Wrap all math operations in parens

Previously if you used local variables or parens in Rust to override the
default order of operations, we would completely lose that information
in the generated SQL. This would at best result in an unexpected value,
but at worse would cause a query like `now + 2.minutes() * (some_int +
1)` to result in a runtime error complaining that there's no operator
for `interval + integer`

I intend to backport this to 1.4

Fixes #2017

CHANGELOG.md

@@ -31,6 +31,12 @@ for Rust libraries in [RFC #1105](https://github.com/rust-lang/rfcs/blob/master/
   have been. All types in Diesel are now correctly only considered
   non-aggregate if their parts are.
 
+* Parenthesis are now inserted around all mathematical operations. This means
+  that `(2.into_sql() + 3) * 4` will correctly evaluate to 20 as expected.
+  Previously we would generate SQL that evaluated to 14. This could even result
+  in runtime errors if multiple types were involved (for example, `interval *
+  (integer + 1)`)
+
 ### Deprecated
 
 * `diesel_(prefix|postfix|infix)_operator!` have been deprecated. These macros

diesel/src/expression/ops/numeric.rs

@@ -37,9 +37,11 @@ macro_rules! numeric_operation {
             Rhs: QueryFragment<DB>,
         {
             fn walk_ast(&self, mut out: AstPass<DB>) -> QueryResult<()> {
+                out.push_sql("(");
                 self.lhs.walk_ast(out.reborrow())?;
                 out.push_sql($op);
                 self.rhs.walk_ast(out.reborrow())?;
+                out.push_sql(")");
                 Ok(())
             }
         }

diesel_tests/tests/expressions/ops.rs

@@ -174,6 +174,7 @@ fn test_dividing_nullables() {
     let data = nullable_table.select(value / value).load(&connection);
     assert_eq!(Ok(expected_data), data);
 }
+
 #[test]
 fn mix_and_match_all_numeric_ops() {
     use schema::users::dsl::*;
@@ -190,3 +191,12 @@ fn mix_and_match_all_numeric_ops() {
     let data = users.select(id * 3 / 2 + 4 - 1).load(&connection);
     assert_eq!(Ok(expected_data), data);
 }
+
+#[test]
+fn precedence_with_parens_is_maintained() {
+    use diesel::sql_types::Integer;
+
+    let x = select((2.into_sql::<Integer>() + 3) * 4)
+        .get_result::<i32>(&connection());
+    assert_eq!(Ok(20), x);
+}