cargo-apk: Use min_sdk_version to select compiler target

According to [1] minSdkVersion is used to determine the compiler target
and ultimately limit what API is available at compile-time.  This is
most likely because using/linking newer API will result in runtime
linker errors on these older platforms.

[1]: https://developer.android.com/ndk/guides/sdk-versions#minsdkversion

cargo-apk/README.md

@@ -51,7 +51,7 @@ runtime_libs = "path/to/libs_folder"
 #
 # Defaults to a `min_sdk_version` of 23 and `target_sdk_version` is based on the ndk's default platform.
 [package.metadata.android.sdk]
-min_sdk_version = 16
+min_sdk_version = 23
 target_sdk_version = 29
 max_sdk_version = 29
 

cargo-apk/src/apk.rs

@@ -79,13 +79,8 @@ impl<'a> ApkBuilder<'a> {
     pub fn check(&self) -> Result<(), Error> {
         for target in &self.build_targets {
             let triple = target.rust_triple();
-            let target_sdk_version = self
-                .manifest
-                .android_manifest
-                .sdk
-                .target_sdk_version
-                .unwrap();
-            let mut cargo = cargo_ndk(&self.ndk, *target, target_sdk_version)?;
+            let min_sdk_version = self.manifest.android_manifest.sdk.min_sdk_version.unwrap();
+            let mut cargo = cargo_ndk(&self.ndk, *target, min_sdk_version)?;
             cargo.arg("check");
             if self.cmd.target().is_none() {
                 cargo.arg("--target").arg(triple);
@@ -158,9 +153,11 @@ impl<'a> ApkBuilder<'a> {
                 .join(artifact)
                 .join(artifact.file_name(CrateType::Cdylib, triple));
 
-            let target_sdk_version = config.manifest.sdk.target_sdk_version.unwrap();
+            // Use minSdkVersion to select the right compiler target:
+            // https://developer.android.com/ndk/guides/sdk-versions#minsdkversion
+            let min_sdk_version = config.manifest.sdk.min_sdk_version.unwrap();
 
-            let mut cargo = cargo_ndk(&config.ndk, *target, target_sdk_version)?;
+            let mut cargo = cargo_ndk(&config.ndk, *target, min_sdk_version)?;
             cargo.arg("rustc");
             if self.cmd.target().is_none() {
                 cargo.arg("--target").arg(triple);
@@ -233,14 +230,14 @@ impl<'a> ApkBuilder<'a> {
 
     pub fn default(&self) -> Result<(), Error> {
         let ndk = Ndk::from_env()?;
-        let target_sdk_version = self
+        let min_sdk_version = self
             .manifest
             .android_manifest
             .sdk
-            .target_sdk_version
+            .min_sdk_version
             .unwrap_or_else(|| ndk.default_platform());
         for target in &self.build_targets {
-            let mut cargo = cargo_ndk(&ndk, *target, target_sdk_version)?;
+            let mut cargo = cargo_ndk(&ndk, *target, min_sdk_version)?;
             cargo.args(self.cmd.args());
             if !cargo.status()?.success() {
                 return Err(NdkError::CmdFailed(cargo).into());

ndk-examples/Cargo.toml

@@ -25,5 +25,5 @@ name = "looper"
 crate-type = ["cdylib"]
 
 [package.metadata.android.sdk]
-min_sdk_version = 16
+min_sdk_version = 23
 target_sdk_version = 29