Add test for rustc-cfg-placeholder interaction with proc-macro

Ensure that the cfg-placeholder isn't visible by proc-macros.

tests/ui/proc-macro/auxiliary/cfg-placeholder.rs

@@ -0,0 +1,28 @@
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+
+#[proc_macro_attribute]
+pub fn my_proc_macro(_: TokenStream, input: TokenStream) -> TokenStream {
+    if format!("{input:#?}").contains("my_attr1") {
+        panic!("found gated attribute my_attr1");
+    }
+    if !format!("{input:#?}").contains("my_attr2") {
+        panic!("didn't if gated my_attr2");
+    }
+    input
+}
+
+#[proc_macro_attribute]
+pub fn my_attr1(_: TokenStream, input: TokenStream) -> TokenStream {
+    panic!("my_attr1 was called");
+    input
+}
+
+#[proc_macro_attribute]
+pub fn my_attr2(_: TokenStream, input: TokenStream) -> TokenStream {
+    if format!("{input:#?}").contains("my_attr1") {
+        panic!("found gated attribute my_attr1");
+    }
+    input
+}

tests/ui/proc-macro/cfg-attr-placeholder-invisible-to-proc-macro.rs

@@ -0,0 +1,13 @@
+// Ensure that `rustc-cfg-placeholder` isn't visible to proc-macros.
+//@proc-macro: cfg-placeholder.rs
+//@check-pass
+#![feature(cfg_eval)]
+#[macro_use] extern crate cfg_placeholder;
+
+#[cfg_eval]
+#[my_proc_macro]
+#[cfg_attr(FALSE, my_attr1)]
+#[cfg_attr(all(), my_attr2)]
+struct S {}
+
+fn main() {}