render the rust version into std/compiler/test docs

src/bootstrap/bin/rustdoc.rs

@@ -48,6 +48,14 @@ fn main() {
         cmd.arg("-Z").arg("force-unstable-if-unmarked");
     }
 
+    // Bootstrap's Cargo-command builder sets this variable to the current Rust version; let's pick
+    // it up so we can make rustdoc print this into the docs
+    if let Some(version) = env::var_os("RUSTDOC_CRATE_VERSION") {
+        // This "unstable-options" can be removed when `--crate-version` is stabilized
+        cmd.arg("-Z").arg("unstable-options")
+           .arg("--crate-version").arg(version);
+    }
+
     std::process::exit(match cmd.status() {
         Ok(s) => s.code().unwrap_or(1),
         Err(e) => panic!("\n\nfailed to run {:?}: {}\n\n", cmd, e),