Auto merge of #120576 - nnethercote:merge-Diagnostic-DiagnosticBuilder, r=davidtwco

Overhaul `Diagnostic` and `DiagnosticBuilder`

Implements the first part of https://github.com/rust-lang/compiler-team/issues/722, which moves functionality and use away from `Diagnostic`, onto `DiagnosticBuilder`.

Likely follow-ups:
- Move things around, because this PR was written to minimize diff size, so some things end up in sub-optimal places. E.g. `DiagnosticBuilder` has impls in both `diagnostic.rs` and `diagnostic_builder.rs`.
- Rename `Diagnostic` as `DiagInner` and `DiagnosticBuilder` as `Diag`.

r? `@davidtwco`

compiler/rustc_session/src/config.rs

@@ -2468,6 +2468,9 @@ pub fn parse_externs(
             ));
             let adjusted_name = name.replace('-', "_");
             if is_ascii_ident(&adjusted_name) {
+                // FIXME: make this translatable
+                #[allow(rustc::diagnostic_outside_of_impl)]
+                #[allow(rustc::untranslatable_diagnostic)]
                 error.help(format!(
                     "consider replacing the dashes with underscores: `{adjusted_name}`"
                 ));

compiler/rustc_session/src/parse.rs

@@ -15,7 +15,8 @@ use rustc_data_structures::fx::{FxHashMap, FxIndexMap, FxIndexSet};
 use rustc_data_structures::sync::{AppendOnlyVec, Lock, Lrc};
 use rustc_errors::{emitter::SilentEmitter, DiagCtxt};
 use rustc_errors::{
-    fallback_fluent_bundle, Diagnostic, DiagnosticBuilder, DiagnosticMessage, MultiSpan, StashKey,
+    fallback_fluent_bundle, DiagnosticBuilder, DiagnosticMessage, EmissionGuarantee, MultiSpan,
+    StashKey,
 };
 use rustc_feature::{find_feature_issue, GateIssue, UnstableFeatures};
 use rustc_span::edition::Edition;
@@ -156,7 +157,11 @@ pub fn feature_warn_issue(
 }
 
 /// Adds the diagnostics for a feature to an existing error.
-pub fn add_feature_diagnostics(err: &mut Diagnostic, sess: &Session, feature: Symbol) {
+pub fn add_feature_diagnostics<G: EmissionGuarantee>(
+    err: &mut DiagnosticBuilder<'_, G>,
+    sess: &Session,
+    feature: Symbol,
+) {
     add_feature_diagnostics_for_issue(err, sess, feature, GateIssue::Language, false);
 }
 
@@ -165,8 +170,8 @@ pub fn add_feature_diagnostics(err: &mut Diagnostic, sess: &Session, feature: Sy
 /// This variant allows you to control whether it is a library or language feature.
 /// Almost always, you want to use this for a language feature. If so, prefer
 /// `add_feature_diagnostics`.
-pub fn add_feature_diagnostics_for_issue(
-    err: &mut Diagnostic,
+pub fn add_feature_diagnostics_for_issue<G: EmissionGuarantee>(
+    err: &mut DiagnosticBuilder<'_, G>,
     sess: &Session,
     feature: Symbol,
     issue: GateIssue,