Use CARGO_PRIMARY_PACKAGE to skip dependencies

See also rust-lang/cargo#8758 and rust-lang/rust-clippy#6188

Author: fpoli

prusti/src/driver.rs

@@ -125,8 +125,8 @@ fn main() {
 
     // If the environment asks us to actually be rustc, then do that.
     // If cargo is compiling a dependency, then be rustc.
-    let prusti_be_rustc = config::be_rustc()
-        || arg_value(&rustc_args, "--cap-lints", |val| val == "allow").is_some();
+    let in_primary_package = env::var("CARGO_PRIMARY_PACKAGE").is_ok();
+    let prusti_be_rustc = config::be_rustc() || !in_primary_package;
     if prusti_be_rustc {
         rustc_driver::main();
     }