Improve coverage accounting for runtime paths (#280)

Author: glenn-jocher

Cargo.toml

@@ -48,6 +48,7 @@ targets = ["x86_64-unknown-linux-gnu"]
 [lints.rust]
 unsafe_code = "warn"
 missing_docs = "warn"
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(coverage_nightly)'] }
 
 [lints.clippy]
 all = "warn"