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),

src/bootstrap/builder.rs

@@ -418,7 +418,8 @@ impl<'a> Builder<'a> {
             .env("RUSTC_SYSROOT", self.sysroot(compiler))
             .env("RUSTC_LIBDIR", self.sysroot_libdir(compiler, self.build.build))
             .env("CFG_RELEASE_CHANNEL", &self.build.config.channel)
-            .env("RUSTDOC_REAL", self.rustdoc(host));
+            .env("RUSTDOC_REAL", self.rustdoc(host))
+            .env("RUSTDOC_CRATE_VERSION", self.build.rust_version());
         cmd
     }
 
@@ -574,6 +575,9 @@ impl<'a> Builder<'a> {
             cargo.env("RUSTC_SAVE_ANALYSIS", "api".to_string());
         }
 
+        // For `cargo doc` invocations, make rustdoc print the Rust version into the docs
+        cargo.env("RUSTDOC_CRATE_VERSION", self.build.rust_version());
+
         // Environment variables *required* throughout the build
         //
         // FIXME: should update code to not require this env var