Change build field to Option<StringOrBool>

[Turbo87]

https://doc.rust-lang.org/cargo/reference/manifest.html#the-build-field says that valid values for the build field are paths and false, which basically matches the readme field (except for the workspace inheritance). Since readme uses the StringOrBool enum, it makes sense to use the same for the build field as well.

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[Turbo87]

@LukeMathWalker since StringOrBool is exclusively used for paths, any thoughts on changing it to PathBuf instead of String?

[LukeMathWalker]

Can you have a string field in a TOML document that is a valid PathBuf but not a valid String? @Turbo87

[Turbo87]

Can you have a string field in a TOML document that is a valid PathBuf but not a valid String?

TOML is defined as only allowing UTF8 and the thing that makes conversion between String and PathBuf tricky is non-UTF8 characters. In other words, a valid TOML file should already ensure that String can be converted to PathBuf and vice-versa without issues.

[LukeMathWalker]

OK, that's what I remembered. Then I think moving to PathBuf would be a step backwards, since we no longer signal in the type system the guarantee that the value is valid UTF8.

A potential alternative would be to use something like Utf8PathBuf from camino to give us the best of both worlds: precise modelling of the guarantees and path-related methods and conversions.

[Turbo87]

Since Path::new(String) is essentially free I guess it's indeed fine to keep it as a String. I'm wondering about adding something like fn as_path(&self) -> Option<&Path> to StringOrBool though (Option because of the Bool variant).

[LukeMathWalker]

I'd rather have a specialized PathBufOrBool, since an as_path method on StringOrBool would be a bit weird.
I'll merge this one in the meantime.

[LukeMathWalker]

Reviewing the docs, I'm not sure we should use StringOrBool here, but rather StringOrFalse, since true isn't a valid value for the build field.

[Turbo87]

https://github.com/rust-lang/cargo/blob/b644a40a1f8406a08bc64305a6a8fd6bb5a34071/src/cargo/util/toml/mod.rs#L1510 suggests that this is mostly a documentation issue. apparently it behaves similar to https://github.com/rust-lang/cargo/blob/b644a40a1f8406a08bc64305a6a8fd6bb5a34071/src/cargo/util/toml/mod.rs#L1532, except for the inheritance.