feat: Add custom completer for cargo +<TAB> to complete toolchain name

Author: nooma-42

src/bin/cargo/cli.rs

@@ -690,10 +690,39 @@ See '<cyan,bold>cargo help</> <cyan><<command>></>' for more information on a sp
                     help.into()
                 }))
             }).collect()
+        })))
+                .arg( Arg::new("toolchain")
+            .help("Specifies the toolchain to use for this command")
+            .value_name("TOOLCHAIN")
+            .action(ArgAction::Set)
+        .add(clap_complete::engine::ArgValueCompleter::new(|partial: &OsStr| {
+            let toolchains = get_toolchains_from_rustup();
+
+            toolchains
+                .into_iter()
+                .filter(|t| OsStr::new(t).as_encoded_bytes().starts_with(partial.as_encoded_bytes()))
+                .map(|t| clap_complete::CompletionCandidate::new(t))
+                .collect()
         })))
         .subcommands(commands::builtin())
 }
 
+fn get_toolchains_from_rustup() -> Vec<String> {
+    let output = std::process::Command::new("rustup")
+        .arg("toolchain")
+        .arg("list")
+        .output()
+        .unwrap();
+
+    if !output.status.success() {
+        return vec![];
+    }
+
+    let stdout = String::from_utf8(output.stdout).unwrap();
+
+    stdout.lines().map(|line| format!("+{}", line)).collect()
+}
+
 #[test]
 fn verify_cli() {
     let gctx = GlobalContext::default().unwrap();