Rewrite #[derive(Associations)] in derives2

This was a pretty straightforward rewrite. Overall I feel much better
about this code structure than what we had before. We're having to do a
bit of funkyness to avoid
rust-lang/rust#47941, but other than that
everything here was pretty straightforward. I had the UI tests guide me
the entire time.

Author: sgrif

diesel_compile_tests/tests/compile-fail/associations_errors.rs

@@ -0,0 +1,34 @@
+#[macro_use]
+extern crate diesel;
+
+table! {
+    foo {
+        id -> Integer,
+    }
+}
+
+#[derive(Identifiable)]
+#[table_name = "foo"]
+struct Bar {
+    id: i32,
+}
+
+#[derive(Identifiable)]
+#[table_name = "foo"]
+struct Baz {
+    id: i32,
+}
+
+#[derive(Associations)]
+#[belongs_to]
+#[belongs_to = "Bar"]
+#[belongs_to()]
+#[belongs_to(foreign_key = "bar_id")]
+#[belongs_to(Bar, foreign_key)]
+#[belongs_to(Bar, foreign_key(bar_id))]
+#[belongs_to(Baz, foreign_key = "bar_id", random_option)]
+struct Foo {
+    bar_id: i32,
+}
+
+fn main() {}

diesel_compile_tests/tests/ui/belongs_to_invalid_option_syntax.rs

@@ -0,0 +1,48 @@
+error: `belongs_to` must be in the form `belongs_to(...)`
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to]
+  | ^
+
+error: `belongs_to` must be in the form `belongs_to(...)`
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to = "Bar"]
+  | ^
+
+error: Expected a struct name
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to()]
+  | ^
+  |
+  = help: e.g. `#[belongs_to(User)]`
+
+error: Expected a struct name
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(foreign_key = "bar_id")]
+  | ^
+  |
+  = help: e.g. `#[belongs_to(User)]`
+
+error: `foreign_key` must be in the form `foreign_key = "value"`
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(Bar, foreign_key)]
+  | ^
+
+warning: The form `foreign_key(value)` is deprecated. Use `foreign_key = "value"` instead
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(Bar, foreign_key(bar_id))]
+  | ^
+
+warning: Unrecognized option random_option
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(Baz, foreign_key = "bar_id", random_option)]
+  | ^
+
+error: aborting due to 5 previous errors
+

diesel_compile_tests/tests/ui/belongs_to_invalid_option_syntax.stderr

@@ -0,0 +1,42 @@
+#[macro_use]
+extern crate diesel;
+
+struct Bar;
+
+table! {
+    foo {
+        id -> Integer,
+    }
+}
+
+#[derive(Associations)]
+#[belongs_to(Bar)]
+#[table_name = "foo"]
+struct Foo1 {
+    bar_id: i32,
+}
+
+#[derive(Associations)]
+#[belongs_to(Bar, foreign_key = "bar_id")]
+#[table_name = "foo"]
+struct Foo2 {
+    bar_id: i32,
+}
+
+#[derive(Associations)]
+#[belongs_to(Bar)]
+#[table_name = "foo"]
+struct Foo3 {
+    #[column_name = "bar_id"]
+    baz_id: i32,
+}
+
+#[derive(Associations)]
+#[belongs_to(Bar, foreign_key = "bar_id")]
+#[table_name = "foo"]
+struct Foo4 {
+    #[column_name = "bar_id"]
+    baz_id: i32,
+}
+
+fn main() {}

diesel_compile_tests/tests/ui/belongs_to_missing_foreign_key_column.rs

@@ -0,0 +1,50 @@
+error[E0412]: cannot find type `bar_id` in module `foo`
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(Bar)]
+  | ^ not found in `foo`
+
+error[E0425]: cannot find value `bar_id` in module `foo`
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(Bar)]
+  | ^ not found in `foo`
+
+error[E0412]: cannot find type `bar_id` in module `foo`
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(Bar, foreign_key = "bar_id")]
+  | ^ not found in `foo`
+
+error[E0425]: cannot find value `bar_id` in module `foo`
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(Bar, foreign_key = "bar_id")]
+  | ^ not found in `foo`
+
+error[E0412]: cannot find type `bar_id` in module `foo`
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(Bar)]
+  | ^ not found in `foo`
+
+error[E0425]: cannot find value `bar_id` in module `foo`
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(Bar)]
+  | ^ not found in `foo`
+
+error[E0412]: cannot find type `bar_id` in module `foo`
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(Bar, foreign_key = "bar_id")]
+  | ^ not found in `foo`
+
+error[E0425]: cannot find value `bar_id` in module `foo`
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(Bar, foreign_key = "bar_id")]
+  | ^ not found in `foo`
+
+error: aborting due to 8 previous errors
+

diesel_compile_tests/tests/ui/belongs_to_missing_foreign_key_column.stderr

@@ -0,0 +1,19 @@
+#[macro_use]
+extern crate diesel;
+
+struct Bar;
+
+#[derive(Associations)]
+#[belongs_to(Bar)]
+#[belongs_to(Bar, foreign_key = "bar_id")]
+struct Foo {}
+
+#[derive(Associations)]
+#[belongs_to(Bar)]
+#[belongs_to(Bar, foreign_key = "bar_id")]
+struct Baz {
+    #[column_name = "baz_id"]
+    bar_id: i32,
+}
+
+fn main() {}

diesel_compile_tests/tests/ui/belongs_to_missing_foreign_key_field.rs

@@ -0,0 +1,26 @@
+error: No field with column name bar_id
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(Bar)]
+  | ^
+
+error: No field with column name bar_id
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(Bar, foreign_key = "bar_id")]
+  | ^
+
+error: No field with column name bar_id
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(Bar)]
+  | ^
+
+error: No field with column name bar_id
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(Bar, foreign_key = "bar_id")]
+  | ^
+
+error: aborting due to 4 previous errors
+

diesel_compile_tests/tests/ui/belongs_to_missing_foreign_key_field.stderr

@@ -0,0 +1,17 @@
+#[macro_use]
+extern crate diesel;
+
+table! {
+    foos {
+        id -> Integer,
+        bar_id -> Integer,
+    }
+}
+
+#[derive(Associations)]
+#[belongs_to(Bar)]
+struct Foo {
+    bar_id: i32,
+}
+
+fn main() {}

diesel_compile_tests/tests/ui/belongs_to_missing_parent_import.rs

@@ -0,0 +1,8 @@
+error[E0412]: cannot find type `Bar` in this scope
+ --> <macro expansion>:1:1
+  |
+1 | #[belongs_to(Bar)]
+  | ^ not found in this scope
+
+error: aborting due to previous error
+

diesel_compile_tests/tests/ui/belongs_to_missing_parent_import.stderr

@@ -25,7 +25,6 @@ extern crate quote;
 extern crate syn;
 
 mod as_expression;
-mod associations;
 mod ast_builder;
 mod attr;
 mod from_sql_row;
@@ -61,11 +60,6 @@ pub fn derive_insertable(input: TokenStream) -> TokenStream {
     expand_derive(input, insertable::derive_insertable)
 }
 
-#[proc_macro_derive(Associations, attributes(table_name, belongs_to))]
-pub fn derive_associations(input: TokenStream) -> TokenStream {
-    expand_derive(input, associations::derive_associations)
-}
-
 #[proc_macro_derive(QueryId)]
 pub fn derive_query_id(input: TokenStream) -> TokenStream {
     expand_derive(input, query_id::derive)