nkd-build: Propagate RUSTFLAGS into CARGO_ENCODED_RUSTFLAGS (#357)

ndk-build uses `CARGO_ENCODED_RUSTFLAGS` to get around flags/arguments
with spaces in `RUSTFLAGS`, with no defined way to escape them.  For
correctness this implementation reads `CARGO_ENCODED_RUSTFLAGS` instead
of blindly overwriting its contents, but as per [`build.rustflags`
precedence rules] setting this environment variable would disregard any
contents in the more common `RUSTFLAGS` environment variable. To make
this more convenient for users, propagate the contents of `RUSTFLAGS`
into `CARGO_ENCODED_RUSTFLAGS` to ensure its contents don't get ignored.

[`build.rustflags` precedence rules]: https://doc.rust-lang.org/cargo/reference/config.html#buildrustflags

ndk-build/CHANGELOG.md

@@ -1,6 +1,8 @@
 # Unreleased
 
 - Add `ndk::DEFAULT_DEV_KEYSTORE_PASSWORD` and make `apk::ApkConfig::apk` public. ([#358](https://github.com/rust-windowing/android-ndk-rs/pull/358))
+- `RUSTFLAGS` is now considered if `CARGO_ENCODED_RUSTFLAGS` is not present allowing `cargo apk build` to not break users' builds if they depend on `RUSTFLAGS` being set prior to the build,
+  as `CARGO_ENCODED_RUSTFLAGS` set by `ndk-build` before invoking `cargo` will take precedence over [all other sources of build flags](https://doc.rust-lang.org/cargo/reference/config.html#buildrustflags). ([#357](https://github.com/rust-windowing/android-ndk-rs/pull/357))
 
 (0.8.1, released on 2022-10-14, was yanked due to violating semver.)
 
@@ -34,7 +36,7 @@
 
 - **Breaking:** Default `target_sdk_version` to `30` or lower (instead of the highest supported SDK version by the detected NDK toolchain)
   for more consistent interaction with Android backwards compatibility handling and its increasingly strict usage rules:
-  https://developer.android.com/distribute/best-practices/develop/target-sdk
+  <https://developer.android.com/distribute/best-practices/develop/target-sdk>
 - **Breaking:** Remove default insertion of `MAIN` intent filter through a custom serialization function, this is better filled in by
   the default setup in `cargo-apk`. ([#241](https://github.com/rust-windowing/android-ndk-rs/pull/241))
 - Add `android:exported` attribute to the manifest's `Activity` element. ([#242](https://github.com/rust-windowing/android-ndk-rs/pull/242))

ndk-build/src/cargo.rs

@@ -14,12 +14,40 @@ pub fn cargo_ndk(
     let clang_target = format!("--target={}{}", target.ndk_llvm_triple(), sdk_version);
     let mut cargo = Command::new("cargo");
 
-    // Read initial RUSTFLAGS
+    const SEP: &str = "\x1f";
+
+    // Read initial CARGO_ENCODED_/RUSTFLAGS
     let mut rustflags = match std::env::var("CARGO_ENCODED_RUSTFLAGS") {
-        Ok(val) => val,
-        Err(std::env::VarError::NotPresent) => "".to_string(),
+        Ok(val) => {
+            if std::env::var_os("RUSTFLAGS").is_some() {
+                panic!(
+                    "Both `CARGO_ENCODED_RUSTFLAGS` and `RUSTFLAGS` were found in the environment, please clear one or the other before invoking this script"
+                );
+            }
+
+            val
+        }
+        Err(std::env::VarError::NotPresent) => {
+            match std::env::var("RUSTFLAGS") {
+                Ok(val) => {
+                    cargo.env_remove("RUSTFLAGS");
+
+                    // Same as cargo
+                    // https://github.com/rust-lang/cargo/blob/f6de921a5d807746e972d9d10a4d8e1ca21e1b1f/src/cargo/core/compiler/build_context/target_info.rs#L682-L690
+                    val.split(' ')
+                        .map(str::trim)
+                        .filter(|s| !s.is_empty())
+                        .collect::<Vec<_>>()
+                        .join(SEP)
+                }
+                Err(std::env::VarError::NotPresent) => String::new(),
+                Err(std::env::VarError::NotUnicode(_)) => {
+                    panic!("RUSTFLAGS environment variable contains non-unicode characters")
+                }
+            }
+        }
         Err(std::env::VarError::NotUnicode(_)) => {
-            panic!("RUSTFLAGS environment variable contains non-unicode characters")
+            panic!("CARGO_ENCODED_RUSTFLAGS environment variable contains non-unicode characters")
         }
     };
 
@@ -36,7 +64,7 @@ pub fn cargo_ndk(
     // https://doc.rust-lang.org/beta/cargo/reference/environment-variables.html#configuration-environment-variables
     cargo.env(cargo_env_target_cfg("LINKER", triple), &clang);
     if !rustflags.is_empty() {
-        rustflags.push('\x1f');
+        rustflags.push_str(SEP);
     }
     rustflags.push_str("-Clink-arg=");
     rustflags.push_str(&clang_target);
@@ -67,7 +95,9 @@ pub fn cargo_ndk(
         // suggests to resort to RUSTFLAGS.
         // Note that `rustflags` will never be empty because of an unconditional `.push_str` above,
         // so we can safely start with appending \x1f here.
-        rustflags.push_str("\x1f-L\x1f");
+        rustflags.push_str(SEP);
+        rustflags.push_str("-L");
+        rustflags.push_str(SEP);
         rustflags.push_str(
             cargo_apk_link_dir
                 .to_str()