avoid replacing the definition of CURRENT_RUSTC_VERSION

Before this commit, replace-version-placeholder hardcoded the path
defining CURRENT_RUSTC_VERSION (to avoid replacing it). After a refactor
moved the file defining it without changing the hardcoded path, the tool
started replacing the constant itself with the version number.

To avoid this from happening in the future, this changes the definition
of the constant to avoid the tool from ever matching it.

compiler/rustc_attr_data_structures/src/stability.rs

@@ -9,7 +9,12 @@ use crate::RustcVersion;
 /// `since` field of the `#[stable]` attribute.
 ///
 /// For more, see [this pull request](https://github.com/rust-lang/rust/pull/100591).
-pub const VERSION_PLACEHOLDER: &str = "CURRENT_RUSTC_VERSION";
+pub const VERSION_PLACEHOLDER: &str = concat!("CURRENT_RUSTC_VERSIO", "N");
+// Note that the `concat!` macro above prevents `src/tools/replace-version-placeholder` from
+// replacing the constant with the current version. Hardcoding the tool to skip this file doesn't
+// work as the file can (and at some point will) be moved around.
+//
+// Turning the `concat!` macro into a string literal will make Pietro cry. That'd be sad :(
 
 /// Represents the following attributes:
 ///

src/tools/replace-version-placeholder/src/main.rs

@@ -11,12 +11,7 @@ fn main() {
     let version_str = version_str.trim();
     walk::walk_many(
         &[&root_path.join("compiler"), &root_path.join("library")],
-        |path, _is_dir| {
-            walk::filter_dirs(path)
-                // We exempt these as they require the placeholder
-                // for their operation
-                || path.ends_with("compiler/rustc_attr/src/builtin.rs")
-        },
+        |path, _is_dir| walk::filter_dirs(path),
         &mut |entry, contents| {
             if !contents.contains(VERSION_PLACEHOLDER) {
                 return;