From b38a55331b8b6171305359f618e997a7ce275e4d Mon Sep 17 00:00:00 2001
From: Graeme Coupar <grambo@grambo.me.uk>
Date: Mon, 24 May 2021 16:51:26 +0100
Subject: [PATCH] Support generic default values

I'm trying to `derive(JsonSchema)` on a field with a default that relies
on type inference to determine it's return type.  This causes compile
errors because schemars calls the default function without providing
any types for inference to use.

This changes that - wraps the `default` in a closure with a defined
return value that it immediately calls.  Feels a bit hacky, but I
couldn't think of a better way to fix this.
---
 schemars/tests/default.rs                    | 11 +++++++++++
 schemars/tests/expected/generic_default.json | 14 ++++++++++++++
 schemars_derive/src/schema_exprs.rs          |  2 +-
 3 files changed, 26 insertions(+), 1 deletion(-)
 create mode 100644 schemars/tests/expected/generic_default.json

diff --git a/schemars/tests/default.rs b/schemars/tests/default.rs
index 0d68d0e..4623994 100644
--- a/schemars/tests/default.rs
+++ b/schemars/tests/default.rs
@@ -54,3 +54,14 @@ pub struct NotSerialize;
 fn schema_default_values() -> TestResult {
     test_default_generated_schema::<MyStruct>("default")
 }
+
+#[derive(Default, Deserialize, Serialize, JsonSchema, Debug, PartialEq)]
+pub struct StructWithGenericDefaults {
+    #[serde(default = "Vec::new")]
+    pub a_vec: Vec<String>,
+}
+
+#[test]
+fn schema_with_generic_default_value() -> TestResult {
+    test_default_generated_schema::<StructWithGenericDefaults>("generic_default")
+}
diff --git a/schemars/tests/expected/generic_default.json b/schemars/tests/expected/generic_default.json
new file mode 100644
index 0000000..095b999
--- /dev/null
+++ b/schemars/tests/expected/generic_default.json
@@ -0,0 +1,14 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "StructWithGenericDefaults",
+  "type": "object",
+  "properties": {
+    "a_vec": {
+      "default": [],
+      "type": "array",
+      "items": {
+        "type": "string"
+      }
+    }
+  }
+}
\ No newline at end of file
diff --git a/schemars_derive/src/schema_exprs.rs b/schemars_derive/src/schema_exprs.rs
index c5b1672..6efa9b8 100644
--- a/schemars_derive/src/schema_exprs.rs
+++ b/schemars_derive/src/schema_exprs.rs
@@ -581,7 +581,7 @@ fn field_default_expr(field: &Field, container_has_default: bool) -> Option<Toke
             quote!(container_default.#member)
         }
         SerdeDefault::Default => quote!(<#ty>::default()),
-        SerdeDefault::Path(path) => quote!(#path()),
+        SerdeDefault::Path(path) => quote!(|| -> #ty { #path() }()),
     };
 
     let default_expr = match field.serde_attrs.skip_serializing_if() {