Suggestion for commenting out HTML attributes

text/0000-attribute-comments.md

@@ -0,0 +1,159 @@
+- Start Date: 2020-12-12
+- RFC PR: (leave this empty)
+- Svelte Issue: (leave this empty)
+
+# Commenting out HTML attributes
+
+## Summary
+
+This RFC proposes a way to be able to comment out individual attributes within an HTML tag. It would make this valid:
+
+```html
+<Draggable
+  {x} {y}
+  bind:m={ matrices[i] }
+  //on:some      <-- this whole line would be ignored
+>
+```
+
+The current implementation (Svelte 3.31.0) passes the `//on:some` tag to Rollup, which breaks the build:
+
+```
+[rollup] [!] (plugin svelte) ParseError: Expected >
+[rollup] app/App.svelte
+[rollup] 150:           x={x} y={y}
+[rollup] 151:           bind:m={ matrices[i] }
+[rollup] 152:           //on:some
+[rollup]                 ^
+[rollup] 153:           {validate}
+[rollup] 154:           {onBan}
+```
+
+## Motivation
+
+Comments are commonplace in programming but not allowed by the XML syntax, within a tag. Svelte, being a compiler, could extend the syntax by allowing them and simply remove such lines from the text passed on to Rollup.
+
+The proposal is:
+
+- Within a tag
+   - if a line matches `^\s*\/\/.*$` regex (starts with white space, followed by `//`
+      - such line is replaced by an empty line (to keep line numbering unchanged)
+
+Sample:
+
+```
+<Sample a=10 b=12
+  some="42 //"
+  //other    tbd.
+  //further=13   <-- disabled
+/>
+```
+
+Use cases:
+
+- one wants to temporarily disable an attribute
+- reminder that something needs to be done, in the future
+
+
+### Prior discussions
+
+Prior to making this RFC, I mentioned the idea at the [Svelte Discord](https://discord.com/channels/457912077277855764/653341885674553345/786609908807499796) and got two favourable comments, so decided to push further.
+
+>I second your idea @asko ...I find myself wanting to do this often :slight_smile:
+
+>me too, it's kinda annoying to move the property to other line and comment it and then put it back to where it was when I want to uncomment it.
+
+
+### Why not proper end-of-line comments?
+
+The author feels that the beginning whitespace + `//` is good enough to cover all current real world use cases. Proper end-of-line comments would allow eg. `a=12  // 12 = 4*3` but attributes rarely need commenting like that. It is, instead, the disable/enable use case that is leading this RFC.
+
+### Why `//`?
+
+XML itself does not sport an end-of-line comment syntax.
+
+`//` should be intuitive from JavaScript and (as we have seen in the summary) it does not break existing working code, since Rollup builds choke on it.
+
+`#` (bash, Makefile) is already being used for something else.
+
+`--` (Lua) is suggested in the [style properties RFC](https://github.com/sveltejs/rfcs/blob/master/text/0000-style-properties.md) for other purposes.
+
+
+## Detailed design
+
+The implementation would be a single-line-ish change in the Svelte parser.
+
+The larger implications are with the ecosystem: Svelte syntax highlighters (VS Code, WebStorm et.al.) should eventually render such within-tag lines as comments. The author is not aware, whether the Svelte parser is used by these highlighters (easy to change) or whether they implement their own (takes longer).
+
+### Early prototyping
+
+One could try to make custom highlighting rules for WebStorm, to dim down the suggested lines.
+
+One could do a filter to Svelte parsing (if there is support for such?), to make an early implemantation of this RFC.
+
+
+## How we teach this
+
+Adding `//` to the beginning of attributes seems intuitive to web developers so they might just find it to work, at least if IDE syntax highlighting supports it.
+
+However, since this would be a non-standard HTML feature, it likely deserves to be mentioned in the docs, maybe between [HTML tags](https://svelte.dev/tutorial/html-tags).
+
+Sample text:
+
+>Svelte allows you to disable/enable HTML attributes by preceding them with `//`. Such lines are ignored. [Next]
+
+For existing users, a notice in the [Changelog](https://github.com/sveltejs/svelte/blob/master/CHANGELOG.md) should do, since it's a non-breaking change.
+
+
+## Drawbacks
+
+It feels wrong to make XML (HTML) do dance steps it doesn't otherwise do.
+
+This kind of changes can be seen as further distancing Svelte from the HTML/CSS/JS knowledge base, and that may not be a good thing. 
+
+However, it's mostly (intended to be) a development-level feature. This is the author's only objection, and therefore the RFC, to have a discussion and decision, whether this is aligned with Svelte's general trajectory.
+
+
+## Alternatives
+
+### Use of current HTML commenting (after tag)
+
+```
+<Sample a=10 b=12
+  some="42 //"
+/>
+  <!-- other tbd. -->
+  <!-- further=13 - disabled -->
+```
+
+The context of such lines is lost, and seems confusing. It can be done, but requires a team convention to know that comments after a tag refer to the tag attributes.
+
+### Preceding attributes with `__` or `--`
+
+This passes Rollup builds, unlike `//`.
+
+```
+<Sample a=10 b=12
+  some="42 //"
+  __other
+  __further=13
+/>
+```
+
+Pros:
+
+- already works; no changes to Svelte needed
+
+Cons: 
+
+- no additional explanation at the end of line can be made
+- the attributes are just renamed - they are still there in build output. So this is not really commenting.
+
+### Prior art
+
+The author is not aware of preceding art that introduces within-HTML-tag commenting. References to such are a welcome addition.
+
+
+## Unresolved questions
+
+> Optional, but suggested for first drafts. What parts of the design are still TBD?