Make rustc use `RUST_BACKTRACE=full` by default

Compiler panics should be rare - when they do occur, we want the report
filed by the user to contain as much information as possible. This is
especially important when the panic is due to an incremental compilation
bug, since we may not have enough information to reproduce it.

This PR sets `RUST_BACKTRACE=full` inside `rustc` if the user has not
explicitly set `RUST_BACKTRACE`. This is more verbose than
`RUST_BACKTRACE=1`, but this may make it easier to debug incremental
compilation issues. Users who find this too verbose can still manually
set `RUST_BACKTRACE` before invoking the compiler.

This only affects `rustc` (and any tool using `rustc_driver::install_ice_hook`).
It does *not* affect any user crates or the standard library -
backtraces will continue to be off by default in any application
*compiled* by rustc.

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