avoid an unreachable fallback

Author: RalfJung

src/librustc_mir/interpret/intrinsics/caller_location.rs

@@ -12,21 +12,21 @@ use crate::interpret::{
 
 impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
     /// Walks up the callstack from the intrinsic's callsite, searching for the first callsite in a
-    /// frame which is not `#[track_caller]`. If the first frame found lacks `#[track_caller]`, then
-    /// `None` is returned and the callsite of the function invocation itself should be used.
+    /// frame which is not `#[track_caller]`.
     crate fn find_closest_untracked_caller_location(&self) -> Span {
         self.stack
             .iter()
             .rev()
-            // Skip `#[track_caller]` frames.
-            .skip_while(|frame| frame.instance.def.requires_caller_location(*self.tcx))
-            // Find next frame with source info.
-            .find_map(|frame| frame.current_source_info())
-            .map(|si| si.span)
-            // Fallback to current frame. That one has to have source_info as only
-            // currently unwinding frames without cleanup do *not* have it -- and those
-            // frames do not call this intrinsic.
-            .unwrap_or_else(|| self.frame().current_source_info().unwrap().span)
+            // Find first non-`#[track_caller]` frame.
+            .find(|frame| !frame.instance.def.requires_caller_location(*self.tcx))
+            // Assert that there is always such a frame.
+            .unwrap()
+            .current_source_info()
+            // Assert that the frame we look at is actually executing code currently
+            // (`current_source_info` is None when we are unwinding and the frame does
+            // not require cleanup).
+            .unwrap()
+            .span
     }
 
     /// Allocate a `const core::panic::Location` with the provided filename and line/column numbers.