Add new build option to support Windows DLLs

[cwshugg]

Hi - this PR implements a new build argument: --no-include-main-msvc - in order to help enable support for building and fuzzing Windows DLLs.

I recently ran into an issue when attempting to fuzz a Windows-only DLL (#386) where the /include:main linker argument that is added during FuzzProject::cargo() for MSVC-based builds gave the DLL a main symbol it could not resolve, which caused linking to fail:

LINK : error LNK2001: unresolved external symbol main
C:\....\depedency.dll : fatal error LNK1120: 1 unresolved externals

The DLL is, by nature, completely separate from the fuzzing targets in my cargo-fuzz repository, but I still wanted it to be a dependency listed in fuzz/Cargo.toml, so that it would be built and instrumented in the exact same way as the fuzzing target binaries. I found that by controlling whether or not /include:main is added to the cargo build arguments executed by cargo-fuzz, I was able to set up a "dummy" fuzzing target that provides its own main function:

# fuzz/Cargo.toml
# ...
[dependencies]
# the DLL is optional; only to be built with a specific feature
my_dll = { path = "..", optional = true }
# ...
[features]
build_dll = ["dep:my_dll"]
# ...
[[bin]]
name = "dummy_target_build_dll"
path = "fuzz_targets/dummy_target_build_dll.rs"
required-features = ["build_dll"]
test = false
doc = false
bench = false
# ... other binaries ...

// dummy_target_build_dll.rs
fn main()
{
    println!("DLL build complete! Now, copy the DLL to your desired location, `
              and use `cargo fuzz run` to run your other fuzzing targets.");
    // ... do other work, such as copying the DLL into the desired location ...
}

Then, by executing cargo fuzz run --no-include-main-msvc --features=build_dll dummy_target, the DLL would be built successfully. I could then proceed to run all other fuzzing targets, which would load in the instrumented DLL and fuzz it.

TL;DR: This solution allows Windows DLLs to be instrumented & fuzzed, but only if manual control over /include:main is allowed (hence, --no-include-main-msvc). This resolves #386.

[fitzgen]

Thanks!

[fitzgen]

https://github.com/rust-fuzz/cargo-fuzz/actions/runs/11371974196/job/31744642568?pr=388#step:6:231

error[E0063]: missing field `no_include_main_msvc` in initializer of `options::BuildOptions`
Error:    --> src/options.rs:263:28
    |
263 |         let default_opts = BuildOptions {
    |                            ^^^^^^^^^^^^ missing `no_include_main_msvc`

[cwshugg]

@fitzgen - Thanks for your patience; I was out of town the past few days and haven't had a chance to address the two PRs. Just added the fix to set that argument's default value!

[cwshugg]

Hi @fitzgen - just wanted to check in and see if you've been able to give this a second look!

[fitzgen]

I was on vacation for a bit, re-running CI now. Thanks for your patience!

[fitzgen]

Thanks!

[cwshugg]

Thank you very much! Appreciate your help with getting this merged in 😄

[cwshugg]

@fitzgen - Will this be built into a future release? (When do you think that release would come?) It would be great to be able to pull this new code down via an updated version number in Cargo.toml.

Let me know if there is anything I can do to help there.