proc_macro: don't use DiagnosticBuilder for building up Diagnostics.

src/libproc_macro/diagnostic.rs

@@ -10,7 +10,8 @@
 
 use Span;
 
-use rustc_errors as rustc;
+use rustc_errors as errors;
+use syntax_pos::MultiSpan;
 
 /// An enum representing a diagnostic level.
 #[unstable(feature = "proc_macro_diagnostic", issue = "38356")]
@@ -97,37 +98,32 @@ impl Diagnostic {
     /// Emit the diagnostic.
     #[unstable(feature = "proc_macro_diagnostic", issue = "38356")]
     pub fn emit(self) {
+        let level = self.level.to_internal();
+        let mut diag = errors::Diagnostic::new(level, &*self.message);
+
+        if let Some(span) = self.span {
+            diag.set_span(span.0);
+        }
+
+        for child in self.children {
+            let span = child.span.map_or(MultiSpan::new(), |s| s.0.into());
+            let level = child.level.to_internal();
+            diag.sub(level, &*child.message, span, None);
+        }
+
         ::__internal::with_sess(move |sess, _| {
-            let handler = &sess.span_diagnostic;
+            errors::DiagnosticBuilder::new_diagnostic(&sess.span_diagnostic, diag).emit();
-            let level = __internal::level_to_internal_level(self.level);
-            let mut diag = rustc::DiagnosticBuilder::new(handler, level, &*self.message);
-
-            if let Some(span) = self.span {
-                diag.set_span(span.0);
-            }
-
-            for child in self.children {
-                let span = child.span.map(|s| s.0);
-                let level = __internal::level_to_internal_level(child.level);
-                diag.sub(level, &*child.message, span);
-            }
-
-            diag.emit();
         });
     }
 }
 
-#[unstable(feature = "proc_macro_internals", issue = "27812")]
+impl Level {
-#[doc(hidden)]
+    fn to_internal(self) -> errors::Level {
-pub mod __internal {
+        match self {
-    use super::{Level, rustc};
+            Level::Error => errors::Level::Error,
-
+            Level::Warning => errors::Level::Warning,
-    pub fn level_to_internal_level(level: Level) -> rustc::Level {
+            Level::Note => errors::Level::Note,
-        match level {
+            Level::Help => errors::Level::Help,
-            Level::Error => rustc::Level::Error,
-            Level::Warning => rustc::Level::Warning,
-            Level::Note => rustc::Level::Note,
-            Level::Help => rustc::Level::Help,
             Level::__Nonexhaustive => unreachable!("Level::__Nonexhaustive")
         }
     }

src/librustc_errors/diagnostic.rs

@@ -379,7 +379,7 @@ impl Diagnostic {
 
     /// Convenience function for internal use, clients should use one of the
     /// public methods above.
-    pub(crate) fn sub(&mut self,
+    pub fn sub(&mut self,
            level: Level,
            message: &str,
            span: MultiSpan,