perf: Attempt to prevent people joining known busted rooms

src/core/config/mod.rs

@@ -1751,6 +1751,25 @@ pub struct Config {
 	#[serde(default, with = "serde_regex")]
 	pub forbidden_alias_names: RegexSet,
 
+	/// Allow joining rooms that are known to be broken or have a history of
+	/// causing issues.
+	///
+	/// The rooms that are banned in this way are hardcoded and set by the
+	/// maintainers, and cannot be configured. This method is a last-resort to
+	/// prevent people who are just setting up Matrix from joining these huge,
+	/// old rooms that have been recommended to them, only to watch their
+	/// server turn into a space heater and have horrific performance issues
+	/// that are unresolvable due to the completely broken state of the rooms.
+	///
+	/// If you enable this option, you acknowledge that joining rooms banned by
+	/// this feature will likely cause you severe performance issues, and you
+	/// forgo your right to complain about any slowdowns or inflated resource
+	/// usage you encounter.
+	///
+	/// default: false
+	#[serde(default)]
+	pub allow_joining_broken_rooms: bool,
+
 	/// List of forbidden username patterns/strings.
 	///
 	/// Regex can be used or explicit contains matches can be done by just