Use compile_error! instead of panicking

[adamcrume]

As a side effect, this changes error messages from:

error: proc-macro derive panicked
   --> test_suite/tests/test_ser.rs:597:10
    |
597 | #[derive(Serialize)]
    |          ^^^^^^^^^
    |
    = help: message: 2 errors:
                # unknown serde field attribute `typo`
                # expected serde with attribute to be a string: `with = "..."`

to

error: 2 errors:
    # unknown serde field attribute `typo`
    # expected serde with attribute to be a string: `with = "..."`
   --> test_suite/tests/test_ser.rs:597:10
    |
597 | #[derive(Serialize)]
    |          ^^^^^^^^^

[dtolnay]

Thanks! That improvement to the error message is exactly what we were looking for.

The CI build is failing because we have some compile-fail tests that check the error messages as they existed before this PR, for example this one. All of those tests will need to be updated. The //~ and //~^^ compile-fail comment syntax is explained in COMPILER_TESTS.md.

Running the compile-fail tests locally is somewhat hairy right now. You will need a nightly compiler and the workflow I would recommend is:

# in serde/test_suite/deps, after any change to serde_derive
cargo clean && cargo update && cargo build

# in serde/test_suite
cargo test --features unstable compile_fail

[dtolnay]

Typically macros try to avoid working with Rust source code as strings -- strings are unfriendly to our text editor or IDE when it comes to brace matching, syntax highlighting, indentation, etc. Serde's derive currently uses the quote crate instead of strings of source code.

Here it's just one line so there may not be an appreciable advantage, but I think it would be better to be consistent with the code generation done in the rest of serde_derive and use quote! here as well.