[rust] fix: map-typed query parameters do not compile (.to_string() on a HashMap) (#24896)

* [rust] fix: map-typed query parameters called .to_string() on a HashMap

HashMap implements neither Display nor ToString, so the generated client
did not compile (E0599) wherever a map-typed query parameter fell through
to the scalar path: a required non-nullable map in both reqwest and
reqwest-trait, and the nullable/optional non-deepObject fallthroughs in
reqwest-trait. Serialize the map as one json-encoded parameter with
serde_json::to_string, exactly how reqwest's optional branch already
handles non-primitive parameters.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GcwZ1arjLZNpetHz2a3TJz

* [rust] test: pin the required map-query assertion to its parameter

`serde_json::to_string(&` matched any call in the file; match on the
`labels` key instead, which both libraries emit.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: wiebren

modules/openapi-generator/src/main/resources/rust/reqwest-trait/api.mustache

@@ -188,7 +188,12 @@ impl {{classname}} for {{classname}}Client {
         {{/isArray}}
         {{^isArray}}
         {{^isNullable}}
+        {{#isMap}}
+        local_var_req_builder = local_var_req_builder.query(&[("{{{baseName}}}", &serde_json::to_string(&{{{paramName}}})?)]);
+        {{/isMap}}
+        {{^isMap}}
         local_var_req_builder = local_var_req_builder.query(&[("{{{baseName}}}", &{{{paramName}}}.to_string())]);
+        {{/isMap}}
         {{/isNullable}}
         {{#isNullable}}
         {{#isDeepObject}}
@@ -225,9 +230,16 @@ impl {{classname}} for {{classname}}Client {
         {{/isModel}}
         {{^isObject}}
         {{^isModel}}
+        {{#isMap}}
+        if let Some(ref param_value) = {{{paramName}}} {
+            local_var_req_builder = local_var_req_builder.query(&[("{{{baseName}}}", &serde_json::to_string(param_value)?)]);
+        };
+        {{/isMap}}
+        {{^isMap}}
         if let Some(ref param_value) = {{{paramName}}} {
             local_var_req_builder = local_var_req_builder.query(&[("{{{baseName}}}", &param_value.to_string())]);
         };
+        {{/isMap}}
         {{/isModel}}
         {{/isObject}}
         {{/isDeepObject}}
@@ -267,7 +279,12 @@ impl {{classname}} for {{classname}}Client {
             {{/isModel}}
             {{^isObject}}
             {{^isModel}}
+            {{#isMap}}
+            local_var_req_builder = local_var_req_builder.query(&[("{{{baseName}}}", &serde_json::to_string(param_value)?)]);
+            {{/isMap}}
+            {{^isMap}}
             local_var_req_builder = local_var_req_builder.query(&[("{{{baseName}}}", &param_value.to_string())]);
+            {{/isMap}}
             {{/isModel}}
             {{/isObject}}
             {{/isDeepObject}}

modules/openapi-generator/src/main/resources/rust/reqwest/api.mustache

@@ -166,7 +166,12 @@ pub {{#supportAsync}}async {{/supportAsync}}fn {{{operationId}}}(configuration:
     {{/isArray}}
     {{^isArray}}
     {{^isNullable}}
+    {{#isMap}}
+    req_builder = req_builder.query(&[("{{{baseName}}}", &serde_json::to_string(&{{{vendorExtensions.x-rust-param-identifier}}})?)]);
+    {{/isMap}}
+    {{^isMap}}
     req_builder = req_builder.query(&[("{{{baseName}}}", &{{{vendorExtensions.x-rust-param-identifier}}}.to_string())]);
+    {{/isMap}}
     {{/isNullable}}
     {{#isNullable}}
     {{#isDeepObject}}

modules/openapi-generator/src/test/java/org/openapitools/codegen/rust/RustClientCodegenTest.java

@@ -449,4 +449,26 @@ private Path generateReqwestConfiguration(String library, boolean supportTokenSo
         TestUtils.assertFileExists(outputPath);
         return outputPath;
     }
+
+    @Test
+    public void testMapQueryParamsSerializeAsJson() throws IOException {
+        // HashMap has no Display: map-typed query params must serialize via serde_json::to_string
+        for (String library : new String[] {"reqwest", "reqwest-trait"}) {
+            Path target = Files.createTempDirectory("test");
+            target.toFile().deleteOnExit();
+            final CodegenConfigurator configurator = new CodegenConfigurator()
+                    .setGeneratorName("rust")
+                    .setLibrary(library)
+                    .setInputSpec("src/test/resources/3_0/rust/map-query-params.yaml")
+                    .setSkipOverwrite(false)
+                    .setOutputDir(target.toAbsolutePath().toString().replace("\\", "/"));
+            new DefaultGenerator().opts(configurator.toClientOptInput()).generate();
+            Path outputPath = Path.of(target.toString(), "/src/apis/default_api.rs");
+            TestUtils.assertFileExists(outputPath);
+            TestUtils.assertFileContains(outputPath, "(\"labels\", &serde_json::to_string(&");
+            TestUtils.assertFileContains(outputPath, "(\"counts\", &serde_json::to_string(param_value)?)");
+            TestUtils.assertFileNotContains(outputPath, "labels.to_string()");
+            TestUtils.assertFileNotContains(outputPath, "param_value.to_string()");
+        }
+    }
 }

modules/openapi-generator/src/test/resources/3_0/rust/map-query-params.yaml

@@ -0,0 +1,25 @@
+openapi: 3.0.3
+info:
+  title: map query params
+  version: 1.0.0
+paths:
+  /items:
+    get:
+      operationId: listItems
+      parameters:
+        - name: labels
+          in: query
+          required: true
+          schema:
+            type: object
+            additionalProperties:
+              type: string
+        - name: counts
+          in: query
+          schema:
+            type: object
+            additionalProperties:
+              type: integer
+      responses:
+        '200':
+          description: ok