fix(error): Include -- in more cases

Inspired by rust-lang/cargo#12494.
Part of this is that our "did you mean" does prefix checks so it can be
overly aggressive in providing suggestions.
To avoid providing needless suggestions I limited this change to `last`
/ `trailing_var_arg` as those convey that `--` is more likely a valid
suggestion.

Author: epage

clap_builder/src/parser/parser.rs

@@ -1570,10 +1570,16 @@ impl<'cmd> Parser<'cmd> {
             .collect();
 
         // `did_you_mean` is a lot more likely and should cause us to skip the `--` suggestion
+        // with the one exception being that the CLI is trying to capture arguments
         //
         // In theory, this is only called for `--long`s, so we don't need to check
-        let suggested_trailing_arg =
-            did_you_mean.is_none() && !trailing_values && self.cmd.has_positionals();
+        let suggested_trailing_arg = (did_you_mean.is_none()
+            || self
+                .cmd
+                .get_positionals()
+                .any(|arg| arg.is_last_set() || arg.is_trailing_var_arg_set()))
+            && !trailing_values
+            && self.cmd.has_positionals();
         ClapError::unknown_argument(
             self.cmd,
             format!("--{arg}"),

tests/builder/error.rs

@@ -165,6 +165,7 @@ fn suggest_trailing_last() {
 error: unexpected argument '--ignored' found
 
   tip: a similar argument exists: '--ignore-rust-version'
+  tip: to pass '--ignored' as a value, use '-- --ignored'
 
 Usage: cargo --ignore-rust-version [-- <TESTNAME>]