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Document cross compilation target == host bugfix. #6803

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Document cross compilation target == host bugfix. #6803

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12 changes: 12 additions & 0 deletions docs/installation.rst
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Expand Up @@ -160,6 +160,18 @@ use ``--no-binary``.

$ pip install cryptography --no-binary cryptography

Cross Compiling
~~~~~~~~~~~~~~~

If you are cross compiling for a target system that has the same target triple
as the build host you should add these variables in your environment to prevent
cargo from linking host build-scripts against the target libraries.

.. code-block:: console

__CARGO_TEST_CHANNEL_OVERRIDE_DO_NOT_USE_THIS="nightly"
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The DO_NOT_USE_THIS is kind of a red-flag that this probably isn't the right way to do this 😂

I'm not sure I understand the use case here, what does it mean to cross-compile for a target with the same triple as the host?

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@jameshilliard jameshilliard Jan 9, 2022 •
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The DO_NOT_USE_THIS is kind of a red-flag that this probably isn't the right way to do this 😂

It's the correct way to do this on a stable rust toolchain, because the fix is behind a nightly flag and that's the only way to enable a nightly flag on the stable toolchain. By policy nightly flags are not supported/documented as available on stable toolchains but there's not actually a technical reason that they won't work(they work effectively the same as they do on a nightly toolchain if you set __CARGO_TEST_CHANNEL_OVERRIDE_DO_NOT_USE_THIS="nightly"), this particular nightly only feature is needed to fix a critical cross compilation toolchain bug. I've been trying to stabilize this for a while but that's stalled for unclear reasons.

I'm not sure I understand the use case here, what does it mean to cross-compile for a target with the same triple as the host?

For example if the host triple(ie x86_64-unknown-linux-gnu) is identical to the target triple(ie x86_64-unknown-linux-gnu) then you need to set this in the env when cross compiling otherwise the build will likely fail with cryptic build script linker errors due to cargo using the wrong linker for build scripts.

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Why do you want to cross compile at all if the host triple is the target triple? Wouldn't you just compile it? I'm assuming there's a nuance I'm missing here 😄

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@jameshilliard jameshilliard Jan 9, 2022 •
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Why do you want to cross compile at all if the host triple is the target triple?

For example I'm trying to cross compile a rootfs image for an embedded x86_64 board using buildroot.

Wouldn't you just compile it?

We can't compile on the target because the target does not have a compiler.

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We are currently using this fix with our infrastructure to build python-cryptography.

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(Apologies for not following up here more promptly.)

I guess the thing I don't understand is why this is considered cross-compilation at all? If the host and target are both "the same", what's being solved here?

CARGO_UNSTABLE_TARGET_APPLIES_TO_HOST="true"
CARGO_TARGET_APPLIES_TO_HOST="false"

Using your own OpenSSL on Linux
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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