Auto merge of #12206 - ehuss:index-version-unique, r=weihanglo

Add a small note about indexes ignoring SemVer build metadata.

This adds some clarification to the documentation on how registries need to handle SemVer build metadata, and the uniqueness constraint around version entries. IIRC, some issues in cargo have been fixed, but some still linger (like #7180). crates.io just fixed it so that new duplicates will be rejected.

src/doc/src/reference/registry-index.md

@@ -80,6 +80,8 @@ harder to support older versions of Cargo that lack `{prefix}`/`{lowerprefix}`.
 For example, nginx rewrite rules can easily construct `{prefix}` but can't
 perform case-conversion to construct `{lowerprefix}`.
 
+### Name restrictions
+
 Registries should consider enforcing limitations on package names added to
 their index. Cargo itself allows names with any [alphanumeric], `-`, or `_`
 characters. [crates.io] imposes its own limitations, including the following:
@@ -98,6 +100,14 @@ attacks](https://en.wikipedia.org/wiki/IDN_homograph_attack) and other
 concerns in [UTR36](https://www.unicode.org/reports/tr36/) and
 [UTS39](https://www.unicode.org/reports/tr39/).
 
+### Version uniqueness
+
+Indexes *must* ensure that each version only appears once for each package.
+This includes ignoring SemVer build metadata.
+For example, the index must *not* contain two entries with a version `1.0.7` and `1.0.7+extra`.
+
+### JSON schema
+
 Each line in a package file contains a JSON object that describes a published
 version of the package. The following is a pretty-printed example with comments
 explaining the format of the entry.