cargo-apk: Reimplement "default" subcommand trailing args for cargo with -- separator

`clap` [does not currently support] parsing unknown arguments into a
side `Vec`, which is exactly what `cargo apk --` needs to parse a few
known `cargo` arguments (such as `--target` and `-p`) but forward the
rest verbatim to the underlying `cargo` subcommand.

`allow_hyphen_values = true` could partially help us out with this, but
it parses all remaining arguments into that `Vec` upon encountering the
first unknown flag/arg, resulting in all known flags after that to also
be treated as "unknown" instead of filling up our `args: Args` struct.

Since [a workaround for this isn't currently functioning], introduce
pure trailing args with an additional `--` separator to make it clear
which arguments go to `cargo-apk` (and are almost all, except
`--device`, forwarded to `cargo`) and which are only passed to `cargo
<subcommand>`.

[does not currently support]: clap-rs/clap#1404
[a workaround for this isn't currently functioning]: clap-rs/clap#1404 (comment)

Author: MarijnS95

cargo-apk/src/apk.rs

@@ -306,7 +306,7 @@ impl<'a> ApkBuilder<'a> {
         Ok(())
     }
 
-    pub fn default(&self, cargo_cmd: &str) -> Result<(), Error> {
+    pub fn default(&self, cargo_cmd: &str, additional_args: &[String]) -> Result<(), Error> {
         for target in &self.build_targets {
             let mut cargo = cargo_ndk(
                 &self.ndk,
@@ -322,6 +322,10 @@ impl<'a> ApkBuilder<'a> {
                 cargo.arg("--target").arg(triple);
             }
 
+            for additional_arg in additional_args {
+                cargo.arg(additional_arg);
+            }
+
             if !cargo.status()?.success() {
                 return Err(NdkError::CmdFailed(cargo).into());
             }

cargo-apk/src/main.rs

@@ -27,7 +27,6 @@ struct Args {
 }
 
 #[derive(clap::Subcommand)]
-#[clap(trailing_var_arg = true)]
 enum ApkSubCmd {
     /// Analyze the current package and report errors, but don't build object files nor an apk
     #[clap(visible_alias = "c")]
@@ -44,9 +43,20 @@ enum ApkSubCmd {
     /// Invoke `cargo` under the detected NDK environment
     #[clap(name = "--")]
     Ndk {
+        /// `cargo` subcommand to run
         cargo_cmd: String,
+
+        /// Arguments used to deduce the current environment. Also passed to `cargo`
         #[clap(flatten)]
         args: Args,
+
+        /// Additional arguments passed to `cargo`
+        // TODO: Arguments in this vec should be intermixable with other arguments;
+        // this is somewhat possible with `allow_hyphen_values = true` but any argument
+        // after the first unknown arg/flag ends up inside `cargo_args` instead of being
+        // parsed into `args: Args`.
+        #[clap(trailing_var_arg = true, last = true)]
+        cargo_args: Vec<String>,
     },
     /// Run a binary or example apk of the local package
     #[clap(visible_alias = "r")]
@@ -84,10 +94,14 @@ fn main() -> anyhow::Result<()> {
                 builder.build(artifact)?;
             }
         }
-        ApkSubCmd::Ndk { cargo_cmd, args } => {
+        ApkSubCmd::Ndk {
+            cargo_cmd,
+            args,
+            cargo_args,
+        } => {
             let cmd = Subcommand::new(args.subcommand_args)?;
             let builder = ApkBuilder::from_subcommand(&cmd, args.device)?;
-            builder.default(&cargo_cmd)?;
+            builder.default(&cargo_cmd, &cargo_args)?;
         }
         ApkSubCmd::Run { args, no_logcat } => {
             let cmd = Subcommand::new(args.subcommand_args)?;