Writing tests for nom parsers with VerboseError and convert_error - rust

Question

I'm trying to write tests for nom parsers that provide better messages when they fail.
I have a macro similar to assert_eq! called assert_parses_to! that I want to provide a user-friendly display string when the parser fails:
#[macro_export]
macro_rules! assert_parses_to {
($parser:ident, $input:expr, $expectation:expr) => {
match (&$parser($input), &$expectation) {
(Err(candidate_error), expectation_val) => {
panic!(
"Failed to parse the candidate\n\
{}",
convert_error($input, *candidate_error)
);
}
(Ok(candidate_val), expectation_val) => {
if !(*candidate_val == *expectation_val) {
panic!(
"Failed to parse to expected value\n\
Got: {:?}\n\
Expected: {:?}",
candidate_val, expectation_val
)
}
}
}
};
}
However, when trying to compile, I get this error:
error[E0308]: mismatched types
--> src/parser/tests.rs:27:43
|
27 | convert_error($input, *candidate_error)
| ------------- ^^^^^^^^^^^^^^^^ expected struct `VerboseError`, found enum `nom::Err`
| |
| arguments to this function are incorrect
|
::: src/parser/ansi_2016/lexical.rs:237:9
|
237 | / assert_parses_to!(
238 | | regular_identifier,
239 | | "identifier ",
240 | | (
... |
246 | | )
247 | | )
| |_________- in this macro invocation
|
= note: expected struct `VerboseError<&str>`
found enum `nom::Err<VerboseError<&str>>`
I'm assuming that I need to modify the match statement to extract the VerboseError from the error, but I haven't figured out how.
I'm new to rust and nom - any help appreciated!

Answer 1

Resolved by matching on the inner value of the Err enum:
#[macro_export]
macro_rules! assert_parses_to {
($parser:ident, $input:expr, $expectation:expr) => {
match ($parser($input), $expectation) {
// Deeper match here
(Err(::nom::Err::Error(e) | ::nom::Err::Failure(e)), _) => {
panic!("{}", ::nom::error::convert_error($input, e));
}
(Err(e), _) => {
panic!("Failed to parse {:?}", e)
}
(Ok(candidate_val), expected_val) => {
if !(candidate_val == expected_val) {
panic!(
"Failed to parse to expected value\n\
Got: {:?}\n\
Expected: {:?}",
candidate_val, expected_val
)
}
}
}
};
}