Do not attempt to access HIR parent of Synthetic node

compiler/rustc_middle/src/ty/context.rs

@@ -2251,9 +2251,15 @@ impl<'tcx> TyCtxt<'tcx> {
     /// Find the crate root and the appropriate span where `use` and outer attributes can be
     /// inserted at.
     pub fn crate_level_attribute_injection_span(self, hir_id: HirId) -> Option<Span> {
-        for (_hir_id, node) in self.hir().parent_iter(hir_id) {
-            if let hir::Node::Crate(m) = node {
-                return Some(m.spans.inject_use_span.shrink_to_lo());
+        for (_hir_id, node) in
+            [(hir_id, self.hir_node(hir_id))].into_iter().chain(self.hir().parent_iter(hir_id))
+        {
+            match node {
+                hir::Node::Synthetic => return None,
+                hir::Node::Crate(m) => {
+                    return Some(m.spans.inject_use_span.shrink_to_lo());
+                }
+                _ => {}
             }
         }
         None

tests/ui/suggestions/dont-panic-by-accessing-parent-hir-of-synthetic.rs

@@ -0,0 +1,12 @@
+pub trait Foo<'a> {
+    type Assoc;
+
+    fn demo() -> impl Foo
+    //~^ ERROR missing lifetime specifier
+    //~| ERROR the trait bound `String: Copy` is not satisfied
+    where
+        String: Copy;
+    //~^ ERROR the trait bound `String: Copy` is not satisfied
+}
+
+fn main() {}

tests/ui/suggestions/dont-panic-by-accessing-parent-hir-of-synthetic.stderr

@@ -0,0 +1,37 @@
+error[E0106]: missing lifetime specifier
+  --> $DIR/dont-panic-by-accessing-parent-hir-of-synthetic.rs:4:23
+   |
+LL |     fn demo() -> impl Foo
+   |                       ^^^ expected named lifetime parameter
+   |
+   = help: this function's return type contains a borrowed value, but there is no value for it to be borrowed from
+help: consider using the `'a` lifetime
+   |
+LL |     fn demo() -> impl Foo<'a>
+   |                          ++++
+
+error[E0277]: the trait bound `String: Copy` is not satisfied
+  --> $DIR/dont-panic-by-accessing-parent-hir-of-synthetic.rs:8:9
+   |
+LL |         String: Copy;
+   |         ^^^^^^^^^^^^ the trait `Copy` is not implemented for `String`
+   |
+   = help: see issue #48214
+help: add `#![feature(trivial_bounds)]` to the crate attributes to enable
+   |
+LL + #![feature(trivial_bounds)]
+   |
+
+error[E0277]: the trait bound `String: Copy` is not satisfied
+  --> $DIR/dont-panic-by-accessing-parent-hir-of-synthetic.rs:4:18
+   |
+LL |     fn demo() -> impl Foo
+   |                  ^^^^^^^^ the trait `Copy` is not implemented for `String`
+   |
+   = help: see issue #48214
+   = help: add `#![feature(trivial_bounds)]` to the crate attributes to enable
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0106, E0277.
+For more information about an error, try `rustc --explain E0106`.