Warn when alias shadows external subcommand

As per rust-lang#10049, we start by emitting a warning when an alias shadows an
existing external subcommand. After a transition period (duration not
specified), we will make this a hard error.

src/bin/cargo/cli.rs

@@ -261,6 +261,18 @@ fn expand_aliases(
             }
             (None, None) => {}
             (_, Some(mut alias)) => {
+                // Check if this alias is shadowing an external subcommand
+                // (binary of the form `cargo-<subcommand>`)
+                // Currently this is only a warning, but after a transition period this will become
+                // a hard error.
+                if let Some(path) = super::find_external_subcommand(config, cmd) {
+                    config.shell().warn(format!(
+                        "user-defined alias `{}` is shadowing an external subcommand found at: `{}`",
+                        cmd,
+                        path.display(),
+                    ))?;
+                }
+
                 alias.extend(
                     args.values_of("")
                         .unwrap_or_default()

src/bin/cargo/main.rs

@@ -147,12 +147,16 @@ fn list_commands(config: &Config) -> BTreeMap<String, CommandInfo> {
     commands
 }
 
-fn execute_external_subcommand(config: &Config, cmd: &str, args: &[&str]) -> CliResult {
+fn find_external_subcommand(config: &Config, cmd: &str) -> Option<PathBuf> {
     let command_exe = format!("cargo-{}{}", cmd, env::consts::EXE_SUFFIX);
-    let path = search_directories(config)
+    search_directories(config)
         .iter()
         .map(|dir| dir.join(&command_exe))
-        .find(|file| is_executable(file));
+        .find(|file| is_executable(file))
+}
+
+fn execute_external_subcommand(config: &Config, cmd: &str, args: &[&str]) -> CliResult {
+    let path = find_external_subcommand(config, cmd);
     let command = match path {
         Some(command) => command,
         None => {