Currently the default release profile enables LTO and single CGU builds,
which is very slow to build. Most tests are better run with
optimizations enabled since it allows testing a much larger number of
inputs, so it is inconvenient that building can sometimes take
significantly longer than the tests.
Remedy this by doing the following:
* Move the existing `release` profile to `release-opt`.
* With the above, the default `release` profile is untouched (16 CGUs
and thin local LTO).
* `release-checked` inherits `release`, so no LTO or single CGU.
This means that the simple `cargo test --release` becomes much faster
for local development. We are able to enable the other profiles as
needed in CI.
Tests should ideally still be run with `--profile release-checked` to
ensure there are no debug assetions or unexpected wrapping math hit.
`no-panic` still needs a single CGU, so must be run with `--profile
release-opt`. Since it is not possible to detect CGU or profilel
configuration from within build scripts, the `ENSURE_NO_PANIC`
environment variable must now always be set.
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