Fix compiling -sys crates on docs.rs

We only have control over features when we're the crate being compiled, in other cases the parent crate should control which runtime to use, so we can't just check DOCS_RS - otherwise they'll error out because the wrong runtime was selected.

Author: madsmtm

block-sys/Cargo.toml

@@ -46,12 +46,16 @@ unstable-winobjc = ["objc-sys/unstable-winobjc", "gnustep-1-8"]
 # Link to ObjFW
 unstable-objfw = []
 
+# Private
+unstable-docsrs = ["objc-sys"] # Need `objc-sys` on certain platforms
+
 [dependencies]
 objc-sys = { path = "../objc-sys", version = "=0.2.0-alpha.1", default-features = false, optional = true }
 
 [package.metadata.docs.rs]
 default-target = "x86_64-apple-darwin"
 no-default-features = true
+features = ["unstable-docsrs"]
 
 targets = [
     # MacOS

block-sys/build.rs

@@ -12,7 +12,8 @@ fn main() {
     let mut gnustep = env::var_os("CARGO_FEATURE_GNUSTEP_1_7").is_some();
     let objfw = env::var_os("CARGO_FEATURE_UNSTABLE_OBJFW").is_some();
 
-    if std::env::var("DOCS_RS").is_ok() {
+    // Only when the crate is being compiled directly
+    if cfg!(feature = "unstable-docsrs") {
         if let "macos" | "ios" | "tvos" | "watchos" = &*target_os {
             apple = true;
             // Add cheaty #[cfg(feature = "apple")] directive

objc-sys/Cargo.toml

@@ -47,13 +47,15 @@ unstable-objfw = []
 
 # Private
 unstable-exception = ["cc"]
+unstable-docsrs = []
 
 [build-dependencies]
 cc = { version = "1", optional = true }
 
 [package.metadata.docs.rs]
 default-target = "x86_64-apple-darwin"
 no-default-features = true
+features = ["unstable-docsrs"]
 
 targets = [
     # MacOS

objc-sys/build.rs

@@ -74,7 +74,8 @@ fn main() {
     let objfw = env::var_os("CARGO_FEATURE_UNSTABLE_OBJFW").is_some();
 
     // Choose defaults when generating docs
-    if std::env::var("DOCS_RS").is_ok() {
+    // Only when the crate is being compiled directly
+    if cfg!(feature = "unstable-docsrs") {
         if let "macos" | "ios" | "tvos" | "watchos" = &*target_os {
             apple = true;
         } else {
@@ -100,7 +101,7 @@ fn main() {
         }
         (false, true, false) => {
             // Choose defaults when generating docs
-            if std::env::var("DOCS_RS").is_ok() {
+            if cfg!(feature = "unstable-docsrs") {
                 if "windows" == target_os {
                     WinObjC
                 } else {
@@ -229,6 +230,11 @@ fn main() {
         if std::env::var("DOCS_RS").is_ok() {
             // docs.rs doesn't have clang, so skip building this. The
             // documentation will still work since it doesn't need to link.
+            //
+            // This is independent of the `unstable-docsrs` feature flag; we
+            // never want to try invoking clang on docs.rs, whether we're the
+            // crate being documented currently, or a dependency of another
+            // crate.
             return;
         }
         println!("cargo:rerun-if-changed=extern/exception.m");