go/analysis/passes/errorsas: clarify message

Make it clear that the second argument must be a non-nil pointer.

The new message text matches the phrasing used in the errors.As doc.

Updates golang/go#37625.

Change-Id: I69dc2e34a5f3a5573030ba0f63f20e0821be1816
Reviewed-on: https://go-review.googlesource.com/c/tools/+/221877
Reviewed-by: Dmitri Shuralyov <[email protected]>

Author: jba

go/analysis/passes/errorsas/errorsas.go

@@ -51,7 +51,7 @@ func run(pass *analysis.Pass) (interface{}, error) {
 			return // not enough arguments, e.g. called with return values of another function
 		}
 		if fn.FullName() == "errors.As" && !pointerToInterfaceOrError(pass, call.Args[1]) {
-			pass.ReportRangef(call, "second argument to errors.As must be a pointer to an interface or a type implementing error")
+			pass.ReportRangef(call, "second argument to errors.As must be a non-nil pointer to either a type that implements error, or to any interface type")
 		}
 	})
 	return nil, nil

go/analysis/passes/errorsas/testdata/src/a/a.go

@@ -34,10 +34,10 @@ func _() {
 	errors.As(nil, perr()) // *error, via a call
 	errors.As(nil, ei)     //  empty interface
 
-	errors.As(nil, nil) // want `second argument to errors.As must be a pointer to an interface or a type implementing error`
-	errors.As(nil, e)   // want `second argument to errors.As must be a pointer to an interface or a type implementing error`
-	errors.As(nil, m)   // want `second argument to errors.As must be a pointer to an interface or a type implementing error`
-	errors.As(nil, f)   // want `second argument to errors.As must be a pointer to an interface or a type implementing error`
-	errors.As(nil, &i)  // want `second argument to errors.As must be a pointer to an interface or a type implementing error`
+	errors.As(nil, nil) // want `second argument to errors.As must be a non-nil pointer to either a type that implements error, or to any interface type`
+	errors.As(nil, e)   // want `second argument to errors.As must be a non-nil pointer to either a type that implements error, or to any interface type`
+	errors.As(nil, m)   // want `second argument to errors.As must be a non-nil pointer to either a type that implements error, or to any interface type`
+	errors.As(nil, f)   // want `second argument to errors.As must be a non-nil pointer to either a type that implements error, or to any interface type`
+	errors.As(nil, &i)  // want `second argument to errors.As must be a non-nil pointer to either a type that implements error, or to any interface type`
 	errors.As(two())
 }