MSC3758: Add `event_property_is` push rule condition kind (#3758)

* Create xxxx-expand-push-rule-conditions.md

* Rename to proper MSC number

* Add PR number to title & wrap

* Remove list matching part of MSC3758, will propose in a new MSC

* Link through to the spec for current match type

* Switch to `value` instead of `pattern` for match key

* Expand missing sections

* Fix typo.

* Clarifications from review.

* Update title in MSC to match PR.

Co-authored-by: Hubert Chathi <hubert@uhoreg.ca>

* Update the name of the push rule condition.

---------

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>
Co-authored-by: Hubert Chathi <hubert@uhoreg.ca>

proposals/3758-expand-push-rule-conditions.md

@@ -0,0 +1,69 @@
+# MSC3758: Add `event_property_is` push rule condition kind 
+
+Currently the only condition used to match event content for push rules is the `event_match` kind.
+This compares a glob-style string against a string value within the event dictionary. The event
+dictionary is flattened before conditions are checked to enable matching nested values.
+
+This approach is currently limited to only checking for string values within the event dictionary
+(at any level). This MSC proposes a new exact match type for event content that works with all
+JSON types.
+
+
+## Proposal
+
+### Exact matching event data
+
+We propose a new type of condition, `event_property_is`. Similar to  the current `event_match`
+([link to spec](https://spec.matrix.org/v1.3/client-server-api/#conditions-1)), this condition
+takes two parameters: `value` and `key`. The exact match compares the `value` to the event data
+associated with `key` exactly. Both type and content (when a string) should be identical
+(include case). This allows for matching all non-compound JSON types allowed by
+[canonical JSON](https://spec.matrix.org/v1.5/appendices/#canonical-json):
+i.e. strings, `null`, `true`, `false` and integers. This also provides a simpler
+exact string matching mechanism (and any associated performance gains on implementation side without
+globbing).
+
+An example condition may look like (encoded as a JSON object):
+
+```json
+{
+  "kind": "event_property_is",
+  "key": "event.content.is_something",
+  "value": true
+}
+```
+
+
+## Alternatives
+
+[MSC3862](https://github.com/matrix-org/matrix-spec-proposals/pull/3862) proposes an alternative
+solution by converting non-string JSON objects to strings in the `event_match` condition type.
+
+## Security considerations
+
+None.
+
+## Future extensions
+
+A future MSC may wish to define the behavior of `event_property_is` when
+used with a JSON object or array.
+
+[MSC3887](https://github.com/matrix-org/matrix-spec-proposals/pull/3887) is a
+related MSC which attempts to define behavior for searching for a value inside of
+an array.
+
+## Unstable prefix
+
+While still not part of the Matrix spec, the new push rule condition should be
+`com.beeper.msc3758.exact_event_match` instead of `event_property_is`, e.g.:
+
+```json
+{
+  "kind": "com.beeper.msc3758.exact_event_match",
+  "key": "..."
+}
+```
+
+## Dependencies
+
+None.