cargo-fuzz

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#279 includes changes necessary to start supporting Windows once support also lands in rustc (see https://github.com/rust-lang/rust/pull/89369). I wanted to explore the current state of support by running the cargo-fuzz test suite with the rust toolchain built with Windows support for asan symbols enabled. I've run in to two issues so far.

First Issue - Addressed

The test suite generates a set of fuzz targets and associated Cargo.toml manifest files. Those files include path configuration that, up until this point, has leveraged the PathBuf display functionality to render the paths as strings for the generated toml file. This breaks on Windows. toml needs \ in Windows paths to be escaped, which display does not do. Using the toml serializer address this in a cross platform way. I have tested this change on Windows and Linux. While I don't have a Mac system to test on I have no reason to think this is a risky change there.

Second Issue - Work In Progress

With a dev toolchain built and linked including the changes from https://github.com/rust-lang/rust/pull/89369 and the above path issue addressed there are still failures of the test cases on Windows. All test cases that execute fuzz tests fail with:

error: unable to unlink old fallback exe: Access is denied. (os error 5)

References to this error I've found suggest it has to do with calling cargo from within another execution of cargo on Windows, which is the behavior we're attempting in these test cases. Reference https://github.com/rust-lang/rust-clippy/issues/3118. Clippy addressed this with hardcoding the +nightly toolchain in their calls to cargo which might work here when/if https://github.com/rust-lang/rust/pull/89369 or other support hits nightly. I'm investigating other options.