Improve wrong flag help (#16427)

# Description

Currently, when Nushell encounters an unknown flag, it prints all
options in the help string. This is pretty verbose and uses the
`formatted_flags` signature method, which isn't used anywhere else. This
commit refactors the parser to use `did_you_mean` instead, which only
suggest one closest option or sends the user to `help` if nothing close
is found.


# User-Facing Changes (Bug fixes and other changes)

## Improved error messages for misspelled flags

Previously, the help text for a missing flag would list all of them,
which could get verbose on a single line:

```nushell
~> ls --full-path
Error: nu::parser::unknown_flag

  × The `ls` command doesn't have flag `full-path`.
   ╭─[entry #8:1:4]
 1 │ ls --full-path
   ·    ─────┬─────
   ·         ╰── unknown flag
   ╰────
  help: Available flags: --help(-h), --all(-a), --long(-l), --short-names(-s), --full-paths(-f), --du(-d), --directory(-D), --mime-type(-m), --threads(-t). Use
        `--help` for more information.
```

The new error message only suggests the closest flag:

```nushell
> ls --full-path
Error: nu::parser::unknown_flag

  × The `ls` command doesn't have flag `full-path`.
   ╭─[entry #23:1:4]
 1 │ ls --full-path
   ·    ─────┬─────
   ·         ╰── unknown flag
   ╰────
  help: Did you mean: `--full-paths`?
```


---

Closes #16418

crates/nu-command/tests/commands/ls.rs

@@ -661,14 +661,17 @@ fn list_ignores_ansi() {
 }
 
 #[test]
-fn list_unknown_flag() {
+fn list_unknown_long_flag() {
+    let actual = nu!("ls --full-path");
+
+    assert!(actual.err.contains("Did you mean: `--full-paths`?"));
+}
+
+#[test]
+fn list_unknown_short_flag() {
     let actual = nu!("ls -r");
 
-    assert!(
-        actual
-            .err
-            .contains("Available flags: --help(-h), --all(-a),")
-    );
+    assert!(actual.err.contains("Use `--help` to see available flags"));
 }
 
 #[test]

crates/nu-parser/src/parser.rs

@@ -15,7 +15,7 @@ use nu_engine::DIR_VAR_PARSER_INFO;
 use nu_protocol::{
     BlockId, DeclId, DidYouMean, ENV_VARIABLE_ID, FilesizeUnit, Flag, IN_VARIABLE_ID, ParseError,
     PositionalArg, ShellError, Signature, Span, Spanned, SyntaxShape, Type, Value, VarId, ast::*,
-    casing::Casing, engine::StateWorkingSet, eval_const::eval_constant,
+    casing::Casing, did_you_mean, engine::StateWorkingSet, eval_const::eval_constant,
 };
 use std::{
     collections::{HashMap, HashSet},
@@ -641,11 +641,14 @@ fn parse_long_flag(
                     }
                 }
             } else {
+                let suggestion = did_you_mean(sig.get_names(), &long_name)
+                    .map(|name| format!("Did you mean: `--{name}`?"))
+                    .unwrap_or("Use `--help` to see available flags".to_owned());
                 working_set.error(ParseError::UnknownFlag(
                     sig.name.clone(),
                     long_name.clone(),
                     arg_span,
-                    sig.clone().formatted_flags(),
+                    suggestion,
                 ));
                 (
                     Some(Spanned {
@@ -725,7 +728,7 @@ fn parse_short_flags(
                     sig.name.clone(),
                     format!("-{}", String::from_utf8_lossy(contents)),
                     *first,
-                    sig.clone().formatted_flags(),
+                    "Use `--help` to see available flags".to_owned(),
                 ));
             }
 

crates/nu-protocol/src/signature.rs

@@ -624,37 +624,6 @@ impl Signature {
             examples,
         })
     }
-
-    pub fn formatted_flags(self) -> String {
-        if self.named.len() < 11 {
-            let mut s = "Available flags:".to_string();
-            for flag in self.named {
-                if let Some(short) = flag.short {
-                    let _ = write!(s, " --{}(-{}),", flag.long, short);
-                } else {
-                    let _ = write!(s, " --{},", flag.long);
-                }
-            }
-            s.remove(s.len() - 1);
-            let _ = write!(s, ". Use `--help` for more information.");
-            s
-        } else {
-            let mut s = "Some available flags:".to_string();
-            for flag in self.named {
-                if let Some(short) = flag.short {
-                    let _ = write!(s, " --{}(-{}),", flag.long, short);
-                } else {
-                    let _ = write!(s, " --{},", flag.long);
-                }
-            }
-            s.remove(s.len() - 1);
-            let _ = write!(
-                s,
-                "... Use `--help` for a full list of flags and more information."
-            );
-            s
-        }
-    }
 }
 
 #[derive(Clone)]