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wasm-bindgen/crates/macro/README.md
Alex Crichton c4dcaee1b9
Prepare to have targeted error diagnostics (#604) 
This commit starts to add infrastructure for targeted diagnostics in the
`#[wasm_bindgen]` attribute, intended eventually at providing much better errors
as they'll be pointing to exactly the code in question rather than always to a
`#[wasm_bindgen]` attribute.

The general changes are are:

* A new `Diagnostic` error type is added to the backend. A `Diagnostic` is
  created with a textual error or with a span, and it can also be created from a
  list of diagnostics. A `Diagnostic` implements `ToTokens` which emits a bunch
  of invocations of `compile_error!` that will cause rustc to later generate
  errors.

* Fallible implementations of `ToTokens` have switched to using a new trait,
  `TryToTokens`, which returns a `Result` to use `?` with.

* The `MacroParse` trait has changed to returning a `Result` to propagate errors
  upwards.

* A new `ui-tests` crate was added which uses `compiletest_rs` to add UI tests.
  These UI tests will verify that our output improves over time and does not
  regress. This test suite is added to CI as a new builder as well.

* No `Diagnostic` instances are created just yet, everything continues to panic
  and return `Ok`, with the one exception of the top-level invocations of
  `syn::parse` which now create a `Diagnostic` and pass it along.

This commit does not immediately improve diagnostics but the intention is that
it is laying the groundwork for improving diagnostics over time. It should
ideally be much easier to contribute improved diagnostics after this commit!

cc #601
2018-08-01 17:15:27 -05:00

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wasm-bindgen-macro

Implementation of the #[wasm_bindgen] attribute. See the wasm-bindgen documentation for more information about what this macro does.

Testing

Testing of this macro is done through "ui tests" in the ui-tests subdirectory of this crate. Each Rust file in this folder is compiled with the wasm_bindgen crate, and the *.stderr file sitting next to it is the asserted output of the compiler. If the output matches, the test passes, and if the output doesn't match the test fails. Note that it is also considered a failure if a test actually compiles successfully.

To add a test:

  • Create ui-tests/my-awesome-test.rs
  • Write an invalid #[wasm_bindgen] invocation, testing the error you're generating
  • Execute cargo test -p ui-tests, the test will fail
  • From within the ui-tests folder, execute ./update-all-references.sh. This should create a my-awesome-test.stderr file.
  • Inspect my-awesome-test.stderr to make sure it looks ok
  • Rerun cargo test -p ui-tests and your tests should pass!

Testing here is a work in progress, see #601 for more information.