Add callstack_nth_location

Summary:
I wanted this for some testing and it seems fine to generally add it,
even though i have no immediate plans to use it.

Reviewed By: JakobDegen

Differential Revision: D84766605

fbshipit-source-id: 98e341ad0099ec1e5c9134a842f29d0e098e0656

Author: cjhopman

starlark/src/eval/runtime/cheap_call_stack.rs

@@ -196,6 +196,14 @@ impl<'v> CheapCallStack<'v> {
         }
     }
 
+    pub(crate) fn nth_location(&self, n: usize) -> Option<FileSpan> {
+        if n >= self.count {
+            None
+        } else {
+            self.stack[self.count - 1 - n].location()
+        }
+    }
+
     /// `n`-th element from the top of the stack.
     pub(crate) fn top_nth_function(&self, n: usize) -> anyhow::Result<Value<'v>> {
         self.top_nth_function_opt(n)

starlark/src/eval/runtime/evaluator.rs

@@ -452,6 +452,12 @@ impl<'v, 'a, 'e: 'a> Evaluator<'v, 'a, 'e> {
         self.call_stack.top_location()
     }
 
+    /// Obtain the nth location on the call-stack. May be [`None`] if the
+    /// stack is not that deep. n=0 is the top of the stack.
+    pub fn call_stack_nth_location(&self, n: usize) -> Option<FileSpan> {
+        self.call_stack.nth_location(n)
+    }
+
     pub(crate) fn before_stmt_fn(
         &mut self,
         f: &'a dyn for<'v1, 'a2> Fn(FileSpanRef, bool, &mut Evaluator<'v1, 'a2, 'e>),