Move benchmark assigned err to global exported variable (#106)

toperr is not used, but the go compiler itself doesn't detect this
because it's within an anonymous function. However, go/types does
detect this as being unused, which causes any static analysis tools
which uses go/types' type checker to fail with the message "toperr
assigned and not used".

The final result of the benchmarked function is instead assigned to
an exported global variable to ensure the compiler cannot now, nor
in the future optimise away the function calls due to no observable
side effects.

It was chosen to assign the final result, after the benchmark loop,
to the global variable, as this best follows the example set in the
CL https://go-review.googlesource.com/#/c/37195/. As opposed to
having each call to f assign to the global. This also appears to
better align with the original author's intention of toperr.

This change had no observable impact on the benchmark.

Related https://github.com/golang/go/issues/3059.
Related https://github.com/golang/go/issues/8560.

Thanks dominikh for additional clarifications.

bench_test.go

@@ -23,8 +23,11 @@ func yesErrors(at, depth int) error {
 	return yesErrors(at+1, depth)
 }
 
+// GlobalE is an exported global to store the result of benchmark results,
+// preventing the compiler from optimising the benchmark functions away.
+var GlobalE error
+
 func BenchmarkErrors(b *testing.B) {
-	var toperr error
 	type run struct {
 		stack int
 		std   bool
@@ -54,7 +57,7 @@ func BenchmarkErrors(b *testing.B) {
 				err = f(0, r.stack)
 			}
 			b.StopTimer()
-			toperr = err
+			GlobalE = err
 		})
 	}
 }