Rollup merge of #107580 - lenko-d:default_value_for_a_lifetime_generic_parameter_produces_confusing_diagnostic, r=compiler-errors

Recover from lifetimes with default lifetimes in generic args

Fixes [#107492](https://github.com/rust-lang/rust/issues/107492)

2 files changed, 24 insertions, 1 deletions

diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index fc7c839f1c4..0c11e002690 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -1602,6 +1602,14 @@ pub(crate) struct UnexpectedSelfInGenericParameters {
 }
 
 #[derive(Diagnostic)]
+#[diag(parse_unexpected_default_value_for_lifetime_in_generic_parameters)]
+pub(crate) struct UnexpectedDefaultValueForLifetimeInGenericParameters {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+}
+
+#[derive(Diagnostic)]
 #[diag(parse_multiple_where_clauses)]
 pub(crate) struct MultipleWhereClauses {
     #[primary_span]
diff --git a/compiler/rustc_parse/src/parser/generics.rs b/compiler/rustc_parse/src/parser/generics.rs
index 585dfc518b3..23f49ec55a1 100644
--- a/compiler/rustc_parse/src/parser/generics.rs
+++ b/compiler/rustc_parse/src/parser/generics.rs
@@ -1,5 +1,6 @@
 use crate::errors::{
-    MultipleWhereClauses, UnexpectedSelfInGenericParameters, WhereClauseBeforeTupleStructBody,
+    MultipleWhereClauses, UnexpectedDefaultValueForLifetimeInGenericParameters,
+    UnexpectedSelfInGenericParameters, WhereClauseBeforeTupleStructBody,
     WhereClauseBeforeTupleStructBodySugg,
 };
 
@@ -145,6 +146,20 @@ impl<'a> Parser<'a> {
                         } else {
                             (None, Vec::new())
                         };
+
+                        if this.check_noexpect(&token::Eq)
+                            && this.look_ahead(1, |t| t.is_lifetime())
+                        {
+                            let lo = this.token.span;
+                            // Parse `= 'lifetime`.
+                            this.bump(); // `=`
+                            this.bump(); // `'lifetime`
+                            let span = lo.to(this.prev_token.span);
+                            this.sess.emit_err(
+                                UnexpectedDefaultValueForLifetimeInGenericParameters { span },
+                            );
+                        }
+
                         Some(ast::GenericParam {
                             ident: lifetime.ident,
                             id: lifetime.id,