Malformed cfg value or key/value pair with cargo-prusti

[fpoli]

After PR #48, cargo-prusti fails with "error: malformed cfg value or key/value pair: lib___.rlib".

That PR runs the compiler twice, to collect type system information that are not available in the parse phase of the second run. The preliminar run probably has some side effects (stdout messages? file system changes?) that break cargo.

The error is not catched by the CI tests because they don't yet run cargo-prusti (see also: #3).

[fpoli]

The error is raised when trying to compile crates libc, bitflags, log, ...

[fpoli]

It turns out that cargo executes

rustc - --crate-name ___ --print=file-names --crate-type bin --crate-type rlib --crate-type dylib --crate-type cdylib --crate-type staticlib --crate-type proc-macro --print=sysroot --print=cfg

to print some information of the compiled crate. Prusti prints the same output twice, due to the double call to the compiler, and this breaks cargo's parsing.