Deprecate cURL

[das-sein]

Part of the simplification project: #1611

Deprecates cURL in favor of reqwest.

This is the preliminary step for the complete disuse of cURL by rustup; on-going discussion: #1657

cc's from previous PR:
@dwijnand
@brson
@briansmith
@jnicholls
@seanmonstar

[dwijnand]

I don't think RUSTUP_USE_CURL should be introduced and on usage it emits a warning message. I think the old UsingCurl notification should remain.

[das-sein]

@dwijnand Does this mean we should have the user compile themselves with the default feature in the Cargo.toml set to cURL if they want to use it?

[dwijnand]

No, no compilation necessary, as the rustup binary crate depends on both backends/features:

https://github.com/rust-lang/rustup.rs/blob/194026d2cac3d06c8ef19dc6e23dabda5d3900c4/Cargo.toml#L23

[das-sein]

How would a user enable one backend over the other without some kind of environment variable? I think I'm missing something here.

[dwijnand]

Ah, I see the confusion. I meant we should introduce RUSTUP_USE_CURL but, if a user sets it, it shouldn't emit a deprecation warning message. Just "downloading with curl".

[das-sein]

@dwijnand Removed the deprecation warning.

[dwijnand]

LGTM. Thanks!

[kinnison]

This is good, though I'd prefer the odd bogon of a diff hunk was fixed.

[das-sein]

@kinnison Weird delta removed from commits.

[nrc]

LGTM other than the Hyper thing

Odd bogon of a diff hunk fixed