Better error message when --config is given a table key and a non-inline-table value

[InSyncWithFoo]

Summary

Resolves #13995.

Test Plan

cargo nextest run and cargo insta test.

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

[AlexWaygood]

This is great! I think we can do even better with a few tweaks, though. With your branch currently, this is the error message I get:

~/dev/ruff (cli-config)⚡ % cargo run -- check . --config='lint.flake8-pytest-style="csv"'
   Compiling ruff v0.8.6 (/Users/alexw/dev/ruff/crates/ruff)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 2.51s
     Running `target/debug/ruff check . '--config=lint.flake8-pytest-style="csv"'`
error: invalid value 'lint.flake8-pytest-style="csv"' for '--config <CONFIG_OPTION>'

  tip: A `--config` flag must either be a path to a `.toml` configuration file
       or a TOML `<KEY> = <VALUE>` pair overriding a specific configuration
       option

`lint.flake8-pytest-style` is a table.
Did you mean to use one of its subkeys instead? Possible choices:
lint.flake8-pytest-style.fixture-parentheses
lint.flake8-pytest-style.parametrize-names-type
lint.flake8-pytest-style.parametrize-values-type
lint.flake8-pytest-style.parametrize-values-row-type
lint.flake8-pytest-style.raises-require-match-for
lint.flake8-pytest-style.raises-extend-require-match-for
lint.flake8-pytest-style.mark-parentheses

Could not parse the supplied argument as a `ruff.toml` configuration option:

invalid type: string "csv", expected struct Flake8PytestStyleOptions
in `lint`

For more information, try '--help'.

Colorized screenshot

But if I apply this diff to your branch:

--- a/crates/ruff/src/args.rs
+++ b/crates/ruff/src/args.rs
@@ -978,23 +978,25 @@ The path `{value}` does not point to a configuration file"
                     let prefixed_subkeys = format!("{set}")
                         .trim_ascii()
                         .split('\n')
-                        .map(|child| format!("{key}.{child}"))
+                        .map(|child| format!(" - `{key}.{child}`"))
                         .join("\n");
 
                     tip.push_str(&format!(
                         "
 
-`{key}` is a table.
-Did you mean to use one of its subkeys instead? Possible choices:
+`{key}` is a table of configuration options, but only a single option can be
+overridden with `--config`. Did you want to override one of the table's subkeys?
+
+Possible choices:
 {prefixed_subkeys}"
                     ));
                 }
-            };
-
-            tip.push_str(&format!(
-                "\n\n{}:\n\n{underlying_error}",
-                config_parse_error.description()
-            ));
+            } else {
+                tip.push_str(&format!(
+                    "\n\n{}:\n\n{underlying_error}",
+                    config_parse_error.description()
+                ));
+            }
         }

Then it becomes:

~/dev/ruff (cli-config)⚡ [2] % cargo run -- check . --config='lint.flake8-pytest-style="csv"'
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.36s
     Running `target/debug/ruff check . '--config=lint.flake8-pytest-style="csv"'`
error: invalid value 'lint.flake8-pytest-style="csv"' for '--config <CONFIG_OPTION>'

  tip: A `--config` flag must either be a path to a `.toml` configuration file
       or a TOML `<KEY> = <VALUE>` pair overriding a specific configuration
       option

`lint.flake8-pytest-style` is a table of configuration options, but only a single option can be
overridden with `--config`. Did you want to override one of the table's subkeys?

Possible choices:
 - `lint.flake8-pytest-style.fixture-parentheses`
 - `lint.flake8-pytest-style.parametrize-names-type`
 - `lint.flake8-pytest-style.parametrize-values-type`
 - `lint.flake8-pytest-style.parametrize-values-row-type`
 - `lint.flake8-pytest-style.raises-require-match-for`
 - `lint.flake8-pytest-style.raises-extend-require-match-for`
 - `lint.flake8-pytest-style.mark-parentheses`

For more information, try '--help'.

Colorized screenshot

which feels significantly more readable to me. WDYT?

[InSyncWithFoo]

@AlexWaygood I was actually aiming for copyability, but readability is fine too.

[...] but only a single option can be overridden with --config.

This is rather misleading, because you can pass an inline table as the value:

# Same as "lint.select = ['I001']"
$ ruff check --isolated --config="lint = { select = ['I001'] }"

[AlexWaygood]

This is rather misleading, because you can pass an inline table as the value:

Oh, great point! I forgot how my own feature worked :-) We can scrap that clause, then.

[AlexWaygood]

LGTM, thank you!

[MichaReiser]

This looks great. I'm a bit worried about the output parsing and I think we can improve the handling if set itself has sub-table entries.