[Partial Namespace] Add a proposal for fixing partial namespaces. (sass#2812)

See sass#2800

Author: nex3

proposal/partial-namespace.md

@@ -0,0 +1,77 @@
+# Partial Namespaces: Draft 1.0
+
+*([Issue](https://github.com/sass/sass/issues/2800))*
+
+## Table of Contents
+
+* [Background](#background)
+* [Summary](#summary)
+* [Procedures](#procedures)
+  * [Determining a `@use` Rule's Namespace](#determining-a-use-rules-namespace)
+* [Deprecation Process](#deprecation-process)
+
+## Background
+
+> This section is non-normative.
+
+In the new Sass [module system][] as written, the implicit namespace of a `@use`
+rule that loads a partial with an explicit leading underscore contains that
+underscore. This was unintended and confusing, making the namespace look like a
+private identifier and making it different than the same file's namespace
+imported without the underscore.
+
+[module system]: ../accepted/module-system.md
+
+## Summary
+
+> This section is non-normative.
+
+The underscore will be stripped from the namespace, so `@use "_styles"` will
+have the implicit namespace `styles` rather than `_styles`.
+
+## Procedures
+
+### Determining a `@use` Rule's Namespace
+
+This modifies the existing algorithm for [Determining a `@use` Rule's
+Namespace][] to read as follows (new text in bold):
+
+[Determining a `@use` Rule's Namespace]: ../spec/at-rules/use.md#determining-a-use-rules-namespace
+
+* If `rule` has an `'as'` clause `as`:
+
+  * If `as` has an identifier, return it.
+
+  * Otherwise, return `null`. The rule is global.
+
+* Let `path` be the `rule`'s URL's [path][URL path].
+
+  [URL path]: https://url.spec.whatwg.org/#concept-url-path
+
+* Let `basename` be the text after the final `/` in `path`, or the entire `path`
+  if `path` doesn't contain `/`.
+
+* Let `module-name` be the text before the first `.` in `path`, or the entire
+  `path` if `path` doesn't contain `.`.
+
+* **If `module-name` begins with `_`, remove the leading `_` and set
+  `module-name` to the result.**
+
+* If `module-name` isn't a Sass identifier, throw an error.
+
+* Return `module-name`.
+
+## Deprecation Process
+
+Although this is technically a breaking change, it will be made without a
+deprecation process for the following reasons:
+
+* Including a leading underscore in a loaded URL is unnecessary, and very few
+  stylesheets do it in practice.
+
+* The module system is still very young and not yet super widely used, so it's
+  even more unlikely that leading underscores are used in `@use` statements in
+  particular.
+
+* This is a fix for an unintended bug in the spec rather than a change to
+  intended behavior that users were expected to rely upon.