Rollup merge of #143719 - xizheyin:142812-1, r=jieyouxu

Emit warning when there is no space between `-o` and arg

Closes rust-lang/rust#142812

`getopt` doesn't seem to have an API to check this, so we have to check the args manually.

r? compiler

compiler/rustc_session/src/config.rs

@@ -1708,6 +1708,11 @@ impl RustcOptGroup {
             OptionKind::FlagMulti => options.optflagmulti(short_name, long_name, desc),
         };
     }
+
+    /// This is for diagnostics-only.
+    pub fn long_name(&self) -> &str {
+        self.long_name
+    }
 }
 
 pub fn make_opt(