Add .cargo/config.toml for fuzz cfg flags

Set fuzzing, secp256k1_fuzz, and hashes_fuzz cfg flags in
fuzz/.cargo/config.toml so they are automatically applied to plain
cargo commands (cargo test, cargo run) run from the fuzz directory.
Remove the now-redundant RUSTFLAGS from the README crash reproduction
examples, the CI cargo test step, and generate_fuzz_coverage.sh.

The honggfuzz and cargo-fuzz docs are unchanged because those tools
build their own RUSTFLAGS env var (which overrides config.toml) and
require the flags to be exported separately.

AI tools were used in preparing this commit.

Author: joostjager

.github/workflows/build.yml

@@ -241,7 +241,7 @@ jobs:
       - name: Sanity check fuzz targets on Rust ${{ env.TOOLCHAIN }}
         run: |
           cd fuzz
-          RUSTFLAGS="--cfg=fuzzing --cfg=secp256k1_fuzz --cfg=hashes_fuzz" cargo test --verbose --color always --lib --bins -j8
+          cargo test --verbose --color always --lib --bins -j8
           cargo clean
       - name: Run fuzzers
         run: cd fuzz && ./ci-fuzz.sh && cd ..

contrib/generate_fuzz_coverage.sh

@@ -55,8 +55,6 @@ fi
 # Create output directory if it doesn't exist
 mkdir -p "$OUTPUT_DIR"
 
-export RUSTFLAGS="--cfg=fuzzing --cfg=secp256k1_fuzz --cfg=hashes_fuzz"
-
 # dont run this command when running in CI
 if [ "$OUTPUT_CODECOV_JSON" = "0" ]; then
     cargo llvm-cov --html --ignore-filename-regex "fuzz/" --output-dir "$OUTPUT_DIR"

fuzz/.cargo/config.toml

@@ -0,0 +1,2 @@
+[build]
+rustflags = ["--cfg=fuzzing", "--cfg=secp256k1_fuzz", "--cfg=hashes_fuzz"]

fuzz/README.md

@@ -134,7 +134,6 @@ mkdir -p ./test_cases/$TARGET
 echo $HEX | xxd -r -p > ./test_cases/$TARGET/any_filename_works
 
 export RUST_BACKTRACE=1
-export RUSTFLAGS="--cfg=fuzzing --cfg=secp256k1_fuzz --cfg=hashes_fuzz"
 cargo test
 ```
 
@@ -152,7 +151,7 @@ Alternatively, you can use the `stdin_fuzz` feature to pipe the crash input dire
 creating test case files on disk:
 
 ```shell
-echo -ne '\x2d\x31\x36\x38\x37\x34\x09\x01...' | RUSTFLAGS="--cfg=fuzzing --cfg=secp256k1_fuzz --cfg=hashes_fuzz" cargo run --features stdin_fuzz --bin full_stack_target
+echo -ne '\x2d\x31\x36\x38\x37\x34\x09\x01...' | cargo run --features stdin_fuzz --bin full_stack_target
 ```
 
 Panics will abort the process directly (the crate uses `panic = "abort"`), resulting in a