identifiers: Add a compat flag to allow arbitrary-length IDs

crates/ruma-common/Cargo.toml

@@ -33,6 +33,9 @@ unstable-msc3932 = ["unstable-msc3931"]
 unstable-msc3958 = []
 unstable-unspecified = []
 
+# Allow IDs to exceed 255 bytes.
+compat-arbitrary-length-ids = ["ruma-identifiers-validation/compat-arbitrary-length-ids"]
+
 # Don't validate the version part in `KeyId`.
 compat-key-id = ["ruma-identifiers-validation/compat-key-id"]
 

crates/ruma-identifiers-validation/Cargo.toml

@@ -12,8 +12,12 @@ rust-version = { workspace = true }
 all-features = true
 
 [features]
+# Allow IDs to exceed 255 bytes.
+compat-arbitrary-length-ids = []
+
 # Don't validate the version part in `key_id::validate`.
 compat-key-id = []
+
 # Allow some user IDs that are invalid even with the specified historical
 # user ID scheme.
 compat-user-id = []

crates/ruma-identifiers-validation/src/lib.rs

@@ -18,10 +18,12 @@ pub mod voip_version_id;
 pub use error::Error;
 
 /// All identifiers must be 255 bytes or less.
+#[cfg(not(feature = "compat-arbitrary-length-ids"))]
 const MAX_BYTES: usize = 255;
 
 /// Checks if an identifier is valid.
 fn validate_id(id: &str, valid_sigils: &[char]) -> Result<(), Error> {
+    #[cfg(not(feature = "compat-arbitrary-length-ids"))]
     if id.len() > MAX_BYTES {
         return Err(Error::MaximumLengthExceeded);
     }

crates/ruma/Cargo.toml

@@ -102,6 +102,10 @@ compat = [
     "compat-signature-id",
     "compat-tag-info",
 ]
+
+# Allow IDs to exceed 255 bytes.
+compat-arbitrary-length-ids = ["ruma-common/compat-arbitrary-length-ids"]
+
 # Don't validate the version part in `KeyId`.
 compat-key-id = ["ruma-common/compat-key-id"]