Flag: input format

[steveklabnik]

This issue is part of #125

-r/--input-format: input format

This flag is currently ignored; the idea is that rustdoc would support various
input formats, and you could specify them via this flag.

Rustdoc only supports Rust source code and Markdown input formats. If the
file ends in .md or .markdown, rustdoc treats it as a Markdown file.
Otherwise, it assumes that the input file is Rust.

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Should we keep this flag or not?

[steveklabnik]

Given that this flag is ignored, we should probably continue to do so.

[steveklabnik]

We talked about this at the tools team meeting today and the intention is to remove this flag.

[steveklabnik]

rust-lang/rust#44136 is the issue tracking the deprecation; it may or may not be removed, but it means we don't need to support it.