fix: fix regression in rust-lang#109203

compiler/rustc_parse/src/parser/diagnostics.rs

@@ -423,11 +423,11 @@ impl<'a> Parser<'a> {
         if let token::Literal(Lit {
             kind: token::LitKind::Integer | token::LitKind::Float,
             symbol,
-            suffix,
+            suffix: Some(suffix), // no suffix makes it a valid literal
         }) = self.token.kind
             && rustc_ast::MetaItemLit::from_token(&self.token).is_none()
         {
-            Some((symbol.as_str().len(), suffix.unwrap()))
+            Some((symbol.as_str().len(), suffix))
         } else {
             None
         }

tests/ui/parser/issues/issue-110014.rs

@@ -0,0 +1,3 @@
+fn`2222222222222222222222222222222222222222() {}
+//~^ ERROR unknown start of token: `
+//~^^ ERROR expected identifier, found `2222222222222222222222222222222222222222`

tests/ui/parser/issues/issue-110014.stderr

@@ -0,0 +1,19 @@
+error: unknown start of token: `
+  --> $DIR/issue-110014.rs:1:3
+   |
+LL | fn`2222222222222222222222222222222222222222() {}
+   |   ^
+   |
+help: Unicode character '`' (Grave Accent) looks like ''' (Single Quote), but it is not
+   |
+LL | fn'2222222222222222222222222222222222222222() {}
+   |   ~
+
+error: expected identifier, found `2222222222222222222222222222222222222222`
+  --> $DIR/issue-110014.rs:1:4
+   |
+LL | fn`2222222222222222222222222222222222222222() {}
+   |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected identifier
+
+error: aborting due to 2 previous errors
+