Allow building as dependency on docs.rs with no features enabled

.gitignore

@@ -1 +1,2 @@
-target
+/target
+/Cargo.lock

android-activity/build.rs

@@ -1,5 +1,3 @@
-#![allow(dead_code)]
-
 fn build_glue_for_game_activity() {
     let android_games_sdk =
         std::env::var("ANDROID_GAMES_SDK").unwrap_or_else(|_err| "android-games-sdk".to_string());
@@ -94,6 +92,21 @@ fn build_glue_for_game_activity() {
 }
 
 fn main() {
-    #[cfg(feature = "game-activity")]
-    build_glue_for_game_activity();
+    // Enable Cargo's change-detection to avoid re-running build script if
+    // irrelvant parts changed. Using build.rs here is just a dummy used to
+    // disable the default "rerun on every change" behaviour Cargo has.
+    println!("cargo:rerun-if-changed=build.rs");
+
+    if cfg!(feature = "game-activity") {
+        build_glue_for_game_activity();
+    }
+
+    // Whether this is used directly in or as a dependency on docs.rs.
+    //
+    // `cfg(docsrs)` cannot be used, since it's only set for the crate being
+    // built, and not for any dependent crates.
+    println!("cargo:rustc-check-cfg=cfg(used_on_docsrs)");
+    if std::env::var("DOCS_RS").is_ok() {
+        println!("cargo:rustc-cfg=used_on_docsrs");
+    }
 }

android-activity/src/game_activity/mod.rs

@@ -1,5 +1,3 @@
-#![cfg(feature = "game-activity")]
-
 use std::collections::HashMap;
 use std::marker::PhantomData;
 use std::ops::Deref;

android-activity/src/lib.rs

@@ -141,7 +141,7 @@ compile_error!(
 );
 #[cfg(all(
     not(any(feature = "game-activity", feature = "native-activity")),
-    not(doc)
+    not(any(doc, used_on_docsrs)),
 ))]
 compile_error!(
     r#"Either "game-activity" or "native-activity" must be enabled as features
@@ -160,8 +160,18 @@ You may need to add a `[patch]` into your Cargo.toml to ensure a specific versio
 android-activity is used across all of your application's crates."#
 );
 
-#[cfg_attr(any(feature = "native-activity", doc), path = "native_activity/mod.rs")]
-#[cfg_attr(any(feature = "game-activity", doc), path = "game_activity/mod.rs")]
+#[cfg_attr(feature = "native-activity", path = "native_activity/mod.rs")]
+#[cfg_attr(feature = "game-activity", path = "game_activity/mod.rs")]
+#[cfg_attr(
+    all(
+        // No activities enabled.
+        not(any(feature = "native-activity", feature = "game-activity")),
+        // And building docs.
+        any(doc, used_on_docsrs),
+    ),
+    // Fall back to documenting native activity.
+    path = "native_activity/mod.rs"
+)]
 pub(crate) mod activity_impl;
 
 pub mod error;

android-activity/src/native_activity/mod.rs

@@ -1,5 +1,3 @@
-#![cfg(any(feature = "native-activity", doc))]
-
 use std::collections::HashMap;
 use std::marker::PhantomData;
 use std::panic::AssertUnwindSafe;