docs: use doc(cfg) to display which methods are unstable

Author: pietroalbini

build.rs

@@ -9,5 +9,10 @@ fn main() -> Result<(), Box<dyn Error>> {
     let output = std::env::var("OUT_DIR")?;
     ::std::fs::write(format!("{}/target", output), target.as_bytes())?;
 
+    println!("cargo:rerun-if-env-changed=DOCS_RS");
+    if std::env::var_os("DOCS_RS").as_deref() == Some(std::ffi::OsStr::new("1")) {
+        println!("cargo:rustc-cfg=docs_rs");
+    }
+
     Ok(())
 }

src/lib.rs

@@ -1,11 +1,14 @@
 #![warn(missing_docs)]
 #![allow(clippy::new_without_default)]
 
+#![cfg_attr(docs_rs, feature(doc_cfg))]
+
 //! Rustwide is a library to execute your code on the Rust ecosystem, powering projects like
 //! [Crater][crater] and [docs.rs][docsrs].
 //!
-//! Rustwide uses feature flags to enable or disable parts of its code at compile time. The
-//! available flags are:
+//! ## Feature flags
+//!
+//! Rustwide provides some optional features that can be enabled with Cargo:
 //!
 //! * **unstable**: allow Rustwide to use unstable Rust and Cargo features. While this feature also
 //!   works on Rust stable it might cause Rustwide to break, and **no stability guarantee is

src/workspace.rs

@@ -101,10 +101,8 @@ impl WorkspaceBuilder {
     /// to build a lot of crates in a batch, but having the option disabled might cause trouble if
     /// you need to build recently published crates, as they might be missing from the cached
     /// index.
-    ///
-    /// **To call this method the `unstable` rustwide feature flag needs to be enabled**, as it
-    /// relies on unstable Cargo features.
-    #[cfg(feature = "unstable")]
+    #[cfg(any(feature = "unstable", doc))]
+    #[cfg_attr(docs_rs, doc(cfg(feature = "unstable")))]
     pub fn fetch_registry_index_during_builds(mut self, enable: bool) -> Self {
         self.fetch_registry_index_during_builds = enable;
         self