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
* Fix importing the same identifier from two modules
This needed a fix in two locations:
* First the generated descriptor function needed its hash to include the module
that the import came from in order to generate unique descriptor functions.
* Second the generation of the JS shim needed to handle duplicate identifiers in
a more uniform fashion, ensuring that imported names didn't clash.
* Fix importing the same name in two modules
Previously two descriptor functions with duplicate symbols were emitted, and now
only one function is emitted by using a global table to keep track of state
across macro invocations.
