Store rlink data in opaque binary format on disk

[Mark-Simulacrum]

This removes one of the only uses of JSON decoding (to Rust structs) from the compiler, and fixes the FIXME comment. It's not clear to me what the reason for using JSON here originally was, and from what I can tell nothing outside of rustc expects to read the emitted information, so it seems like a reasonable step to move it to the metadata-encoding format (rustc_serialize::opaque).

Mostly intended as a FIXME fix, though potentially a stepping stone to dropping the support for Decodable to be used to decode JSON entirely (allowing for better/faster APIs on the Decoder trait).

cc #64191

[rust-highfive]

r? @davidtwco

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[bjorn3]

It's not clear to me what the reason for using JSON here originally was

Not sure.

and from what I can tell nothing outside of rustc expects to read the emitted information

It isn't meant to be read outside of rustc.

[davidtwco]

@bors r=davidtwco,bjorn3

[bors]

📌 Commit a1c261c has been approved by davidtwco,bjorn3

[joshtriplett]

@bors rollup=never

Seems like it might have performance implications (hopefully positive ones).

[Mark-Simulacrum]

rlink isn't currently used by cargo afaik, which is why I didn't queue up perf, but it's possible it'll affect pgo or so.

[bors]

⌛ Testing commit a1c261c with merge b7cd0f7...

[bors]

☀️ Test successful - checks-actions
Approved by: davidtwco,bjorn3
Pushing b7cd0f7 to master...

[rust-timer]

Finished benchmarking commit (b7cd0f7): comparison url.

Summary: This benchmark run did not return any relevant results.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression