Reduce capabilities of Diagnostic.

Currently many diagnostic modifier methods are available on both
`Diagnostic` and `DiagnosticBuilder`. This commit removes most of them
from `Diagnostic`. To minimize the diff size, it keeps them within
`diagnostic.rs` but changes the surrounding `impl Diagnostic` block to
`impl DiagnosticBuilder`. (I intend to move things around later, to give
a more sensible code layout.)

`Diagnostic` keeps a few methods that it still needs, like `sub`,
`arg`, and `replace_args`.

The `forward!` macro, which defined two additional methods per call
(e.g. `note` and `with_note`), is replaced by the `with_fn!` macro,
which defines one additional method per call (e.g. `with_note`). It's
now also only used when necessary -- not all modifier methods currently
need a `with_*` form. (New ones can be easily added as necessary.)

All this also requires changing `trait AddToDiagnostic` so its methods
take `DiagnosticBuilder` instead of `Diagnostic`, which leads to many
mechanical changes. `SubdiagnosticMessageOp` gains a type parameter `G`.

There are three subdiagnostics -- `DelayedAtWithoutNewline`,
`DelayedAtWithNewline`, and `InvalidFlushedDelayedDiagnosticLevel` --
that are created within the diagnostics machinery and appended to
external diagnostics. These are handled at the `Diagnostic` level, which
means it's now hard to construct them via `derive(Diagnostic)`, so
instead we construct them by hand. This has no effect on what they look
like when printed.

There are lots of new `allow` markers for `untranslatable_diagnostics`
and `diagnostics_outside_of_impl`. This is because
`#[rustc_lint_diagnostics]` annotations were present on the `Diagnostic`
modifier methods, but missing from the `DiagnosticBuilder` modifier
methods. They're now present.

compiler/rustc_expand/src/base.rs

@@ -1176,6 +1176,8 @@ impl<'a> ExtCtxt<'a> {
         for (span, notes) in self.expansions.iter() {
             let mut db = self.dcx().create_note(errors::TraceMacro { span: *span });
             for note in notes {
+                // FIXME: make this translatable
+                #[allow(rustc::untranslatable_diagnostic)]
                 db.note(note.clone());
             }
             db.emit();

compiler/rustc_expand/src/config.rs

@@ -384,6 +384,7 @@ impl<'a> StripUnconfigured<'a> {
             );
 
             if attr.is_doc_comment() {
+                #[allow(rustc::untranslatable_diagnostic)]
                 err.help("`///` is for documentation comments. For a plain comment, use `//`.");
             }
 

compiler/rustc_expand/src/proc_macro_server.rs

@@ -10,7 +10,7 @@ use rustc_ast::util::literal::escape_byte_str_symbol;
 use rustc_ast_pretty::pprust;
 use rustc_data_structures::fx::FxHashMap;
 use rustc_data_structures::sync::Lrc;
-use rustc_errors::{ErrorGuaranteed, MultiSpan, PResult};
+use rustc_errors::{DiagnosticBuilder, ErrorGuaranteed, MultiSpan, PResult};
 use rustc_parse::lexer::nfc_normalize;
 use rustc_parse::parse_stream_from_source_str;
 use rustc_session::parse::ParseSess;
@@ -509,13 +509,14 @@ impl server::FreeFunctions for Rustc<'_, '_> {
     }
 
     fn emit_diagnostic(&mut self, diagnostic: Diagnostic<Self::Span>) {
-        let mut diag =
-            rustc_errors::Diagnostic::new(diagnostic.level.to_internal(), diagnostic.message);
+        let message = rustc_errors::DiagnosticMessage::from(diagnostic.message);
+        let mut diag: DiagnosticBuilder<'_, rustc_errors::ErrorGuaranteed> =
+            DiagnosticBuilder::new(&self.sess().dcx, diagnostic.level.to_internal(), message);
         diag.span(MultiSpan::from_spans(diagnostic.spans));
         for child in diagnostic.children {
             diag.sub(child.level.to_internal(), child.message, MultiSpan::from_spans(child.spans));
         }
-        self.sess().dcx.emit_diagnostic(diag);
+        diag.emit();
     }
 }