Clippy (https://github.com/rust-lang/rust-clippy) is a Rust linter to catch common mistakes and improve Rust code, with over 450 lints included.

The Rust playground has support for it under "Tools".

Running it should be as easy as calling clippy-driver instead of rustc, see https://github.com/rust-lang/rust-clippy#as-a-rustc-replacement-clippy-driver. Since it still calls rustc, CE could still show the assembly/LLVM IR/etc.