Make rustc use RUST_BACKTRACE=full by default

compiler/rustc_driver/src/lib.rs

@@ -1240,6 +1240,15 @@ pub fn report_ice(info: &panic::PanicInfo<'_>, bug_report_url: &str) {
 ///
 /// A custom rustc driver can skip calling this to set up a custom ICE hook.
 pub fn install_ice_hook() {
+    // If the user has not explicitly overriden "RUST_BACKTRACE", then produce
+    // full backtraces. When a compiler ICE happens, we want to gather
+    // as much information as possible to present in the issue opened
+    // by the user. Compiler developers and other rustc users can
+    // opt in to less-verbose backtraces by manually setting "RUST_BACKTRACE"
+    // (e.g. `RUST_BACKTRACE=1`)
+    if std::env::var("RUST_BACKTRACE").is_err() {
+        std::env::set_var("RUST_BACKTRACE", "full");
+    }
     SyncLazy::force(&DEFAULT_HOOK);
 }
 

src/test/ui/panics/default-backtrace-ice.rs

@@ -0,0 +1,9 @@
+// unset-rustc-env:RUST_BACKTRACE
+// compile-flags:-Z treat-err-as-bug=1
+// error-pattern:stack backtrace:
+// failure-status:101
+// normalize-stderr-test "note: .*" -> ""
+// normalize-stderr-test "thread 'rustc' .*" -> ""
+// normalize-stderr-test "  .*\n" -> ""
+
+fn main() { missing_ident; }

src/test/ui/panics/default-backtrace-ice.stderr

@@ -0,0 +1,18 @@
+error[E0425]: cannot find value `missing_ident` in this scope
+LL | fn main() { missing_ident; }
+
+
+stack backtrace:
+
+error: internal compiler error: unexpected panic
+
+
+
+
+
+
+
+
+
+query stack during panic:
+end of query stack