Add EmitResult associated type to EmissionGuarantee.

This lets different error levels share the same return type from
`emit_*`.

- A lot of inconsistencies in the `DiagCtxt` API are removed.
- `Noted` is removed.
- `FatalAbort` is introduced for fatal errors (abort via `raise`),
  replacing the `EmissionGuarantee` impl for `!`.
- `Bug` is renamed `BugAbort` (to avoid clashing with `Level::Bug` and
  to mirror `FatalAbort`), and modified to work in the new way with bug
  errors (abort via panic).
- Various diagnostic creators and emitters updated to the new, better
  signatures. Note that `DiagCtxt::bug` no longer needs to call
  `panic_any`, because `emit` handles that.

Also shorten the obnoxiously long
`diagnostic_builder_emit_producing_guarantee` name.

compiler/rustc_session/src/parse.rs

@@ -14,7 +14,7 @@ use rustc_data_structures::sync::{AppendOnlyVec, Lock, Lrc};
 use rustc_errors::{emitter::SilentEmitter, DiagCtxt};
 use rustc_errors::{
     fallback_fluent_bundle, Diagnostic, DiagnosticBuilder, DiagnosticId, DiagnosticMessage,
-    ErrorGuaranteed, IntoDiagnostic, Level, MultiSpan, Noted, StashKey,
+    ErrorGuaranteed, FatalAbort, IntoDiagnostic, Level, MultiSpan, StashKey,
 };
 use rustc_feature::{find_feature_issue, GateIssue, UnstableFeatures};
 use rustc_span::edition::Edition;
@@ -346,26 +346,26 @@ impl ParseSess {
     #[track_caller]
     pub fn create_note<'a>(
         &'a self,
-        note: impl IntoDiagnostic<'a, Noted>,
-    ) -> DiagnosticBuilder<'a, Noted> {
+        note: impl IntoDiagnostic<'a, ()>,
+    ) -> DiagnosticBuilder<'a, ()> {
         note.into_diagnostic(&self.dcx, Level::Note)
     }
 
     #[track_caller]
-    pub fn emit_note<'a>(&'a self, note: impl IntoDiagnostic<'a, Noted>) -> Noted {
+    pub fn emit_note<'a>(&'a self, note: impl IntoDiagnostic<'a, ()>) {
         self.create_note(note).emit()
     }
 
     #[track_caller]
     pub fn create_fatal<'a>(
         &'a self,
-        fatal: impl IntoDiagnostic<'a, !>,
-    ) -> DiagnosticBuilder<'a, !> {
+        fatal: impl IntoDiagnostic<'a, FatalAbort>,
+    ) -> DiagnosticBuilder<'a, FatalAbort> {
         fatal.into_diagnostic(&self.dcx, Level::Fatal)
     }
 
     #[track_caller]
-    pub fn emit_fatal<'a>(&'a self, fatal: impl IntoDiagnostic<'a, !>) -> ! {
+    pub fn emit_fatal<'a>(&'a self, fatal: impl IntoDiagnostic<'a, FatalAbort>) -> ! {
         self.create_fatal(fatal).emit()
     }
 
@@ -386,7 +386,10 @@ impl ParseSess {
 
     #[rustc_lint_diagnostics]
     #[track_caller]
-    pub fn struct_fatal(&self, msg: impl Into<DiagnosticMessage>) -> DiagnosticBuilder<'_, !> {
+    pub fn struct_fatal(
+        &self,
+        msg: impl Into<DiagnosticMessage>,
+    ) -> DiagnosticBuilder<'_, FatalAbort> {
         self.dcx.struct_fatal(msg)
     }
 }