bugfix: local variable 'data' colliding with argument name (#291)

This PR makes [THIS ISSUE](#279)
a lot less likely to occur.
The local name "data" in generated functions was colliding with
arguments called "data".

Note that no explicit name collision check was added, but "data" inside
the function was just renamed to "data_" to make it less likely to
collide with arguments.

---------

Co-authored-by: Ben Kraft <ben@benkraft.org>

docs/CHANGELOG.md

@@ -29,6 +29,7 @@ When releasing a new version:
 
 ### Bug fixes:
 - The presence of negative pointer directives, i.e., `# @genqlient(pointer: false)` are now respected even in the when `optional: pointer` is set in the configuration file.
+- Made name collisions between query/mutation arguments and local function variables less likely.
 
 ## v0.6.0
 

generate/operation.go.tmpl

@@ -4,10 +4,10 @@ const {{.Name}}_Operation = `{{$.Body}}`
 {{.Doc}}
 func {{.Name}}(
     {{if ne .Config.ContextType "-" -}}
-    ctx {{ref .Config.ContextType}},
+    ctx_ {{ref .Config.ContextType}},
     {{end}}
     {{- if not .Config.ClientGetter -}}
-    client {{ref "github.com/Khan/genqlient/graphql.Client"}},
+    client_ {{ref "github.com/Khan/genqlient/graphql.Client"}},
     {{end}}
     {{- if .Input -}}
     {{- range .Input.Fields -}}
@@ -16,7 +16,7 @@ func {{.Name}}(
     {{end -}}
     {{end -}}
 ) (*{{.ResponseName}}, {{if .Config.Extensions -}}map[string]interface{},{{end}} error) {
-    req := &graphql.Request{
+    req_ := &graphql.Request{
         OpName: "{{.Name}}",
         Query:  {{.Name}}_Operation,
     {{if .Input -}}
@@ -27,23 +27,23 @@ func {{.Name}}(
         },
     {{end -}}
     }
-    var err error
+    var err_ error
     {{if .Config.ClientGetter -}}
-    var client graphql.Client
+    var client_ graphql.Client
 
-    client, err = {{ref .Config.ClientGetter}}({{if ne .Config.ContextType "-"}}ctx{{else}}{{end}})
-    if err != nil {
-        return nil, {{if .Config.Extensions -}}nil,{{end -}} err
+    client_, err_ = {{ref .Config.ClientGetter}}({{if ne .Config.ContextType "-"}}ctx_{{else}}{{end}})
+    if err_ != nil {
+        return nil, {{if .Config.Extensions -}}nil,{{end -}} err_
     }
     {{end}}
-    var data {{.ResponseName}}
-    resp := &graphql.Response{Data: &data}
+    var data_ {{.ResponseName}}
+    resp_ := &graphql.Response{Data: &data_}
 
-    err = client.MakeRequest(
-        {{if ne .Config.ContextType "-"}}ctx{{else}}nil{{end}},
-        req,
-        resp,
+    err_ = client_.MakeRequest(
+        {{if ne .Config.ContextType "-"}}ctx_{{else}}nil{{end}},
+        req_,
+        resp_,
     )
 
-    return &data, {{if .Config.Extensions -}}resp.Extensions,{{end -}} err
+    return &data_, {{if .Config.Extensions -}}resp_.Extensions,{{end -}} err_
 }

generate/testdata/queries/MutationArgsWithCollidingNames.graphql

@@ -0,0 +1,5 @@
+mutation MutationArgsWithCollidingNames($data: String!, $req: Int, $resp: Int, $client: String) {
+  updateUser(data: $data, req: $req, resp: $resp, client: $client) {
+    id
+  }
+}

generate/testdata/queries/schema.graphql

@@ -184,7 +184,10 @@ type Query {
 }
 
 type Mutation {
-  createUser(name: String!, email: String): User 
+  createUser(name: String!, email: String): User
+  # The following query is non-sensical, but tests that argument names don't 
+  # collide with local var names in generated functions
+  updateUser(data: String!, req: Int, resp: Int, client: String): User
 }
 
 input getPokemonBoolExp {