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

2 files changed, 14 insertions, 1 deletions

diff --git a/src/librustc_errors/diagnostic.rs b/src/librustc_errors/diagnostic.rs
index 0f063542383..9aae188f9ec 100644
--- a/src/librustc_errors/diagnostic.rs
+++ b/src/librustc_errors/diagnostic.rs
@@ -288,7 +288,7 @@ impl Diagnostic {
 
     /// Convenience function for internal use, clients should use one of the
     /// public methods above.
-    fn sub(&mut self,
+    pub(crate) fn sub(&mut self,
            level: Level,
            message: &str,
            span: MultiSpan,
diff --git a/src/librustc_errors/diagnostic_builder.rs b/src/librustc_errors/diagnostic_builder.rs
index 2c8d8b4691f..2cd433bfe3a 100644
--- a/src/librustc_errors/diagnostic_builder.rs
+++ b/src/librustc_errors/diagnostic_builder.rs
@@ -110,6 +110,19 @@ impl<'a> DiagnosticBuilder<'a> {
         // }
     }
 
+    /// Convenience function for internal use, clients should use one of the
+    /// span_* methods instead.
+    pub fn sub<S: Into<MultiSpan>>(
+        &mut self,
+        level: Level,
+        message: &str,
+        span: Option<S>,
+    ) -> &mut Self {
+        let span = span.map(|s| s.into()).unwrap_or(MultiSpan::new());
+        self.diagnostic.sub(level, message, span, None);
+        self
+    }
+
     /// Delay emission of this diagnostic as a bug.
     ///
     /// This can be useful in contexts where an error indicates a bug but