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 created at the `Diagnostic` level, which
means it's now hard to construct them via `derive(Diagnostic)`, so
instead we build them up 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_ast_lowering/src/errors.rs

@@ -1,5 +1,6 @@
 use rustc_errors::{
-    codes::*, AddToDiagnostic, Diagnostic, DiagnosticArgFromDisplay, SubdiagnosticMessageOp,
+    codes::*, AddToDiagnostic, DiagnosticArgFromDisplay, DiagnosticBuilder, EmissionGuarantee,
+    SubdiagnosticMessageOp,
 };
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{symbol::Ident, Span, Symbol};
@@ -41,7 +42,11 @@ pub struct InvalidAbi {
 pub struct InvalidAbiReason(pub &'static str);
 
 impl AddToDiagnostic for InvalidAbiReason {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _: F,
+    ) {
         #[allow(rustc::untranslatable_diagnostic)]
         diag.note(self.0);
     }

compiler/rustc_ast_passes/src/errors.rs

@@ -1,7 +1,10 @@
 //! Errors emitted by ast_passes.
 
 use rustc_ast::ParamKindOrd;
-use rustc_errors::{codes::*, AddToDiagnostic, Applicability, Diagnostic, SubdiagnosticMessageOp};
+use rustc_errors::{
+    codes::*, AddToDiagnostic, Applicability, DiagnosticBuilder, EmissionGuarantee,
+    SubdiagnosticMessageOp,
+};
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{symbol::Ident, Span, Symbol};
 
@@ -372,7 +375,11 @@ pub struct EmptyLabelManySpans(pub Vec<Span>);
 
 // The derive for `Vec<Span>` does multiple calls to `span_label`, adding commas between each
 impl AddToDiagnostic for EmptyLabelManySpans {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _: F,
+    ) {
         diag.span_labels(self.0, "");
     }
 }
@@ -729,7 +736,11 @@ pub struct StableFeature {
 }
 
 impl AddToDiagnostic for StableFeature {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, _: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        _: F,
+    ) {
         diag.arg("name", self.name);
         diag.arg("since", self.since);
         diag.help(fluent::ast_passes_stable_since);

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -22,6 +22,9 @@ macro_rules! gate {
     }};
     ($visitor:expr, $feature:ident, $span:expr, $explain:expr, $help:expr) => {{
         if !$visitor.features.$feature && !$span.allows_unstable(sym::$feature) {
+            // FIXME: make this translatable
+            #[allow(rustc::diagnostic_outside_of_impl)]
+            #[allow(rustc::untranslatable_diagnostic)]
             feature_err(&$visitor.sess, sym::$feature, $span, $explain).with_help($help).emit();
         }
     }};

compiler/rustc_borrowck/src/diagnostics/region_errors.rs

@@ -249,6 +249,9 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
 
         hrtb_bounds.iter().for_each(|bound| {
             let Trait(PolyTraitRef { trait_ref, span: trait_span, .. }, _) = bound else { return; };
+            // FIXME: make this translatable
+            #[allow(rustc::diagnostic_outside_of_impl)]
+            #[allow(rustc::untranslatable_diagnostic)]
             diag.span_note(
                 *trait_span,
                 "due to current limitations in the borrow checker, this implies a `'static` lifetime"
@@ -419,6 +422,9 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
     /// ```
     ///
     /// Here we would be invoked with `fr = 'a` and `outlived_fr = 'b`.
+    // FIXME: make this translatable
+    #[allow(rustc::diagnostic_outside_of_impl)]
+    #[allow(rustc::untranslatable_diagnostic)]
     pub(crate) fn report_region_error(
         &mut self,
         fr: RegionVid,
@@ -683,12 +689,18 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
             borrowck_errors::borrowed_data_escapes_closure(self.infcx.tcx, *span, escapes_from);
 
         if let Some((Some(outlived_fr_name), outlived_fr_span)) = outlived_fr_name_and_span {
+            // FIXME: make this translatable
+            #[allow(rustc::diagnostic_outside_of_impl)]
+            #[allow(rustc::untranslatable_diagnostic)]
             diag.span_label(
                 outlived_fr_span,
                 format!("`{outlived_fr_name}` declared here, outside of the {escapes_from} body",),
             );
         }
 
