Protect against panic retrieving form_ast

Summary:
We are seeing panics when retrieving the ast syntax node from a form_id, triggered when the function of interest is defined in a different module.

Make this process safe by explicitly checking that the `form_id` and the assist context source both come from the same file.

Reviewed By: jcpetruzza

Differential Revision: D67764074

fbshipit-source-id: 855edd91b9be7846b841aaf14073e679e323d7dd

crates/ide_assists/src/assist_context.rs

@@ -221,11 +221,15 @@ impl<'a> AssistContext<'a> {
         SymbolClass::classify(&self.sema, InFile::new(self.file_id(), token))
     }
 
-    pub(crate) fn form_ast<T>(&self, form_id: FormId<T>) -> T
+    pub(crate) fn form_ast<T>(&self, form_id: InFile<FormId<T>>) -> Option<T>
     where
         T: AstNode,
     {
-        form_id.get(&self.source_file)
+        if self.file_id() == form_id.file_id {
+            Some(form_id.value.get(&self.source_file))
+        } else {
+            None
+        }
     }
 
     pub(crate) fn ast_ptr_get<T>(&self, ptr: InFileAstPtr<T>) -> Option<T>

crates/ide_assists/src/helpers.rs

@@ -43,6 +43,7 @@ use hir::ExprId;
 use hir::FormList;
 use hir::FunctionClauseBody;
 use hir::FunctionDef;
+use hir::InFile;
 use hir::InFileAstPtr;
 use hir::InFunctionClauseBody;
 use hir::NameArity;
@@ -385,9 +386,9 @@ pub(crate) fn ranges_for_delete_function(
         })
         .collect();
 
-    let spec_range = function_def.spec.map(|spec| {
-        let ast_spec = ctx.form_ast(spec.spec.form_id);
-        extend_form_range_for_delete(ast_spec.syntax())
+    let spec_range = function_def.spec.and_then(|spec| {
+        let ast_spec = ctx.form_ast(InFile::new(spec.file.file_id, spec.spec.form_id));
+        ast_spec.map(|ast_spec| extend_form_range_for_delete(ast_spec.syntax()))
     });
 
     Some(FunctionRanges {