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_expand/src/base.rs

@@ -1118,15 +1118,12 @@ impl<'a> ExtCtxt<'a> {
         &self,
         sp: S,
         msg: impl Into<DiagnosticMessage>,
-    ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
+    ) -> DiagnosticBuilder<'a> {
         self.sess.dcx().struct_span_err(sp, msg)
     }
 
     #[track_caller]
-    pub fn create_err(
-        &self,
-        err: impl IntoDiagnostic<'a>,
-    ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
+    pub fn create_err(&self, err: impl IntoDiagnostic<'a>) -> DiagnosticBuilder<'a> {
         self.sess.create_err(err)
     }
 
@@ -1230,7 +1227,7 @@ pub fn expr_to_spanned_string<'a>(
     cx: &'a mut ExtCtxt<'_>,
     expr: P<ast::Expr>,
     err_msg: &'static str,
-) -> Result<(Symbol, ast::StrStyle, Span), Option<(DiagnosticBuilder<'a, ErrorGuaranteed>, bool)>> {
+) -> Result<(Symbol, ast::StrStyle, Span), Option<(DiagnosticBuilder<'a>, bool)>> {
     // Perform eager expansion on the expression.
     // We want to be able to handle e.g., `concat!("foo", "bar")`.
     let expr = cx.expander().fully_expand_fragment(AstFragment::Expr(expr)).make_expr();

compiler/rustc_expand/src/mbe/diagnostics.rs

@@ -215,7 +215,7 @@ impl<'matcher> Tracker<'matcher> for FailureForwarder {
 }
 
 pub(super) fn emit_frag_parse_err(
-    mut e: DiagnosticBuilder<'_, rustc_errors::ErrorGuaranteed>,
+    mut e: DiagnosticBuilder<'_>,
     parser: &Parser<'_>,
     orig_parser: &mut Parser<'_>,
     site_span: Span,

compiler/rustc_expand/src/mbe/transcribe.rs

@@ -9,8 +9,8 @@ use rustc_ast::mut_visit::{self, MutVisitor};
 use rustc_ast::token::{self, Delimiter, Token, TokenKind};
 use rustc_ast::tokenstream::{DelimSpacing, DelimSpan, Spacing, TokenStream, TokenTree};
 use rustc_data_structures::fx::FxHashMap;
+use rustc_errors::DiagnosticBuilder;
 use rustc_errors::{pluralize, PResult};
-use rustc_errors::{DiagnosticBuilder, ErrorGuaranteed};
 use rustc_span::hygiene::{LocalExpnId, Transparency};
 use rustc_span::symbol::{sym, Ident, MacroRulesNormalizedIdent};
 use rustc_span::Span;
@@ -528,7 +528,7 @@ fn out_of_bounds_err<'a>(
     max: usize,
     span: Span,
     ty: &str,
-) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
+) -> DiagnosticBuilder<'a> {
     let msg = if max == 0 {
         format!(
             "meta-variable expression `{ty}` with depth parameter \

compiler/rustc_expand/src/module.rs

@@ -43,7 +43,7 @@ pub enum ModError<'a> {
     ModInBlock(Option<Ident>),
     FileNotFound(Ident, PathBuf, PathBuf),
     MultipleCandidates(Ident, PathBuf, PathBuf),
-    ParserError(DiagnosticBuilder<'a, ErrorGuaranteed>),
+    ParserError(DiagnosticBuilder<'a>),
 }
 
 pub(crate) fn parse_external_mod(