                  "cannot override a built-in derive macro");
-        }
 
         let attributes_attr = list.get(1);
         let proc_attrs: Vec<_> = if let Some(attr) = attributes_attr {