Add unsupported short suggestion for --out-dir flag

Author: 0xPoe

src/bin/cargo/commands/build.rs

@@ -35,14 +35,7 @@ pub fn cli() -> Command {
         .arg_parallel()
         .arg_target_triple("Build for the target triple")
         .arg_target_dir()
-        .arg(
-            opt(
-                "out-dir",
-                "Copy final artifacts to this directory (unstable)",
-            )
-            .value_name("PATH")
-            .help_heading(heading::COMPILATION_OPTIONS),
-        )
+        .arg_out_dir()
         .arg_build_plan()
         .arg_unit_graph()
         .arg_timings()

src/cargo/util/command_prelude.rs

@@ -357,6 +357,26 @@ pub trait CommandExt: Sized {
             .help_heading(heading::COMPILATION_OPTIONS),
         )
     }
+
+    fn arg_out_dir(self) -> Self {
+        let unsupported_short_arg = {
+            let msg = format!("no short alias is defined for `--out-dir`");
+            let value_parser = UnknownArgumentValueParser::suggest(msg);
+            flag("unsupported-short-out-dir-flag", "")
+                .short('O')
+                .value_parser(value_parser)
+                .hide(true)
+        };
+        self._arg(
+            opt(
+                "out-dir",
+                "Copy final artifacts to this directory (unstable)",
+            )
+            .value_name("PATH")
+            .help_heading(heading::COMPILATION_OPTIONS),
+        )
+        ._arg(unsupported_short_arg)
+    }
 }
 
 impl CommandExt for Command {

tests/testsuite/out_dir.rs

@@ -291,7 +291,9 @@ fn unsupported_short_out_dir_flag() {
         .masquerade_as_nightly_cargo(&["out-dir"])
         .with_stderr(
             "\
-error: unexpected argument '-O' found
+error: unexpected argument '--unsupported-short-out-dir-flag' found
+
+  tip: a similar argument exists: '--out-dir'
 
 Usage: cargo[EXE] build [OPTIONS]