Rollup merge of rust-lang#101215 - est31:rustdoc_version_placeholder, r=Mark-Simulacrum

Also replace the version placeholder in rustc_attr

Replace the version placeholder with the current version in the rustc_attr crate too so that users won't see the placeholder but instead the explicit version. This especially fixes the bug for rustdoc not showing it but instead the placeholder.

Originally reported [here](https://rust-lang.zulipchat.com/#narrow/stream/241545-t-release/topic/libs.20stabilization.20placeholder/near/296057188).

cc rust-lang#100591

![Screenshot_20220830_233727](https://user-images.githubusercontent.com/8872119/187548079-6207776b-4481-4351-afff-607f5b3fe03a.png)

Author: Dylan-DPC

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")
         },