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

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()));

compiler/rustc_session/src/config.rs

@@ -1349,7 +1349,7 @@ enum OptionStability {
 
 pub struct RustcOptGroup {
     pub apply: Box<dyn Fn(&mut getopts::Options) -> &mut getopts::Options>,
-    name: &'static str,
+    pub name: &'static str,
     stability: OptionStability,
 }
 

tests/ui/compiletest-self-test/compile-flags-last.stderr

@@ -1,2 +1,5 @@
 error: Argument to option 'cap-lints' missing
+       Usage:
+           --cap-lints LEVEL   Set the most restrictive lint level. More restrictive
+                               lints are capped at this level
 

tests/ui/invalid-compile-flags/print-without-arg.rs

@@ -0,0 +1 @@
+//@ compile-flags: --print

tests/ui/invalid-compile-flags/print-without-arg.stderr

@@ -0,0 +1,5 @@
+error: Argument to option 'print' missing
+       Usage:
+           --print [crate-name|file-names|sysroot|target-libdir|cfg|check-cfg|calling-conventions|target-list|target-cpus|target-features|relocation-models|code-models|tls-models|target-spec-json|all-target-specs-json|native-static-libs|stack-protector-strategies|link-args|deployment-target]
+                               Compiler information to print on stdout
+