Rule Change: Add languageOnly option to fenced-code-language rule

[TKDev7]

What rule do you want to change?

fenced-code-language

What change do you want to make?

Generate more warnings

How do you think the change should be implemented?

A new option

Example code

```js startline=3
const message = "Hello, world!";
console.log(message);
```

What does the rule currently do for this code?

Currently, the rule only validates the presence and allowance of the language identifier. It ignores any additional content in the info string (e.g., startline=3).

What will the rule do after it's changed?

When languageOnly is true, the rule will report an error if the info string contains anything beyond the language identifier itself.

Participation

• I am willing to submit a pull request to implement this change.

Additional comments

This option aligns with markdownlint's MD040 rule's language_only parameter, offering stricter control and promoting consistent rendering.

[nzakas]

I think the option makes sense, though I'd prefer to come up with a better name than "languageOnly", as I don't think that's particularly clear. Maybe allowInfoString where the default is false?

[snitin315]

allowInfoString makes sense 👍🏻

[TKDev7]

The info string is all the text that follows the opening code fence. This includes the language and and any additional metadata. How about allowMeta?

[lumirlumir]

Actually, I'm leaning toward using checkMeta or similar as the option name for the following reasons:

• infoString -> meta: As @TKDev7 mentioned, info is a general term that includes code fence language and other meta information. Mdast uses meta for the property name we want to check now:

• Throughout the implementation of Markdown rules, the boolean option is usually named check. Here are some references:

markdown/src/rules/no-duplicate-headings.js

Lines 64 to 66 in 614ef3e

	checkSiblingsOnly: {
	type: "boolean",
	},

markdown/src/rules/no-empty-definitions.js

Lines 73 to 75 in 614ef3e

	checkFootnoteDefinitions: {
	type: "boolean",
	},

markdown/src/rules/table-column-count.js

Lines 44 to 46 in 614ef3e

	checkMissingCells: {
	type: "boolean",
	},

[JoshuaKGoldberg]

+1 to having an option to do with meta, as that's often what it's called.

But - does this responsibility belong to fenced-code-language? The rule right now only pertains to language concerns, not additional meta(data). I'd think this would be best as a new rule.

I'm also not confident this option/rule makes sense to have on by default. Meta is pretty common in many Markdown scenarios, such as the docs generators used in Astro, Docusaurus, etc.

[nzakas]

I think @JoshuaKGoldberg's point makes sense. If what we want to do is disallow meta then that does seem outside the scope of this rule and perhaps a new rule is what we need.