Also replace the version placeholder in rustc_attr

This fixes rustdoc not showing the current version as stabilization version
for recently stabilized lang features.

compiler/rustc_attr/src/builtin.rs

@@ -15,6 +15,12 @@ use std::num::NonZeroU32;
 
 use crate::session_diagnostics::{self, IncorrectReprFormatGenericCause};
 
+/// The version placeholder that recently stabilized features contain inside the
+/// `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 fn is_builtin_attr(attr: &Attribute) -> bool {
     attr.is_doc_comment() || attr.ident().filter(|ident| is_builtin_attr_name(ident.name)).is_some()
 }
@@ -483,6 +489,12 @@ where
                         }
                     }
 
+                    if let Some(s) = since && s.as_str() == VERSION_PLACEHOLDER {
+                        let version = option_env!("CFG_VERSION").unwrap_or("<current>");
+                        let version = version.split(' ').next().unwrap();
+                        since = Some(Symbol::intern(&version));
+                    }
+
                     match (feature, since) {
                         (Some(feature), Some(since)) => {
                             let level = Stable { since, allowed_through_unstable_modules: false };

compiler/rustc_passes/src/lib_features.rs

@@ -5,6 +5,7 @@
 //! collect them instead.
 
 use rustc_ast::{Attribute, MetaItemKind};
+use rustc_attr::VERSION_PLACEHOLDER;
 use rustc_errors::struct_span_err;
 use rustc_hir::intravisit::Visitor;
 use rustc_middle::hir::nested_filter;
@@ -54,7 +55,6 @@ impl<'tcx> LibFeatureCollector<'tcx> {
                         }
                     }
                 }
-                const VERSION_PLACEHOLDER: &str = "CURRENT_RUSTC_VERSION";
 
                 if let Some(s) = since && s.as_str() == VERSION_PLACEHOLDER {
                     let version = option_env!("CFG_VERSION").unwrap_or("<current>");

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

@@ -14,7 +14,7 @@ fn main() {
             walk::filter_dirs(path)
                 // We exempt these as they require the placeholder
                 // for their operation
-                || path.ends_with("compiler/rustc_passes/src/lib_features.rs")
+                || path.ends_with("compiler/rustc_attr/src/builtin.rs")
                 || path.ends_with("src/tools/tidy/src/features/version.rs")
                 || path.ends_with("src/tools/replace-version-placeholder")
         },