+        // FIXME: make this translatable
+        #[allow(rustc::diagnostic_outside_of_impl)]
+        #[allow(rustc::untranslatable_diagnostic)]
         if let Some((Some(fr_name), fr_span)) = fr_name_and_span {
             diag.span_label(
                 fr_span,
@@ -712,6 +724,9 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, 'tcx> {
                 let outlived_fr_region_name = self.give_region_a_name(errci.outlived_fr).unwrap();
                 outlived_fr_region_name.highlight_region_name(&mut diag);
 
+                // FIXME: make this translatable
+                #[allow(rustc::diagnostic_outside_of_impl)]
+                #[allow(rustc::untranslatable_diagnostic)]
                 diag.span_label(
                     *span,
                     format!(

compiler/rustc_builtin_macros/src/errors.rs

@@ -1,6 +1,6 @@
 use rustc_errors::{
-    codes::*, AddToDiagnostic, DiagCtxt, Diagnostic, DiagnosticBuilder, EmissionGuarantee,
-    IntoDiagnostic, Level, MultiSpan, SingleLabelManySpans, SubdiagnosticMessageOp,
+    codes::*, AddToDiagnostic, DiagCtxt, DiagnosticBuilder, EmissionGuarantee, IntoDiagnostic,
+    Level, MultiSpan, SingleLabelManySpans, SubdiagnosticMessageOp,
 };
 use rustc_macros::{Diagnostic, Subdiagnostic};
 use rustc_span::{symbol::Ident, Span, Symbol};
@@ -611,7 +611,11 @@ pub(crate) struct FormatUnusedArg {
 // Allow the singular form to be a subdiagnostic of the multiple-unused
 // form of diagnostic.
 impl AddToDiagnostic for FormatUnusedArg {
-    fn add_to_diagnostic_with<F: SubdiagnosticMessageOp>(self, diag: &mut Diagnostic, f: F) {
+    fn add_to_diagnostic_with<G: EmissionGuarantee, F: SubdiagnosticMessageOp<G>>(
+        self,
+        diag: &mut DiagnosticBuilder<'_, G>,
+        f: F,
+    ) {
         diag.arg("named", self.named);
         let msg = f(diag, crate::fluent_generated::builtin_macros_format_unused_arg.into());
         diag.span_label(self.span, msg);

compiler/rustc_codegen_ssa/src/back/write.rs

@@ -1854,9 +1854,7 @@ impl SharedEmitterMain {
                 Ok(SharedEmitterMessage::Diagnostic(diag)) => {
                     let dcx = sess.dcx();
                     let mut d = rustc_errors::Diagnostic::new_with_messages(diag.lvl, diag.msgs);
-                    if let Some(code) = diag.code {
-                        d.code(code);
-                    }
+                    d.code = diag.code; // may be `None`, that's ok
                     d.replace_args(diag.args);
                     dcx.emit_diagnostic(d);
                 }

compiler/rustc_const_eval/src/transform/check_consts/ops.rs

@@ -93,6 +93,9 @@ pub struct FnCallNonConst<'tcx> {
 }
 
 impl<'tcx> NonConstOp<'tcx> for FnCallNonConst<'tcx> {
+    // FIXME: make this translatable
+    #[allow(rustc::diagnostic_outside_of_impl)]
+    #[allow(rustc::untranslatable_diagnostic)]
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, _: Span) -> DiagnosticBuilder<'tcx> {
         let FnCallNonConst { caller, callee, args, span, call_source, feature } = *self;
         let ConstCx { tcx, param_env, .. } = *ccx;
@@ -321,6 +324,8 @@ impl<'tcx> NonConstOp<'tcx> for FnCallUnstable {
             .dcx()
             .create_err(errors::UnstableConstFn { span, def_path: ccx.tcx.def_path_str(def_id) });
 
+        // FIXME: make this translatable
+        #[allow(rustc::untranslatable_diagnostic)]
         if ccx.is_const_stable_const_fn() {
             err.help("const-stable functions can only call other const-stable functions");
         } else if ccx.tcx.sess.is_nightly_build() {