Rollup merge of #92992 - kornelski:backtraceopt, r=Mark-Simulacrum

Help optimize out backtraces when disabled

The comment in `rust_backtrace_env` says:

>    // If the `backtrace` feature of this crate isn't enabled quickly return
>   // `None` so this can be constant propagated all over the place to turn
>  // optimize away callers.

but this optimization has regressed, because the only caller of this function had an alternative path that unconditionally (and pointlessly) asked for a full backtrace, so the disabled state couldn't propagate.

I've added a getter for the full format that respects the feature flag, so that the caller will now be able to really optimize out the disabled backtrace path. I've also made `rust_backtrace_env` trivially inlineable when backtraces are disabled.

library/std/src/panicking.rs

@@ -263,7 +263,7 @@ fn default_hook(info: &PanicInfo<'_>) {
     // If this is a double panic, make sure that we print a backtrace
     // for this panic. Otherwise only print it if logging is enabled.
     let backtrace_env = if panic_count::get_count() >= 2 {
-        RustBacktrace::Print(crate::backtrace_rs::PrintFmt::Full)
+        backtrace::rust_backtrace_print_full()
     } else {
         backtrace::rust_backtrace_env()
     };

library/std/src/sys_common/backtrace.rs

@@ -150,16 +150,18 @@ pub enum RustBacktrace {
     RuntimeDisabled,
 }
 
-// For now logging is turned off by default, and this function checks to see
-// whether the magical environment variable is present to see if it's turned on.
-pub fn rust_backtrace_env() -> RustBacktrace {
 // If the `backtrace` feature of this crate isn't enabled quickly return
-    // `None` so this can be constant propagated all over the place to turn
+// `Disabled` so this can be constant propagated all over the place to
 // optimize away callers.
-    if !cfg!(feature = "backtrace") {
+#[cfg(not(feature = "backtrace"))]
-        return RustBacktrace::Disabled;
+pub fn rust_backtrace_env() -> RustBacktrace {
+    RustBacktrace::Disabled
 }
 
+// For now logging is turned off by default, and this function checks to see
+// whether the magical environment variable is present to see if it's turned on.
+#[cfg(feature = "backtrace")]
+pub fn rust_backtrace_env() -> RustBacktrace {
     // Setting environment variables for Fuchsia components isn't a standard
     // or easily supported workflow. For now, always display backtraces.
     if cfg!(target_os = "fuchsia") {
@@ -189,6 +191,15 @@ pub fn rust_backtrace_env() -> RustBacktrace {
     format
 }
 
+/// Setting for printing the full backtrace, unless backtraces are completely disabled
+pub(crate) fn rust_backtrace_print_full() -> RustBacktrace {
+    if cfg!(feature = "backtrace") {
+        RustBacktrace::Print(PrintFmt::Full)
+    } else {
+        RustBacktrace::Disabled
+    }
+}
+
 /// Prints the filename of the backtrace frame.
 ///
 /// See also `output`.