Fix #128930: Print documentation of CLI options missing their arg

1 files changed, 15 insertions, 2 deletions

diff --git a/compiler/rustc_driver_impl/src/lib.rs b/compiler/rustc_driver_impl/src/lib.rs
index 627a0ebb4e5..fa1b8e7c834 100644
--- a/compiler/rustc_driver_impl/src/lib.rs
+++ b/compiler/rustc_driver_impl/src/lib.rs
@@ -1221,17 +1221,30 @@ pub fn handle_options(early_dcx: &EarlyDiagCtxt, args: &[String]) -> Option<geto
     // Parse with *all* options defined in the compiler, we don't worry about
     // option stability here we just want to parse as much as possible.
     let mut options = getopts::Options::new();
-    for option in config::rustc_optgroups() {
+    let optgroups = config::rustc_optgroups();
+    for option in &optgroups {
         (option.apply)(&mut options);
     }
     let matches = options.parse(args).unwrap_or_else(|e| {
-        let msg = match e {
+        let msg: Option<String> = match e {
             getopts::Fail::UnrecognizedOption(ref opt) => CG_OPTIONS
                 .iter()
                 .map(|&(name, ..)| ('C', name))
                 .chain(Z_OPTIONS.iter().map(|&(name, ..)| ('Z', name)))
                 .find(|&(_, name)| *opt == name.replace('_', "-"))
                 .map(|(flag, _)| format!("{e}. Did you mean `-{flag} {opt}`?")),
+            getopts::Fail::ArgumentMissing(ref opt) => {
+                optgroups.iter().find(|option| option.name == opt).map(|option| {
+                    // Print the help just for the option in question.
+                    let mut options = getopts::Options::new();
+                    (option.apply)(&mut options);
+                    // getopt requires us to pass a function for joining an iterator of
+                    // strings, even though in this case we expect exactly one string.
+                    options.usage_with_format(|it| {
+                        it.fold(format!("{e}\nUsage:"), |a, b| a + "\n" + &b)
+                    })
+                })
+            }
             _ => None,
         };
         early_dcx.early_fatal(msg.unwrap_or_else(|| e.to_string()));