Make UnwindTraceFn take `&UnwindContext` only

src/abi.rs

@@ -78,7 +78,7 @@ pub struct UnwindException {
 }
 
 pub type UnwindTraceFn =
-    extern "C" fn(ctx: &mut UnwindContext<'_>, arg: *mut c_void) -> UnwindReasonCode;
+    extern "C" fn(ctx: &UnwindContext<'_>, arg: *mut c_void) -> UnwindReasonCode;
 
 #[cfg(not(feature = "unwinder"))]
 #[repr(C)]

src/panic_handler.rs

@@ -60,7 +60,7 @@ fn stack_trace() {
         counter: usize,
     }
     extern "C" fn callback(
-        unwind_ctx: &mut UnwindContext<'_>,
+        unwind_ctx: &UnwindContext<'_>,
         arg: *mut c_void,
     ) -> UnwindReasonCode {
         let data = unsafe { &mut *(arg as *mut CallbackData) };

src/unwinder/mod.rs

@@ -390,7 +390,7 @@ pub extern "C-unwind" fn _Unwind_Backtrace(
             let frame = try1!(Frame::from_context(&ctx, signal));
             if !skipping {
                 let code = trace(
-                    &mut UnwindContext {
+                    &UnwindContext {
                         frame: frame.as_ref(),
                         ctx: &mut ctx,
                         signal,