Auto merge of #52552 - eddyb:proc-macro-prep, r=alexcrichton

Prepare proc_macro for decoupling it from the rest of the compiler.

This is #49219 up to the point where the bridge is introduced. Aside from moving some code around, the largest change is the rewrite of `proc_macro::quote` to be simpler and do less introspection.

I'd like to also extend `quote!` with `${stmt;...;expr}` instead of just `$variable` (and maybe even `$(... $iter ...)*`), which seems pretty straight-forward now, but I don't know if/when I should.

r? @alexcrichton or @dtolnay cc @jseyfried @petrochenkov

Author: bors

Diff for: 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")]
@@ -97,38 +98,21 @@ impl Diagnostic {
     /// Emit the diagnostic.
     #[unstable(feature = "proc_macro_diagnostic", issue = "38356")]
     pub fn emit(self) {
-        ::__internal::with_sess(move |sess, _| {
-            let handler = &sess.span_diagnostic;
-            let level = __internal::level_to_internal_level(self.level);
-            let mut diag = rustc::DiagnosticBuilder::new(handler, level, &*self.message);
+        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);
-            }
+        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);
-            }
+        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);
+        }
 
-            diag.emit();
+        ::__internal::with_sess(move |sess, _| {
+            errors::DiagnosticBuilder::new_diagnostic(&sess.span_diagnostic, diag).emit();
         });
     }
 }
-
-#[unstable(feature = "proc_macro_internals", issue = "27812")]
-#[doc(hidden)]
-pub mod __internal {
-    use super::{Level, rustc};
-
-    pub fn level_to_internal_level(level: Level) -> rustc::Level {
-        match level {
-            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")
-        }
-    }
-}