Add fuzz test coverage Makefile target (#75)

.gitignore

@@ -1,2 +1,3 @@
 /target
 /Cargo.lock
+lcov.info

Makefile

@@ -10,6 +10,25 @@ test:
 fuzz:
 	cargo +nightly fuzz run fuzz_roundtrip -j 4
 
+# Generate a lcov.info file for tools like VSCode's Coverage Gutters extension,
+# and output basic coverage information on the command line.
+RUST_LLVM_COV=$(shell find $(shell rustc --print sysroot) -name llvm-cov)
+RUST_TARGET_TRIPLE=$(shell rustc -vV | sed -n 's|host: ||p')
+fuzz-coverage:
+	rustup component add --toolchain nightly llvm-tools-preview
+	cargo +nightly fuzz coverage fuzz_roundtrip
+	@$(RUST_LLVM_COV) export \
+		-instr-profile=fuzz/coverage/fuzz_roundtrip/coverage.profdata \
+		-object target/$(RUST_TARGET_TRIPLE)/coverage/$(RUST_TARGET_TRIPLE)/release/fuzz_roundtrip \
+		--ignore-filename-regex "rustc" \
+		-format=lcov \
+		> lcov.info
+	@$(RUST_LLVM_COV) report \
+		-instr-profile=fuzz/coverage/fuzz_roundtrip/coverage.profdata \
+		-object target/$(RUST_TARGET_TRIPLE)/coverage/$(RUST_TARGET_TRIPLE)/release/fuzz_roundtrip \
+		--ignore-filename-regex ".cargo/registry"
+	@echo "View the coverage in lcov.info in VSCode using the Coverage Gutters extension."
+
 build:
 	cargo build
 	cargo build --features serde