The following presentation at this year's DEF CON was brought to our attention on the Diesel Gitter Channel:
SQL Injection isn't Dead: Smuggling Queries at the Protocol Level
http://web.archive.org/web/20240812130923/https://media.defcon.org/DEF%20CON%2032/DEF%20CON%2032%20presentations/DEF%20CON%2032%20-%20Paul%20Gerste%20-%20SQL%20Injection%20Isn't%20Dead%20Smuggling%20Queries%20at%20the%20Protocol%20Level.pdf
(Archive link for posterity.) Essentially, encoding a value larger than 4GiB can cause the length prefix in the protocol to overflow, causing the server to interpret the rest of the string as binary protocol commands or other data.
It appears Diesel does perform truncating casts in a way that could be problematic, for example: https://github.com/diesel-rs/diesel/blob/ae82c4a5a133db65612b7436356f549bfecda1c7/diesel/src/pg/connection/stmt/mod.rs#L36
This code has existed essentially since the beginning,
so it is reasonable to assume that all published versions <= 2.2.2 are affected.
Mitigation
The prefered migration to the outlined problem is to update to a Diesel version newer than 2.2.2, which includes fixes for the problem.
As always, you should make sure your application is validating untrustworthy user input. Reject any input over 4 GiB, or any input that could encode to a string longer than 4 GiB. Dynamically built queries are also potentially problematic if it pushes the message size over this 4 GiB bound.
For web application backends, consider adding some middleware that limits the size of request bodies by default.
Resolution
Diesel now uses #[deny] directives for the following Clippy lints:
to prevent casts that will lead to precision loss or other trunctations. Additionally we performed an audit of the relevant code.
A fix is included in the 2.2.3 release.