Support configurable backend for ServeDir

[davidpdrsn]

We've heard from several users who want to use ServeDir (or ServeFile) with files that are embedded in the binary, perhaps using rust-embed. By using ServeDir they'd get a lot for free such as finding the file from the path, range requests, pre-compressed files, etc.

That isn't possible today since ServeDir is hardcoded to always fetch files from the filesystem. I've been thinking that it could be solved by making a "backend" trait that tells ServeDir how to open files and get metadata. Something like:

#[async_trait]
pub trait Backend {
    type File: AsyncRead + AsyncSeek;
    type Metadata: FileMetadata;

    async fn open_file(&self, path: Path) -> io::Result<Self::File>;

    async fn metadata(&self, path: Path) -> io::Result<Self::Metadata>;
}

// we need a trait here because users cannot construct `std::fs::Metadata`
pub trait Metadata {
    // whatever methods we need
}

Then tower-http could provide a default backend that uses the file system but allow others to write their own.

This should be possible without breaking changes so not adding it to the 0.4 milestone.

[Folyd]

Wow, thanks! This is an excellent feature I have wanted for a long time.