rustup update fails on FreeBSD for nightly due to missing docs

[bdrewery]

This seems related to #49187:

This breaks rustup update as well with --force.

# rustup update
info: syncing channel updates for 'stable-x86_64-unknown-freebsd'
info: syncing channel updates for 'nightly-x86_64-unknown-freebsd'
info: latest update on 2018-03-28, rust version 1.26.0-nightly (9c9424de5 2018-03-27)
error: component 'rust-docs' for 'x86_64-unknown-freebsd' is unavailable for download
info: checking for self-updates

       stable-x86_64-unknown-freebsd unchanged - rustc 1.24.1 (d3ae9a9e0 2018-02-27)
  nightly-x86_64-unknown-freebsd update failed - rustc 1.26.0-nightly (75af15ee6 2018-03-20)

# RUST_BACKTRACE=1 rustup update --force
info: syncing channel updates for 'stable-x86_64-unknown-freebsd'
info: syncing channel updates for 'nightly-x86_64-unknown-freebsd'
info: latest update on 2018-03-28, rust version 1.26.0-nightly (9c9424de5 2018-03-27)
thread 'main' panicked at 'components available', /checkout/src/libcore/option.rs:874:4
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
             at /checkout/src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:49
   1: std::sys_common::backtrace::_print
             at /checkout/src/libstd/sys_common/backtrace.rs:68
   2: std::panicking::default_hook::{{closure}}
             at /checkout/src/libstd/sys_common/backtrace.rs:57
             at /checkout/src/libstd/panicking.rs:381
   3: std::panicking::default_hook
             at /checkout/src/libstd/panicking.rs:397
   4: std::panicking::rust_panic_with_hook
             at /checkout/src/libstd/panicking.rs:577
   5: std::panicking::begin_panic
             at /checkout/src/libstd/panicking.rs:538
   6: std::panicking::begin_panic_fmt
             at /checkout/src/libstd/panicking.rs:522
   7: rust_begin_unwind
             at /checkout/src/libstd/panicking.rs:498
   8: core::panicking::panic_fmt
             at /checkout/src/libcore/panicking.rs:71
   9: core::option::expect_failed
             at /checkout/src/libcore/option.rs:874
  10: rustup_dist::manifestation::Manifestation::update
  11: rustup_dist::dist::update_from_dist_
fatal runtime error: failed to initiate panic, error 5
zsh: abort (core dumped)  RUST_BACKTRACE=1 rustup update --force

[kennytm]

I'm not sure if this should be fixed at rust-central-station level (release promotion --- hard-link/copy the x86_64 Linux docs to every other targets), or rustup level (make docs not an essential component).

[prisme60]

It fails also on my raspberry pi:

[christian@alarmpi ~]$ rustup update nightly
info: syncing channel updates for 'nightly-arm-unknown-linux-gnueabihf'
260.7 KiB / 260.7 KiB (100 %)  50.7 KiB/s ETA:   0 s
info: latest update on 2018-03-29, rust version 1.26.0-nightly (e5277c145 2018-03-28)
error: component 'rust-docs' for 'arm-unknown-linux-gnueabihf' is unavailable for download

[tarka]

Same with armv7:

$ rustup install nightly
info: syncing channel updates for 'nightly-armv7-unknown-linux-gnueabihf'
info: latest update on 2018-03-31, rust version 1.26.0-nightly (80785a547 2018-03-30)
error: component 'rust-docs' for 'armv7-unknown-linux-gnueabihf' is unavailable for download

[ctrlcctrlv]

Same problem here on x86_64-unknown-freebsd

[shisoft]

Same problem on nightly-aarch64-unknown-linux-gnu

[ctrlcctrlv]

This sucks -- builds broken on major platforms for a week and no fix in sight. If we don't know how to fix it, why not just revert #49187 until it's figured out?

[Mark-Simulacrum]

Cc @alexcrichton