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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lib.rs	Prepare to have targeted error diagnostics (#604)	2018-08-01 17:15:27 -05:00