notes for turning on ASan and UBSan in the Rust test harness

[oconnor663]

(Note that we already use -fsanitize=address,undefined in the Makefile tests for the C implementation.)

Reddit user /u/gzk11059 gave some very helpful pointers. Based on that thread, here's a first start on running sanitizers against the C implementation.

cd c/blake3_c_rust_bindings
CC=clang CFLAGS="-fsanitize=address,undefined -fno-sanitize-recover=all" RUSTFLAGS=-Zsanitizer=address cargo +nightly test

I can confirm that if I add some undefined behavior like this:

int signed_overflow = 2147483647;
signed_overflow++;

and then run the command above, I get a failure like this:

test test::test_compare_reference_impl ... ../blake3.c:11:18: runtime error: signed integer overflow: 2147483647 + 1 cannot be represented in type 'int'
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../blake3.c:11:18 in