Show --help if there is no man page for subcommand

Signed-off-by: hi-rustin <rustin.liu@gmail.com>

src/bin/cargo/commands/help.rs

@@ -22,11 +22,21 @@ pub fn exec(config: &mut Config, args: &ArgMatches) -> CliResult {
     let subcommand = args.get_one::<String>("COMMAND");
     if let Some(subcommand) = subcommand {
         if !try_help(config, subcommand)? {
-            crate::execute_external_subcommand(
-                config,
-                subcommand,
-                &[OsStr::new(subcommand), OsStr::new("--help")],
-            )?;
+            match check_builtin(&subcommand) {
+                Some(s) => {
+                    crate::execute_internal_subcommand(
+                        config,
+                        &[OsStr::new(s), OsStr::new("--help")],
+                    )?;
+                }
+                None => {
+                    crate::execute_external_subcommand(
+                        config,
+                        subcommand,
+                        &[OsStr::new(subcommand), OsStr::new("--help")],
+                    )?;
+                }
+            }
         }
     } else {
         let mut cmd = crate::cli::cli();

src/bin/cargo/main.rs

@@ -189,9 +189,22 @@ fn execute_external_subcommand(config: &Config, cmd: &str, args: &[&OsStr]) -> C
             return Err(CliError::new(err, 101));
         }
     };
+    execute_subcommand(config, Some(&command), args)
+}
+
+fn execute_internal_subcommand(config: &Config, args: &[&OsStr]) -> CliResult {
+    execute_subcommand(config, None, args)
+}
 
+// This function is used to execute a subcommand. It is used to execute both
+// internal and external subcommands.
+// If `cmd_path` is `None`, then the subcommand is an internal subcommand.
+fn execute_subcommand(config: &Config, cmd_path: Option<&PathBuf>, args: &[&OsStr]) -> CliResult {
     let cargo_exe = config.cargo_exe()?;
-    let mut cmd = ProcessBuilder::new(&command);
+    let mut cmd = match cmd_path {
+        Some(cmd_path) => ProcessBuilder::new(cmd_path),
+        None => ProcessBuilder::new(&cargo_exe),
+    };
     cmd.env(cargo::CARGO_ENV, cargo_exe).args(args);
     if let Some(client) = config.jobserver_from_env() {
         cmd.inherit_jobserver(client);