WIP [next]: implement generalized query placeholders

Signed-off-by: Austin Bonander <austin@launchbadge.com>

sqlx-core/Cargo.toml

@@ -52,3 +52,4 @@ memchr = "2.3"
 conquer-once = { version = "0.3.2", optional = true }
 parking_lot = { version = "0.11.1", optional = true }
 crossbeam = { version = "0.8.0", optional = true }
+combine = "4.5.2"

sqlx-core/src/database.rs

@@ -26,6 +26,15 @@ pub trait Database:
 
     /// The concrete [`TypeId`] implementation for this database.
     type TypeId: 'static + PartialEq + Hash + Clone + Copy + Send + Sync;
+
+    /// The character used to prefix bind parameter placeholders, e.g. `$` for Postgres, `?` for MySQL, etc.
+    const PLACEHOLDER_CHAR: char;
+
+    /// The indexing type for bind parameters.
+    ///
+    /// E.g. `Implicit` for MySQL which just does `SELECT 1 FROM foo WHERE bar = ? AND baz = ?`
+    /// or `OneIndexed` for Postgres which does `SELECT 1 FROM foo WHERE bar = $1 AND baz = $2`
+    const PARAM_INDEXING: crate::placeholders::ParamIndexing;
 }
 
 /// Associates [`Database`] with an `Output` of a generic lifetime.

sqlx-core/src/error.rs

@@ -85,6 +85,9 @@ pub enum Error {
     /// An error occurred encoding a value for a specific parameter to
     /// be sent to the database.
     ParameterEncode { parameter: Either<usize, Box<str>>, source: EncodeError },
+
+    /// An error occurred while parsing or expanding the generic placeholder syntax in a query.
+    Placeholders(crate::placeholders::Error),
 }
 
 impl Error {
@@ -179,6 +182,8 @@ impl Display for Error {
             Self::ParameterEncode { parameter: Either::Right(name), source } => {
                 write!(f, "Encode parameter `{}`: {}", name, source)
             }
+
+            Self::Placeholders(e) => e.fmt(f),
         }
     }
 }
@@ -188,7 +193,7 @@ impl StdError for Error {
         match self {
             Self::ConnectOptions { source: Some(source), .. } => Some(&**source),
             Self::Network(source) => Some(source),
-
+            Self::Placeholders(source) => Some(source),
             _ => None,
         }
     }

sqlx-core/src/lib.rs

@@ -49,6 +49,9 @@ pub mod io;
 #[doc(hidden)]
 pub mod net;
 
+#[doc(hidden)]
+pub mod placeholders;
+
 #[doc(hidden)]
 #[cfg(feature = "_mock")]
 pub mod mock;