Issue 1135: Associated constants (diesel-rs#1145)

* 1135: Change has_static_query_id() to be associated const

This replaces `fn has_static_query_id() -> bool` with
`const HAS_STATIC_QUERY_ID: bool` on the trait `QueryId`.

* remove allow associated consts, since this feature is now stable

* remove extra line above const declaration

* change tests to use const HAS_STATIC_QUERY_ID

* 1135: Use const NAME instead of fn name()

On trait `diesel::query_source::Column`, use an associated
constant instead of a method to return the columns name. Also
change all references to this trait to use the constant.

* 1135: use const FIELDS_NEEDED instead of fields_needed()

In `diesel::types::FromSqlRow`, use an associated constant
instead of a function to return the number of fields needed
by structs implementing the FromSqlRow trait.

* Style change

Organize traits and their implementers to that they
have types, then consts, then functions, in that order, and with
blank lines between the groups.

* fix missing line and trailing whitespace

Author: willmurphyscode

diesel/src/expression/bound.rs

@@ -38,9 +38,7 @@ impl<T, U, DB> QueryFragment<DB> for Bound<T, U> where
 impl<T: QueryId, U> QueryId for Bound<T, U> {
     type QueryId = Bound<T::QueryId, ()>;
 
-    fn has_static_query_id() -> bool {
-        T::has_static_query_id()
-    }
+    const HAS_STATIC_QUERY_ID: bool = T::HAS_STATIC_QUERY_ID; 
 }
 
 impl<T, U, QS> SelectableExpression<QS> for Bound<T, U> where

diesel/src/expression/coerce.rs

@@ -60,9 +60,7 @@ impl<T, ST, DB> QueryFragment<DB> for Coerce<T, ST> where
 impl<T: QueryId, ST: 'static> QueryId for Coerce<T, ST> {
     type QueryId = Coerce<T::QueryId, ST>;
 
-    fn has_static_query_id() -> bool {
-        true
-    }
+    const HAS_STATIC_QUERY_ID: bool = true; 
 }
 
 impl<T, ST> NonAggregate for Coerce<T, ST> where

diesel/src/expression/mod.rs

@@ -229,7 +229,5 @@ impl<QS, T, DB> BoxableExpression<QS, DB> for T where
 impl<QS, ST, DB> QueryId for BoxableExpression<QS, DB, SqlType=ST> {
     type QueryId = ();
 
-    fn has_static_query_id() -> bool {
-        false
-    }
+    const HAS_STATIC_QUERY_ID: bool = false; 
 }

diesel/src/expression/nullable.rs

@@ -40,9 +40,7 @@ impl<T, QS> AppearsOnTable<QS> for Nullable<T> where
 impl<T: QueryId> QueryId for Nullable<T> {
     type QueryId = T::QueryId;
 
-    fn has_static_query_id() -> bool {
-        T::has_static_query_id()
-    }
+    const HAS_STATIC_QUERY_ID: bool = T::HAS_STATIC_QUERY_ID;
 }
 
 impl<T> NonAggregate for Nullable<T> where

diesel/src/expression/operators.rs

@@ -329,7 +329,7 @@ impl<T, U, DB> Changeset<DB> for Eq<T, U> where
     }
 
     fn walk_ast(&self, mut out: AstPass<DB>) -> QueryResult<()> {
-        try!(out.push_identifier(T::name()));
+        try!(out.push_identifier(T::NAME));
         out.push_sql(" = ");
         QueryFragment::walk_ast(&self.right, out)
     }

diesel/src/expression/unchecked_bind.rs

@@ -33,9 +33,7 @@ impl<Query, Value, ST> QueryId for UncheckedBind<Query, Value, ST> where
 {
     type QueryId = UncheckedBind<Query::QueryId, (), ST::QueryId>;
 
-    fn has_static_query_id() -> bool {
-        Query::has_static_query_id() && ST::has_static_query_id()
-    }
+    const HAS_STATIC_QUERY_ID: bool = Query::HAS_STATIC_QUERY_ID && ST::HAS_STATIC_QUERY_ID; 
 }
 
 impl<Query, Value, ST, DB> QueryFragment<DB> for UncheckedBind<Query, Value, ST> where

diesel/src/macros/mod.rs

@@ -68,9 +68,7 @@ macro_rules! __diesel_column {
         impl $crate::query_source::Column for $column_name {
             type Table = $($table)::*;
 
-            fn name() -> &'static str {
-                $sql_name
-            }
+            const NAME: &'static str = $sql_name;
         }
 
         impl<T> $crate::EqAll<T> for $column_name where

diesel/src/macros/query_id.rs

@@ -5,9 +5,7 @@ macro_rules! impl_query_id {
         impl $crate::query_builder::QueryId for $name {
             type QueryId = Self;
 
-            fn has_static_query_id() -> bool {
-                true
-            }
+            const HAS_STATIC_QUERY_ID: bool = true;
         }
     };
 
@@ -18,19 +16,15 @@ macro_rules! impl_query_id {
         {
             type QueryId = $name<$($ty_param::QueryId),*>;
 
-            fn has_static_query_id() -> bool {
-                $($ty_param::has_static_query_id() &&)* true
-            }
+            const HAS_STATIC_QUERY_ID: bool = $($ty_param::HAS_STATIC_QUERY_ID &&)* true;
         }
     };
 
     (noop: $name: ident) => {
         impl $crate::query_builder::QueryId for $name {
             type QueryId = ();
 
-            fn has_static_query_id() -> bool {
-                false
-            }
+            const HAS_STATIC_QUERY_ID: bool = false;
         }
     };
 
@@ -39,9 +33,7 @@ macro_rules! impl_query_id {
         impl<$($ty_param),*> $crate::query_builder::QueryId for $name<$($ty_param),*> {
             type QueryId = ();
 
-            fn has_static_query_id() -> bool {
-                false
-            }
+            const HAS_STATIC_QUERY_ID: bool = false;
         }
     }
 }

diesel/src/mysql/types/mod.rs

@@ -78,9 +78,7 @@ impl HasSqlType<::types::Numeric> for Mysql {
 impl QueryId for ::types::Numeric {
     type QueryId = Self;
 
-    fn has_static_query_id() -> bool {
-        true
-    }
+    const HAS_STATIC_QUERY_ID: bool = true;
 }
 
 /// Represents the MySQL datetime type.

diesel/src/pg/upsert/on_conflict_actions.rs

@@ -169,7 +169,7 @@ impl<T> QueryFragment<Pg> for Excluded<T> where
 {
     fn walk_ast(&self, mut out: AstPass<Pg>) -> QueryResult<()> {
         out.push_sql("excluded.");
-        try!(out.push_identifier(T::name()));
+        try!(out.push_identifier(T::NAME));
         Ok(())
     }
 }