Ability to gather print statements (#179)

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

src/builtins/debugging.rs

@@ -17,10 +17,12 @@ pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
     m.insert("print", (print, MAX_ARGS));
 }
 
-// Symbol analyzer must ensure that vars used by print are defined before
-// the print statement. Scheduler must ensure the above constraint.
-// Additionally interpreter must allow undefined inputs to print.
-fn print(span: &Span, _params: &[Ref<Expr>], args: &[Value], _strict: bool) -> Result<Value> {
+pub fn print_to_string(
+    span: &Span,
+    _params: &[Ref<Expr>],
+    args: &[Value],
+    _strict: bool,
+) -> Result<String> {
     if args.len() > MAX_ARGS as usize {
         bail!(span.error("print supports up to 100 arguments"));
     }
@@ -34,6 +36,15 @@ fn print(span: &Span, _params: &[Ref<Expr>], args: &[Value], _strict: bool) -> R
         };
     }
 
+    Ok(msg)
+}
+
+// Symbol analyzer must ensure that vars used by print are defined before
+// the print statement. Scheduler must ensure the above constraint.
+// Additionally interpreter must allow undefined inputs to print.
+fn print(span: &Span, params: &[Ref<Expr>], args: &[Value], strict: bool) -> Result<Value> {
+    let msg = print_to_string(span, params, args, strict)?;
+
     if !msg.is_empty() {
         eprintln!("{}", &msg[1..]);
     }