use noop backends on Miri (#360)

Author: RalfJung

src/backtrace/mod.rs

@@ -121,7 +121,11 @@ impl fmt::Debug for Frame {
 }
 
 cfg_if::cfg_if! {
-    if #[cfg(
+    if #[cfg(miri)] {
+        mod noop;
+        use self::noop::trace as trace_imp;
+        pub(crate) use self::noop::Frame as FrameImp;
+    } else if #[cfg(
         any(
             all(
                 unix,

src/symbolize/mod.rs

@@ -456,7 +456,13 @@ pub fn clear_symbol_cache() {
 }
 
 cfg_if::cfg_if! {
-    if #[cfg(all(windows, target_env = "msvc", not(target_vendor = "uwp")))] {
+    if #[cfg(miri)] {
+        mod noop;
+        use self::noop::resolve as resolve_imp;
+        use self::noop::Symbol as SymbolImp;
+        #[allow(unused)]
+        unsafe fn clear_symbol_cache_imp() {}
+    } else if #[cfg(all(windows, target_env = "msvc", not(target_vendor = "uwp")))] {
         mod dbghelp;
         use self::dbghelp::resolve as resolve_imp;
         use self::dbghelp::Symbol as SymbolImp;