rustdoc: mechanism for taking/handling arbitrary rustc arguments?

[QuietMisdreavus]

Right now, rustdoc takes a handful of compilation-focused arguments so it can affect the compilation process it uses to extract docs or run tests. A discussion in #cargo today made me wonder if it was possible (or even feasible) to take arbitrary rustc arguments and have some way to parse those into the required structures or features needed for rustdoc's purposes as a compiler driver. Something like having -- and then batching everything that goes after that?

@rust-lang/compiler @rust-lang/dev-tools @rust-lang/t-dev-tools-docs

[QuietMisdreavus]

The idea that brought this up was for Cargo to be able to supply $RUSTFLAGS to rustdoc by simply having the equivalent of rustdoc ... -- $RUSTFLAGS when executing rustdoc.

[QuietMisdreavus]

Duplicate of #43031