Remove obsolete no-op #[main] attribute from compiler.

compiler/rustc_feature/src/builtin_attrs.rs

@@ -339,7 +339,6 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
 
     // Entry point:
-    ungated!(main, Normal, template!(Word), WarnFollowing),
     ungated!(start, Normal, template!(Word), WarnFollowing),
     ungated!(no_start, CrateLevel, template!(Word), WarnFollowing),
     ungated!(no_main, CrateLevel, template!(Word), WarnFollowing),

src/test/ui/attributes/main-removed-1.rs

@@ -0,0 +1,2 @@
+#[main]  //~ ERROR cannot find attribute `main` in this scope
+fn main() {}

src/test/ui/attributes/main-removed-1.stderr

@@ -0,0 +1,10 @@
+error: cannot find attribute `main` in this scope
+  --> $DIR/main-removed-1.rs:1:3
+   |
+LL | #[main]
+   |   ^^^^
+   |
+   = note: `main` is in scope, but it is a function, not an attribute
+
+error: aborting due to previous error
+

src/test/ui/attributes/main-removed-2/auxiliary/tokyo.rs

@@ -0,0 +1,12 @@
+// force-host
+// no-prefer-dynamic
+
+#![crate_type = "proc-macro"]
+
+extern crate proc_macro;
+use proc_macro::TokenStream;
+
+#[proc_macro_attribute]
+pub fn main(_: TokenStream, input: TokenStream) -> TokenStream {
+    "fn main() { println!(\"Hello Tokyo!\"); }".parse().unwrap()
+}

src/test/ui/attributes/main-removed-2/main.rs

@@ -0,0 +1,11 @@
+// run-pass
+// aux-build:tokyo.rs
+// compile-flags:--extern tokyo
+// edition:2021
+
+use tokyo::main;
+
+#[main]
+fn main() {
+    panic!("the #[main] macro should replace this with non-panicking code")
+}