Add --list to known subcommands.

`cross --list` should list the subcommands present for cargo in the
image, rather on the host. This works because any option provided before
a subcommand has priority over the subcommand. For example, `cargo build
--help` prints the help menu for `cargo build`, but `cargo --help build`
ignores `build` and prints the help for `cargo`. Therefore, the options
`--help`, `--version`, and `--list` can be treated as
pseudo-subcommands.

Fixes #715.

CHANGELOG.md

@@ -5,6 +5,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+- #719 - add `--list` to known subcommands.
 - #714 - use host target directory when falling back to host cargo.
 - #713 - convert relative target directories to absolute paths.
 - #709 - Update Emscripten targets to `emcc` version 3.1.10

src/cargo.rs

@@ -18,6 +18,7 @@ pub enum Subcommand {
     Deb,
     Clippy,
     Metadata,
+    List,
 }
 
 impl Subcommand {
@@ -30,7 +31,7 @@ impl Subcommand {
     }
 
     pub fn needs_target_in_command(self) -> bool {
-        !matches!(self, Subcommand::Metadata)
+        !matches!(self, Subcommand::Metadata | Subcommand::List)
     }
 }
 
@@ -47,6 +48,7 @@ impl<'a> From<&'a str> for Subcommand {
             "deb" => Subcommand::Deb,
             "clippy" => Subcommand::Clippy,
             "metadata" => Subcommand::Metadata,
+            "--list" => Subcommand::List,
             _ => Subcommand::Other,
         }
     }

src/cli.rs

@@ -63,7 +63,7 @@ pub fn parse(target_list: &TargetList) -> Result<Args> {
                     all.push("--target-dir=/target".into());
                 }
             } else {
-                if !arg.starts_with('-') && sc.is_none() {
+                if (!arg.starts_with('-') || arg == "--list") && sc.is_none() {
                     sc = Some(Subcommand::from(arg.as_ref()));
                 }