Avoid struct_diagnostic where possible.

It's necessary for `derive(Diagnostic)`, but is best avoided elsewhere
because there are clearer alternatives.

This required adding `Handler::struct_almost_fatal`.

compiler/rustc_builtin_macros/src/errors.rs

@@ -453,7 +453,7 @@ impl<'a> IntoDiagnostic<'a> for EnvNotDefinedWithUserMessage {
             rustc::untranslatable_diagnostic,
             reason = "cannot translate user-provided messages"
         )]
-        let mut diag = handler.struct_diagnostic(self.msg_from_user.to_string());
+        let mut diag = handler.struct_err(self.msg_from_user.to_string());
         diag.set_span(self.span);
         diag
     }
@@ -804,7 +804,7 @@ pub(crate) struct AsmClobberNoReg {
 impl<'a> IntoDiagnostic<'a> for AsmClobberNoReg {
     fn into_diagnostic(self, handler: &'a Handler) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
         let mut diag =
-            handler.struct_diagnostic(crate::fluent_generated::builtin_macros_asm_clobber_no_reg);
+            handler.struct_err(crate::fluent_generated::builtin_macros_asm_clobber_no_reg);
         diag.set_span(self.spans.clone());
         // eager translation as `span_labels` takes `AsRef<str>`
         let lbl1 = handler.eagerly_translate_to_string(

compiler/rustc_codegen_llvm/src/errors.rs

@@ -107,7 +107,8 @@ impl IntoDiagnostic<'_, FatalError> for ParseTargetMachineConfig<'_> {
         let (message, _) = diag.styled_message().first().expect("`LlvmError` with no message");
         let message = handler.eagerly_translate_to_string(message.clone(), diag.args());
 
-        let mut diag = handler.struct_diagnostic(fluent::codegen_llvm_parse_target_machine_config);
+        let mut diag =
+            handler.struct_almost_fatal(fluent::codegen_llvm_parse_target_machine_config);
         diag.set_arg("error", message);
         diag
     }

compiler/rustc_errors/src/lib.rs

@@ -727,7 +727,12 @@ impl Handler {
         self.inner.borrow_mut().emit_stashed_diagnostics()
     }
 
-    /// Construct a builder with the `msg` at the level appropriate for the specific `EmissionGuarantee`.
+    /// Construct a builder with the `msg` at the level appropriate for the
+    /// specific `EmissionGuarantee`.
+    ///
+    /// Note: this is necessary for `derive(Diagnostic)`, but shouldn't be used
+    /// outside of that. Instead use `struct_err`, `struct_warn`, etc., which
+    /// make the diagnostic kind clearer.
     #[rustc_lint_diagnostics]
     #[track_caller]
     pub fn struct_diagnostic<G: EmissionGuarantee>(
@@ -942,13 +947,23 @@ impl Handler {
         result
     }
 
-    /// Construct a builder at the `Error` level with the `msg`.
+    /// Construct a builder at the `Fatal` level with the `msg`.
     #[rustc_lint_diagnostics]
     #[track_caller]
     pub fn struct_fatal(&self, msg: impl Into<DiagnosticMessage>) -> DiagnosticBuilder<'_, !> {
         DiagnosticBuilder::new(self, Level::Fatal, msg)
     }
 
+    /// Construct a builder at the `Fatal` level with the `msg`, that doesn't abort.
+    #[rustc_lint_diagnostics]
+    #[track_caller]
+    pub fn struct_almost_fatal(
+        &self,
+        msg: impl Into<DiagnosticMessage>,
+    ) -> DiagnosticBuilder<'_, FatalError> {
+        DiagnosticBuilder::new(self, Level::Fatal, msg)
+    }
+
     /// Construct a builder at the `Help` level with the `msg`.
     #[rustc_lint_diagnostics]
     pub fn struct_help(&self, msg: impl Into<DiagnosticMessage>) -> DiagnosticBuilder<'_, ()> {

compiler/rustc_mir_transform/src/errors.rs

@@ -65,7 +65,7 @@ pub(crate) struct RequiresUnsafe {
 impl<'sess> IntoDiagnostic<'sess> for RequiresUnsafe {
     #[track_caller]
     fn into_diagnostic(self, handler: &'sess Handler) -> DiagnosticBuilder<'sess, ErrorGuaranteed> {
-        let mut diag = handler.struct_diagnostic(fluent::mir_transform_requires_unsafe);
+        let mut diag = handler.struct_err(fluent::mir_transform_requires_unsafe);
         diag.code(rustc_errors::DiagnosticId::Error("E0133".to_string()));
         diag.set_span(self.span);
         diag.span_label(self.span, self.details.label());

compiler/rustc_parse/src/errors.rs

@@ -1046,7 +1046,7 @@ impl<'a> IntoDiagnostic<'a> for ExpectedIdentifier {
     ) -> rustc_errors::DiagnosticBuilder<'a, ErrorGuaranteed> {
         let token_descr = TokenDescription::from_token(&self.token);
 
-        let mut diag = handler.struct_diagnostic(match token_descr {
+        let mut diag = handler.struct_err(match token_descr {
             Some(TokenDescription::ReservedIdentifier) => {
                 fluent::parse_expected_identifier_found_reserved_identifier_str
             }
@@ -1103,7 +1103,7 @@ impl<'a> IntoDiagnostic<'a> for ExpectedSemi {
     ) -> rustc_errors::DiagnosticBuilder<'a, ErrorGuaranteed> {
         let token_descr = TokenDescription::from_token(&self.token);
 
-        let mut diag = handler.struct_diagnostic(match token_descr {
+        let mut diag = handler.struct_err(match token_descr {
             Some(TokenDescription::ReservedIdentifier) => {
                 fluent::parse_expected_semi_found_reserved_identifier_str
             }

compiler/rustc_session/src/errors.rs

@@ -19,7 +19,7 @@ impl<'a> IntoDiagnostic<'a> for FeatureGateError {
         self,
         handler: &'a rustc_errors::Handler,
     ) -> rustc_errors::DiagnosticBuilder<'a, ErrorGuaranteed> {
-        let mut diag = handler.struct_diagnostic(self.explain);
+        let mut diag = handler.struct_err(self.explain);
         diag.set_span(self.span);
         diag.code(error_code!(E0658));
         diag

compiler/rustc_session/src/parse.rs

@@ -14,7 +14,7 @@ use rustc_data_structures::sync::{AppendOnlyVec, Lock, Lrc};
 use rustc_errors::{emitter::SilentEmitter, Handler};
 use rustc_errors::{
     fallback_fluent_bundle, Diagnostic, DiagnosticBuilder, DiagnosticId, DiagnosticMessage,
-    EmissionGuarantee, ErrorGuaranteed, IntoDiagnostic, MultiSpan, Noted, StashKey,
+    ErrorGuaranteed, IntoDiagnostic, MultiSpan, Noted, StashKey,
 };
 use rustc_feature::{find_feature_issue, GateIssue, UnstableFeatures};
 use rustc_span::edition::Edition;
@@ -390,13 +390,4 @@ impl ParseSess {
     pub fn struct_fatal(&self, msg: impl Into<DiagnosticMessage>) -> DiagnosticBuilder<'_, !> {
         self.span_diagnostic.struct_fatal(msg)
     }
-
-    #[rustc_lint_diagnostics]
-    #[track_caller]
-    pub fn struct_diagnostic<G: EmissionGuarantee>(
-        &self,
-        msg: impl Into<DiagnosticMessage>,
-    ) -> DiagnosticBuilder<'_, G> {
-        self.span_diagnostic.struct_diagnostic(msg)
-    }
 }