rustc prints the warning:
`-C save-temps` might not produce all requested temporary
products when incremental compilation is enabled.
However, incremental compilation isn't even enabled.
A look at the Rust source code confirms that this warning is indeed
printed unconditionally when the `-C save-temps` flag is specified:
5f2b325f64/src/librustc/session/config.rs (L1015-L1018)