Fix #107998, avoid ICE when the generic_span is empty

Author: chenyukang

compiler/rustc_lint/src/context.rs

@@ -837,9 +837,17 @@ pub trait LintContext: Sized {
                             (use_span, "'_".to_owned())
                         };
                         debug!(?deletion_span, ?use_span);
+
+                        // issue 107998 for the case such as a wrong function pointer type
+                        // `deletion_span` is empty and there is no need to report lifetime uses here
+                        let suggestions = if deletion_span.is_empty() {
+                            vec![(use_span, replace_lt)]
+                        } else {
+                            vec![(deletion_span, String::new()), (use_span, replace_lt)]
+                        };
                         db.multipart_suggestion(
                             msg,
-                            vec![(deletion_span, String::new()), (use_span, replace_lt)],
+                            suggestions,
                             Applicability::MachineApplicable,
                         );
                     }

tests/ui/single-use-lifetime/issue-107998.rs

@@ -0,0 +1,9 @@
+#![deny(single_use_lifetimes)]
+
+fn with<R>(f: &fn<'a>(x: &'a i32) -> R) -> R {
+    //~^ ERROR function pointer types may not have generic parameters
+    //~| ERROR lifetime parameter `'a` only used once
+    f(&3)
+}
+
+fn main() {}

tests/ui/single-use-lifetime/issue-107998.stderr

@@ -0,0 +1,30 @@
+error: function pointer types may not have generic parameters
+  --> $DIR/issue-107998.rs:3:18
+   |
+LL | fn with<R>(f: &fn<'a>(x: &'a i32) -> R) -> R {
+   |                  ^^^^
+   |
+help: consider moving the lifetime parameter to a `for` parameter list
+   |
+LL - fn with<R>(f: &fn<'a>(x: &'a i32) -> R) -> R {
+LL + fn with<R>(f: &for<'a> fn(x: &'a i32) -> R) -> R {
+   |
+
+error: lifetime parameter `'a` only used once
+  --> $DIR/issue-107998.rs:3:19
+   |
+LL | fn with<R>(f: &fn<'a>(x: &'a i32) -> R) -> R {
+   |                   ^^      ---
+   |                   |       |
+   |                   |       ...is used only here
+   |                   |       help: elide the single-use lifetime
+   |                   this lifetime...
+   |
+note: the lint level is defined here
+  --> $DIR/issue-107998.rs:1:9
+   |
+LL | #![deny(single_use_lifetimes)]
+   |         ^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+