Allow ? to be used in script files without main()

This is done by specifying the Result<(), Box<dyn std::error::Error+Sync+Send>>
return type.

This aligns with what https://github.com/stevedonovan/runner is doing.

Author: fornwall

src/manifest.rs

@@ -53,7 +53,7 @@ pub fn split_input(
             let source = if source.lines().any(|line| line.starts_with("fn main()")) {
                 source.to_string()
             } else {
-                format!("fn main() {{\n{}\n}}", source)
+                format!("fn main() -> Result<(), Box<dyn std::error::Error+Sync+Send>> {{\n    {{\n    {}    }}\n    Ok(())\n}}", source)
             };
             (manifest, source, templates::get_template("file")?, false)
         }

tests/data/script-without-main-question-mark.rs

@@ -0,0 +1,3 @@
+use std::fs::File;
+
+File::open("__rust-script-this-file-does-not-exist.txt")?;

tests/tests/script.rs

@@ -144,3 +144,11 @@ fn script_with_same_name_as_dependency() {
     )
     .unwrap()
 }
+
+
+#[test]
+fn script_without_main_question_mark() {
+    let out = rust_script!("tests/data/script-without-main-question-mark.rs").unwrap();
+    assert_eq!(out.stderr,
+        "Error: Os { code: 2, kind: NotFound, message: \"No such file or directory\" }\n");
+}