Allow the library consumer to specify the Android API level

build.rs

@@ -1,5 +1,3 @@
-extern crate cc;
-
 use std::env;
 use std::path::Path;
 
@@ -11,14 +9,17 @@ pub fn main() {
  }
 }
 
-// Used to detect the value of the `__ANDROID_API__`
-// builtin #define
-const MARKER: &str = "BACKTRACE_RS_ANDROID_APIVERSION";
-const ANDROID_API_C: &str = "
+#[cfg(not(no_cc))]
+fn android_version_from_c_headers() -> Option<u32> {
+ extern crate cc;
+
+ // Used to detect the value of the `__ANDROID_API__`
+ // builtin #define
+ const MARKER: &str = "BACKTRACE_RS_ANDROID_APIVERSION";
+ const ANDROID_API_C: &str = "
 BACKTRACE_RS_ANDROID_APIVERSION __ANDROID_API__
 ";
 
-fn build_android() {
  // Create `android-api.c` on demand.
  // Required to support calling this from the `std` build script.
  let out_dir = env::var_os("OUT_DIR").unwrap();
@@ -32,26 +33,50 @@ fn build_android() {
  "warning: android version detection failed while running C compiler: {}",
  e
  );
- return;
+ return None;
  }
  };
  let expansion = match std::str::from_utf8(&expansion) {
  Ok(s) => s,
- Err(_) => return,
+ Err(_) => return None,
  };
  eprintln!("expanded android version detection:\n{}", expansion);
  let i = match expansion.find(MARKER) {
  Some(i) => i,
- None => return,
+ None => return None,
  };
  let version = match expansion[i + MARKER.len() + 1..].split_whitespace().next() {
  Some(s) => s,
- None => return,
+ None => return None,
  };
- let version = match version.parse::<u32>() {
- Ok(n) => n,
- Err(_) => return,
+ match version.parse::<u32>() {
+ Ok(n) => Some(n),
+ Err(_) => None,
+ }
+}
+
+/// Sets cfgs that depend on the Android API level.
+///
+/// This depends on the use of a C preprocessor to find the API level in system
+/// headers. For build systems that do not want to use a C processor inside the
+/// execution of build scripts, the build system can specify the API level
+/// through the `__ANDROID_API__` environment variable. When `--cfg=no_cc` is
+/// specified, the environment variable is used instead.
+fn build_android() {
+ let version = {
+ #[cfg(no_cc)]
+ {
+ std::env::var("__ANDROID_API__")
+ .expect("__ANDROID_API__ must be set when building with `--cfg=no_cc`")
+ .parse::<u32>()
+ .expect("__ANDROID_API__ must be a number")
+ }
+ #[cfg(not(no_cc))]
+ {
+ android_version_from_c_headers().unwrap_or_default()
+ }
  };
+
  if version >= 21 {
  println!("cargo:rustc-cfg=feature=\"dl_iterate_phdr\"");
  }