toolchain: Add config settings for debug info and opt level

[ted-logan]

Add two build flags:

• //rust/settings:opt_level
• //rust/settings:debug_info

Each of these flags takes a string list that allows the rustc
optimization level (--codegen=opt-level=x) and debug info
(--codegen=debuginfo=x) to be changed.

If either flag has one value, it will be used for all three compilation
modes (dbg, fastbuild, opt). If the flag has three values (separated by
commas) each value will be used in sequence for dbg, fastbuild, and opt.

Note that if one is changing the opt-level, one should consider changing
debug-assertions (via a separate extra rustc flag). By default, in
rustc, debug-assertions are enabled only if opt-level is 0.

Add tests to verify that the expected codegen flags are provided on the
rustc command line.

[scentini]

Could this PR be replaced by a
.bazelrc

build:opt -Copt-level=X
build:opt -Cdebuginfo=Y

build:dbg -Copt-level=X
build:dbg -Copt-level=Y

...

?

This PR seems to tie in to wanting easier way to customize flags, which was requested in #515. I would rather look at a more general mechanism than individual flags.

[UebelAndre]

Could this PR be replaced by a .bazelrc

build:opt -Copt-level=X
build:opt -Cdebuginfo=Y

build:dbg -Copt-level=X
build:dbg -Copt-level=Y

...

?

This PR seems to tie in to wanting easier way to customize flags, which was requested in #515. I would rather look at a more general mechanism than individual flags.

Is build:dbg and build:opt automatically picked up with the --compilation_mode flag? Or follow transitions? It'd be pretty sweet if it did!

[scentini]

Not really; but the other way around works, eg one could have a --config=opt and put build:opt -c opt. As for following transitions - there is no difference whether --@rules_rust//rust:extra_rustc_flags=-Copt-level=X is set on the command line or via the .bazelrc with build:opt --@rules_rust//rust:extra_rustc_flags=-Copt-level=X.

[ted-logan]

I can use --@rules_rust//:extra_rustc_flags= to add flags to the rustc command line, but they get appended at the end of argv and don't replace the --codegen=opt-level=x added by rust/private/rustc.bzl. I don't see clear documentation from rustc about how it handles multiple instances of the same argument with different values.

This does seem to work in practice, so I'll probably use it instead, though it's still undocumented so it feels like undefined behavior in the complier.