Default to short backtraces for dev builds of rustc itself

A dev build almost certainly means that whoever's built the compiler
has the opportunity to rerun it to collect a more complete trace. So
we don't need to default to a complete trace; we should hide irrelevant
details by default.

compiler/rustc_driver_impl/src/lib.rs

@@ -1388,7 +1388,11 @@ pub fn install_ice_hook(
     // opt in to less-verbose backtraces by manually setting "RUST_BACKTRACE"
     // (e.g. `RUST_BACKTRACE=1`)
     if env::var_os("RUST_BACKTRACE").is_none() {
-        panic::set_backtrace_style(panic::BacktraceStyle::Full);
+        if env!("CFG_RELEASE_CHANNEL") == "dev" {
+            panic::set_backtrace_style(panic::BacktraceStyle::Short);
+        } else {
+            panic::set_backtrace_style(panic::BacktraceStyle::Full);
+        }
     }
 
     let using_internal_features = Arc::new(std::sync::atomic::AtomicBool::default());