Add unsupported lowercase -z flag suggestion for -Z flag

src/bin/cargo/cli.rs

@@ -619,14 +619,7 @@ See '<cyan,bold>cargo help</> <cyan><<command>></>' for more information on a sp
                 .global(true),
         )
         .arg_config()
-        .arg(
-            Arg::new("unstable-features")
-                .help("Unstable (nightly-only) flags to Cargo, see 'cargo -Z help' for details")
-                .short('Z')
-                .value_name("FLAG")
-                .action(ArgAction::Append)
-                .global(true),
-        )
+        .arg_unstable_feature()
         .subcommands(commands::builtin())
 }
 

src/cargo/util/command_prelude.rs

@@ -393,6 +393,28 @@ pub trait CommandExt: Sized {
         self._arg(unsupported_short_arg)
             ._arg(multi_opt("config", "KEY=VALUE", "Override a configuration value").global(true))
     }
+
+    fn arg_unstable_feature(self) -> Self {
+        let unsupported_short_arg = {
+            let value_parser = UnknownArgumentValueParser::suggest_arg("-Z");
+            Arg::new("unsupported-lowercase-unstable-feature-flag")
+                .help("")
+                .short('z')
+                .value_parser(value_parser)
+                .action(ArgAction::SetTrue)
+                .global(true)
+                .hide(true)
+        };
+        self._arg(
+            Arg::new("unstable-features")
+                .help("Unstable (nightly-only) flags to Cargo, see 'cargo -Z help' for details")
+                .short('Z')
+                .value_name("FLAG")
+                .action(ArgAction::Append)
+                .global(true),
+        )
+        ._arg(unsupported_short_arg)
+    }
 }
 
 impl CommandExt for Command {

tests/testsuite/build.rs

@@ -259,6 +259,8 @@ fn cargo_compile_with_unsupported_short_unstable_feature_flag() {
             "\
 error: unexpected argument '-z' found
 
+  tip: a similar argument exists: '-Z'
+
 Usage: cargo [+toolchain] [OPTIONS] [COMMAND]
        cargo [+toolchain] [OPTIONS] -Zscript <MANIFEST_RS> [ARGS]...