MSC3980: Dotted Field Consistency (#3980)

* MSC for consistency between push rules and filters.

* Update proposals/3980-dotted-fields-consistency.md

Co-authored-by: Travis Ralston <travisr@matrix.org>

---------

Co-authored-by: Travis Ralston <travisr@matrix.org>

proposals/3980-dotted-fields-consistency.md

@@ -0,0 +1,60 @@
+# MSC3980: Dotted Field Consistency
+
+[MSC3873](https://github.com/matrix-org/matrix-spec-proposals/pull/3873) disambiguated
+how event properties are matched for push rules and based the proposal on how
+[filters](https://spec.matrix.org/v1.6/client-server-api/#post_matrixclientv3useruseridfilter)
+are currently escaped for consistency.
+
+Through the MSC process the escaping was expanded from just escaping `.` characters
+with a backslash (`\.`) to also escaping backslashes themselves (`\\`). Unfortunately
+that MSC did not propose applying this change *back* to filters for consistency. [^1]
+
+## Proposal
+
+Apply consistent escaping as described in [MSC3873](https://github.com/matrix-org/matrix-spec-proposals/pull/3873)
+to the `event_fields` property of filters.
+This would allow an unambiguous way to describe property names, as currently
+the behavior of backslashes is undefined.
+
+The text given in MSC3873 can apply here (changing `key` to `event_fields`):
+
+> The dot (`.`) character in the [`event_fields`] parameter is changed to be exclusively
+reserved for field separators. Any literal dot in field names are to be
+escaped using a backslash (`\.`) and any literal backslashes are also escaped with
+a backslash (`\\`). A backslash before any other character has no special meaning
+and is left as-is, but it is recommended that implementations do not redundantly
+escape characters, as they may be used for escape sequences in the future.
+
+In short, this would update the
+[description of `event_fields`](https://spec.matrix.org/v1.6/client-server-api/#post_matrixclientv3useruseridfilter)
+(bolded part is new):
+
+> List of event fields to include. If this list is absent then all fields are
+> included. The entries may include ‘.’ characters to indicate sub-fields. So
+> [‘content.body’] will include the ‘body’ field of the ‘content’ object. A
+> literal ‘.’ **or '\\'** character in a field name may be escaped using a ‘\’. A server
+> may include more fields than were requested.
+
+
+## Potential issues
+
+This is slightly backwards incompatible if a property name currently contains a
+backslash in it.
+
+## Alternatives
+
+Leave things as they are and be inconsistent between different parts of the spec.
+
+## Security considerations
+
+N/A
+
+## Unstable prefix
+
+N/A
+
+## Dependencies
+
+N/A
+
+[^1]: This came up while writing the [spec PR for MSC3873](https://github.com/matrix-org/matrix-spec/pull/1464#discussion_r1135712844).