Give DiagnosticBuilder a default type.

`IntoDiagnostic` defaults to `ErrorGuaranteed`, because errors are the
most common diagnostic level. It makes sense to do likewise for the
closely-related (and much more widely used) `DiagnosticBuilder` type,
letting us write `DiagnosticBuilder<'a, ErrorGuaranteed>` as just
`DiagnosticBuilder<'a>`. This cuts over 200 lines of code due to many
multi-line things becoming single line things.

compiler/rustc_resolve/src/diagnostics.rs

@@ -551,7 +551,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         &mut self,
         span: Span,
         resolution_error: ResolutionError<'a>,
-    ) -> DiagnosticBuilder<'_, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'_> {
         match resolution_error {
             ResolutionError::GenericParamsFromOuterItem(outer_res, has_generic_params) => {
                 use errs::GenericParamsFromOuterItemLabel as Label;

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -424,7 +424,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
         span: Span,
         source: PathSource<'_>,
         res: Option<Res>,
-    ) -> (DiagnosticBuilder<'tcx, ErrorGuaranteed>, Vec<ImportSuggestion>) {
+    ) -> (DiagnosticBuilder<'tcx>, Vec<ImportSuggestion>) {
         debug!(?res, ?source);
         let base_error = self.make_base_error(path, span, source, res);
 

compiler/rustc_resolve/src/lib.rs

@@ -37,7 +37,7 @@ use rustc_data_structures::fx::{FxHashMap, FxHashSet, FxIndexMap, FxIndexSet};
 use rustc_data_structures::intern::Interned;
 use rustc_data_structures::steal::Steal;
 use rustc_data_structures::sync::{FreezeReadGuard, Lrc};
-use rustc_errors::{Applicability, DiagnosticBuilder, ErrorGuaranteed};
+use rustc_errors::{Applicability, DiagnosticBuilder};
 use rustc_expand::base::{DeriveResolutions, SyntaxExtension, SyntaxExtensionKind};
 use rustc_feature::BUILTIN_ATTRIBUTES;
 use rustc_hir::def::Namespace::{self, *};
@@ -704,7 +704,7 @@ struct PrivacyError<'a> {
 
 #[derive(Debug)]
 struct UseError<'a> {
-    err: DiagnosticBuilder<'a, ErrorGuaranteed>,
+    err: DiagnosticBuilder<'a>,
     /// Candidates which user could `use` to access the missing type.
     candidates: Vec<ImportSuggestion>,
     /// The `DefId` of the module to place the use-statements in.