Improve the error message when insert is called with an owned value

Prior to this commit, the error will occur when you attempt to call
`execute`, `get_result`, or `get_results`. The message will indicate
that those methods don't exist, because `InsertQuery` doesn't implement
`QueryFragment`, with a very long generic type signature. The reason for
this being that `StructName` doesn't implement `Insertable`,
`&StructName` does.

This now handles this common error as quickly and directly as possible.
The error message will now be:

```
error: mismatched types:
 expected `&_`
    found `StructName`
```

This does cause a minor change in functionality, as it becomes
impossible to pass an owned value to `insert`. I don't think this is a
problem, as really the only thing this removes is the ability to
construct an insert statement and the return the query without executing
it, e.g.:

```rust
fn build_insert_query(name: &str) -> Box<QueryFragment<Pg>> {
    let new_user = NewUser { name: name };
    insert(new_user).into(users::table)
}
```

However, this case just doesn't seem like it matters, and I'm fine with
losing support for it.

Fixes diesel-rs#249.

Author: sgrif

CHANGELOG.md

@@ -18,6 +18,10 @@ for Rust libraries in [RFC #1105](https://github.com/rust-lang/rfcs/blob/master/
   change in the return type of `ToSql` and `FromSql` to have those bounds as
   well.
 
+* It is no longer possible to pass an owned value to `diesel::insert`. `insert`
+  will now give a more helpful error message when you accidentally try to pass
+  an owned value instead of a reference.
+
 ### Fixed
 
 * `#[insertable_into]` can now be used with structs that have lifetimes with

diesel/src/persistable.rs

@@ -8,7 +8,7 @@ use query_source::{Table, Column};
 /// Represents that a structure can be used to to insert a new row into the database.
 /// Implementations can be automatically generated by
 /// [`#[insertable_into]`](https://github.com/sgrif/diesel/tree/master/diesel_codegen#insertable_intotable_name).
-/// This is automatically implemented for `&[T]`, `Vec<T>` and `&Vec<T>` for inserting more than
+/// This is automatically implemented for `&[T]` and `&Vec<T>` for inserting more than
 /// one record.
 pub trait Insertable<T: Table, DB: Backend> {
     type Values: InsertValues<DB>;

diesel/src/query_builder/functions.rs

@@ -108,7 +108,7 @@ pub fn delete<T: UpdateTarget>(source: T) -> DeleteStatement<T> {
 /// Creates an insert statement. Will add the given data to a table. This
 /// function is not exported by default. As with other commands, the resulting
 /// query can return the inserted rows if you choose.
-pub fn insert<T>(records: T) -> IncompleteInsertStatement<T> {
+pub fn insert<'a, T: ?Sized>(records: &'a T) -> IncompleteInsertStatement<&'a T> {
     IncompleteInsertStatement::new(records)
 }
 

diesel_compile_tests/tests/compile-fail/insert_gives_helpful_error_message_when_passing_owned_value.rs

@@ -0,0 +1,31 @@
+#![feature(custom_derive, custom_attribute, plugin)]
+#![plugin(diesel_codegen)]
+
+#[macro_use]
+extern crate diesel;
+
+use diesel::*;
+use diesel::pg::PgConnection;
+
+table! {
+    users {
+        id -> Integer,
+        name -> VarChar,
+    }
+}
+
+#[insertable_into(users)]
+struct NewUser<'a> {
+    name: &'a str,
+}
+
+fn main() {
+    let connection = PgConnection::establish("").unwrap();
+    let new_user = NewUser { name: "Sean" };
+    insert(new_user).into(users::table).execute(&connection).unwrap();
+    //~^ ERROR mismatched types
+    //~| expected `&_`
+    //~| found `NewUser<'_>`
+    //~| ERROR E0275
+    // The E0275 is a quirk in Rust, not something we actually care about
+}