From 6360287fd91c5d02597b45ef65c34388af348638 Mon Sep 17 00:00:00 2001
From: George Bateman <george.bateman16@gmail.com>
Date: Sun, 11 Aug 2024 09:07:52 +0100
Subject: [PATCH 1/2] Fix #128930: Print documentation of CLI options missing
 their arg

---
 compiler/rustc_driver_impl/src/lib.rs           | 17 +++++++++++++++--
 compiler/rustc_session/src/config.rs            |  2 +-
 .../invalid-compile-flags/print-without-arg.rs  |  1 +
 .../print-without-arg.stderr                    |  5 +++++
 4 files changed, 22 insertions(+), 3 deletions(-)
 create mode 100644 tests/ui/invalid-compile-flags/print-without-arg.rs
 create mode 100644 tests/ui/invalid-compile-flags/print-without-arg.stderr

diff --git a/compiler/rustc_driver_impl/src/lib.rs b/compiler/rustc_driver_impl/src/lib.rs
index 627a0ebb4e5e3..fa1b8e7c83497 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()));
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index f58a991a616da..6699fd8332ba0 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/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,
 }
 
diff --git a/tests/ui/invalid-compile-flags/print-without-arg.rs b/tests/ui/invalid-compile-flags/print-without-arg.rs
new file mode 100644
index 0000000000000..a762cb22275ee
--- /dev/null
+++ b/tests/ui/invalid-compile-flags/print-without-arg.rs
@@ -0,0 +1 @@
+//@ compile-flags: --print
diff --git a/tests/ui/invalid-compile-flags/print-without-arg.stderr b/tests/ui/invalid-compile-flags/print-without-arg.stderr
new file mode 100644
index 0000000000000..a18d2779cade9
--- /dev/null
+++ b/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
+

From ae75a9b33aef4bdd03f33bc75bdf44fe3a48356d Mon Sep 17 00:00:00 2001
From: George Bateman <george.bateman16@gmail.com>
Date: Sun, 11 Aug 2024 09:58:11 +0100
Subject: [PATCH 2/2] Update expectations

---
 tests/ui/compiletest-self-test/compile-flags-last.stderr | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tests/ui/compiletest-self-test/compile-flags-last.stderr b/tests/ui/compiletest-self-test/compile-flags-last.stderr
index d8d40a7d9f112..72d92206e2bb4 100644
--- a/tests/ui/compiletest-self-test/compile-flags-last.stderr
+++ b/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