Constrain cyclic associated types (#2702)

Add bounds such that cyclic associated types are equal to themselves.
```
<T as Connection>::Database as Database>::Connection == T
<T as ConnectOptions>::Connection as Connection>::Options == T
<T as Row>::Database as Database>::Row == T
<T as Column>::Database as Database>::Column == T
<T as Value>::Database as Database>::Value == T
```

Author: BadBastion

sqlx-core/src/column.rs

@@ -4,7 +4,7 @@ use crate::error::Error;
 use std::fmt::Debug;
 
 pub trait Column: 'static + Send + Sync + Debug {
-    type Database: Database;
+    type Database: Database<Column = Self>;
 
     /// Gets the column ordinal.
     ///

sqlx-core/src/connection.rs

@@ -11,7 +11,7 @@ use url::Url;
 
 /// Represents a single database connection.
 pub trait Connection: Send {
-    type Database: Database;
+    type Database: Database<Connection = Self>;
 
     type Options: ConnectOptions<Connection = Self>;
 
@@ -184,7 +184,7 @@ impl LogSettings {
 }
 
 pub trait ConnectOptions: 'static + Send + Sync + FromStr<Err = Error> + Debug + Clone {
-    type Connection: Connection + ?Sized;
+    type Connection: Connection<Options = Self> + ?Sized;
 
     /// Parse the `ConnectOptions` from a URL.
     fn from_url(url: &Url) -> Result<Self, Error>;

sqlx-core/src/row.rs

@@ -12,7 +12,7 @@ use crate::value::ValueRef;
 /// [`FromRow`]: crate::row::FromRow
 /// [`Query::fetch`]: crate::query::Query::fetch
 pub trait Row: Unpin + Send + Sync + 'static {
-    type Database: Database;
+    type Database: Database<Row = Self>;
 
     /// Returns `true` if this row has no columns.
     #[inline]

sqlx-core/src/value.rs

@@ -7,7 +7,7 @@ use std::borrow::Cow;
 
 /// An owned value from the database.
 pub trait Value {
-    type Database: Database;
+    type Database: Database<Value = Self>;
 
     /// Get this value as a reference.
     fn as_ref(&self) -> <Self::Database as Database>::ValueRef<'_>;