Formatting without fmt::Debug

[kornelski]

Fixes #75

[lukaslueg]

Thanks for the PR. I guess this is in preparation of rust-lang/rust#129709?!

I'll revert CI back to @stable after this, and add an extra nightly-phase to test this unstable feature specifically (built seems to be used in embedded quite often, and those people do care about binary sizes, so fmt-debug should be supported by built early on).

This PR removes the use of PathBuf's Debug-representation, using Display instead, which the (stable) docs explicitly mention as being lossy; the docs also advertise using Debug. Are we sure .escape_default() on the string-representation is sufficient to do what PathBuf's Debug used to do?

[kornelski]

That suggestion to use Debug is misleading, because Debug is also lossy or incorrect.

Debug has the same problem of not knowing the real encoding of paths on most Unix systems (where a single path could even contain multiple different incompatible encodings when it crosses file systems). You get some escape codes, but they're a combination of misinterpreting bytes as UTF-8 where possible and byte escape codes that are not allowed in unicode strings.

The format of Debug is unspecified and unstable:

Derived Debug formats are not stable, and so may change with future Rust versions. Additionally, Debug implementations of types provided by the standard library (std, core, alloc, etc.) are not stable, and may also change with future Rust versions.

No matter what you do, Rust's strings can't contain any non-UTF-8 encoding, so this is an unsolvable problem for as long as you're using &str. You'd have to generate platform-specific code that serializes and deserializes bytes of the path, and Debug print doesn't do that.