Don't panic in serde_test

[Mingun]

Panics lead to reporting errors in tests inside of serde_test internals, returning errors moves the report location to the corresponding assert_tokens expression

[oli-obk]

How do test failures look now compared to before? It could be less obvious that the error is a test failure and not a failure from serde. If the errors are indeed misleading, we could also use #[track_caller] liberally instead

[Mingun]

Take the first commit of #2437. Without this change the output is:

failures:

---- regression::issue2432::deny_unknown_fields::tuple stdout ----
thread 'regression::issue2432::deny_unknown_fields::tuple' panicked at 'expected Token::F32(345.234) but deserialization wants Token::TupleEnd', D:\Projects\serde\serde_test\src\de.rs:97:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- regression::issue2432::deny_unknown_fields::seq stdout ----
thread 'regression::issue2432::deny_unknown_fields::seq' panicked at 'expected Token::F32(345.234) but deserialization wants Token::SeqEnd', D:\Projects\serde\serde_test\src\de.rs:97:9

---- regression::issue2432::deny_unknown_fields::tuple_struct stdout ----
thread 'regression::issue2432::deny_unknown_fields::tuple_struct' panicked at 'expected Token::F32(345.234) but deserialization wants Token::TupleStructEnd', D:\Projects\serde\serde_test\src\de.rs:97:9

---- regression::issue2432::no_deny_unknown_fields::seq stdout ----
thread 'regression::issue2432::no_deny_unknown_fields::seq' panicked at 'expected Token::F32(345.234) but deserialization wants Token::SeqEnd', D:\Projects\serde\serde_test\src\de.rs:97:9

---- regression::issue2432::no_deny_unknown_fields::tuple stdout ----
thread 'regression::issue2432::no_deny_unknown_fields::tuple' panicked at 'expected Token::F32(345.234) but deserialization wants Token::TupleEnd', D:\Projects\serde\serde_test\src\de.rs:97:9

---- regression::issue2432::no_deny_unknown_fields::tuple_struct stdout ----
thread 'regression::issue2432::no_deny_unknown_fields::tuple_struct' panicked at 'expected Token::F32(345.234) but deserialization wants Token::TupleStructEnd', D:\Projects\serde\serde_test\src\de.rs:97:9


failures:
    regression::issue2432::deny_unknown_fields::seq
    regression::issue2432::deny_unknown_fields::tuple
    regression::issue2432::deny_unknown_fields::tuple_struct
    regression::issue2432::no_deny_unknown_fields::seq
    regression::issue2432::no_deny_unknown_fields::tuple
    regression::issue2432::no_deny_unknown_fields::tuple_struct

test result: FAILED. 4 passed; 6 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.01s

error: test failed, to rerun pass `--test regression`
PS D:\Projects\serde\test_suite>

Not much helpful. All tests points here:

serde/serde_test/src/de.rs

Lines 91 to 99 in f583401

	{
	let value = visitor.visit_seq(DeserializerSeqVisitor {
	de: self,
	len: len,
	end: end,
	})?;
	assert_next_token!(self, end);
	Ok(value)
	}

With this change the output is:

failures:

---- regression::issue2432::deny_unknown_fields::seq stdout ----
thread 'regression::issue2432::deny_unknown_fields::seq' panicked at 'assertion failed: `(left == right)`
  left: `Error { msg: "expected Token::F32(345.234) but deserialization wants Token::SeqEnd" }`,
 right: `"invalid length 2, expected struct Struct with 2 elements"`', test_suite\tests\regression\issue2432.rs:315:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- regression::issue2432::deny_unknown_fields::tuple stdout ----
thread 'regression::issue2432::deny_unknown_fields::tuple' panicked at 'assertion failed: `(left == right)`
  left: `Error { msg: "expected Token::F32(345.234) but deserialization wants Token::TupleEnd" }`,
 right: `"invalid length 2, expected struct Struct with 2 elements"`', test_suite\tests\regression\issue2432.rs:346:9

---- regression::issue2432::deny_unknown_fields::tuple_struct stdout ----
thread 'regression::issue2432::deny_unknown_fields::tuple_struct' panicked at 'assertion failed: `(left == right)`
  left: `Error { msg: "expected Token::F32(345.234) but deserialization wants Token::TupleStructEnd" }`,
 right: `"invalid length 2, expected struct Struct with 2 elements"`', test_suite\tests\regression\issue2432.rs:377:9

---- regression::issue2432::no_deny_unknown_fields::seq stdout ----
thread 'regression::issue2432::no_deny_unknown_fields::seq' panicked at 'tokens failed to deserialize: expected Token::F32(345.234) but deserialization wants Token::SeqEnd', test_suite\tests\regression\issue2432.rs:132:9

---- regression::issue2432::no_deny_unknown_fields::tuple stdout ----
thread 'regression::issue2432::no_deny_unknown_fields::tuple' panicked at 'tokens failed to deserialize: expected Token::F32(345.234) but deserialization wants Token::TupleEnd', test_suite\tests\regression\issue2432.rs:164:9

---- regression::issue2432::no_deny_unknown_fields::tuple_struct stdout ----
thread 'regression::issue2432::no_deny_unknown_fields::tuple_struct' panicked at 'tokens failed to deserialize: expected Token::F32(345.234) but deserialization wants Token::TupleStructEnd', test_suite\tests\regression\issue2432.rs:196:9


failures:
    regression::issue2432::deny_unknown_fields::seq
    regression::issue2432::deny_unknown_fields::tuple
    regression::issue2432::deny_unknown_fields::tuple_struct
    regression::issue2432::no_deny_unknown_fields::seq
    regression::issue2432::no_deny_unknown_fields::tuple
    regression::issue2432::no_deny_unknown_fields::tuple_struct

test result: FAILED. 4 passed; 6 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

error: test failed, to rerun pass `--test regression`
PS D:\Projects\serde\test_suite>

Now locations points to the failed test.

[oli-obk]

Thanks! While we could have gotten the panic locations fixed by sprinkling track_caller everywhere, the fact that it is a very localized change and gives us the assert_eq message, is very useful imo.

[dtolnay]

Thanks!