Add TraceEngine option (hidden)

Added the future TraceEngine option with current options Auto, Llvm and
Ptrace. Check it before doing the coverage instrumentation compiler
flags and warn if the user selects Llvm but they don't have a current
enough compiler

src/cargo.rs

@@ -483,7 +483,9 @@ pub fn rust_flags(config: &Config) -> String {
     if config.release {
         value.push_str("-C debug-assertions=off ");
     }
-    if config.engine == TraceEngine::Llvm && supports_llvm_coverage() {
+    if (config.engine == TraceEngine::Auto || config.engine == TraceEngine::Llvm)
+        && supports_llvm_coverage()
+    {
         value.push_str("-Z instrument-coverage ");
     } else if config.engine == TraceEngine::Llvm {
         error!(

src/config/mod.rs

@@ -229,7 +229,7 @@ impl Default for Config {
             avoid_cfg_tarpaulin: false,
             jobs: None,
             color: Color::Auto,
-            engine: TraceEngine::Ptrace,
+            engine: TraceEngine::Auto,
         }
     }
 }
@@ -303,7 +303,7 @@ impl<'a> From<&'a ArgMatches<'a>> for ConfigWrapper {
             profile: get_profile(args),
             metadata: RefCell::new(None),
             avoid_cfg_tarpaulin: args.is_present("avoid-cfg-tarpaulin"),
-            engine: TraceEngine::Ptrace,
+            engine: TraceEngine::Auto,
         };
         if args.is_present("ignore-config") {
             Self(vec![args_config])

src/config/types.rs

@@ -15,6 +15,7 @@ arg_enum! {
 arg_enum! {
     #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash, Ord, PartialOrd, Deserialize, Serialize)]
     pub enum TraceEngine {
+        Auto,
         Ptrace,
         Llvm,
     }