Rollup merge of #44125 - SergioBenitez:master, r=nrc

Initial diagnostic API for proc-macros.

This commit introduces the ability to create and emit `Diagnostic` structures from proc-macros, allowing for proc-macro authors to emit warning, error, note, and help messages just like the compiler does.

The API is somewhat based on the diagnostic API already present in `rustc` with several changes that improve usability. The entry point into the diagnostic API is a new `Diagnostic` type which is primarily created through new `error`, `warning`, `help`, and `note` methods on `Span`. The `Diagnostic` type records the diagnostic level, message, and optional `Span` for the top-level diagnostic and contains a `Vec` of all of the child diagnostics. Child diagnostics can be added through builder methods on `Diagnostic`.

A typical use of the API may look like:

```rust
let token = parse_token();
let val = parse_val();

val.span
    .error(format!("expected A but found {}", val))
    .span_note(token.span, "because of this token")
    .help("consider using a different token")
    .emit();
```

cc @jseyfried @nrc @dtolnay @alexcrichton

0 files changed, 0 insertions, 0 deletions