feat: Case-insensitive username and email fields

Problem
-------
It's common for typos to occur when entering usernames and emails that
result in random incorrect capitalization. Additionally, it's both a
security vulnerability and a usability issue if multiple users can
have the same username/email just with different capitalization.

Solution
--------
- Add a case-insensitive collation (we only officially support this on
  Postgres
- Assign the Username and Email columns of the User table to use the
  case-insensitive collation.

src/migration/collation/m20241022_065427_case_insensitive_collation.rs

@@ -0,0 +1,25 @@
+//! Migration to create a case-insensitive collation.
+//!
+//! See: <https://www.postgresql.org/docs/current/collation.html#COLLATION-NONDETERMINISTIC>
+//!
+//! Note: Currently only supports Postgres. If another DB is used, will do nothing.
+
+use crate::migration::collation::{
+    exec_create_case_insensitive_collation, exec_drop_case_insensitive_collation,
+};
+use async_trait::async_trait;
+use sea_orm_migration::prelude::*;
+
+#[derive(DeriveMigrationName)]
+pub struct Migration;
+
+#[async_trait]
+impl MigrationTrait for Migration {
+    async fn up(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+        exec_create_case_insensitive_collation(manager).await
+    }
+
+    async fn down(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+        exec_drop_case_insensitive_collation(manager).await
+    }
+}

src/migration/collation/mod.rs

@@ -0,0 +1,173 @@
+//! Utilities and migrations related to collations.
+
+pub mod m20241022_065427_case_insensitive_collation;
+
+use sea_orm::{DbBackend, Statement};
+use sea_orm_migration::prelude::*;
+
+/// Collations available from Roadster.
+#[derive(DeriveIden)]
+#[non_exhaustive]
+pub enum Collation {
+    /// The `default` collation. This comes included in Postgres.
+    ///
+    /// Note: This iden needs to be surrounded in quotes, at least in Postgres.
+    Default,
+    /// A case-insensitive collation.
+    CaseInsensitive,
+}
+
+/// Wrapper around [`create_case_insensitive_collation`] to execute the returned [`Statement`], if
+/// present.
+///
+/// # Examples
+/// ```rust
+/// use roadster::migration::collation::exec_create_case_insensitive_collation;
+/// use sea_orm_migration::prelude::*;
+///
+/// #[derive(DeriveMigrationName)]
+/// pub struct Migration;
+///
+/// #[async_trait::async_trait]
+/// impl MigrationTrait for Migration {
+///     async fn up(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+///         exec_create_case_insensitive_collation(manager).await
+///     }
+/// #
+/// #    async fn down(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+/// #        todo!()
+/// #    }
+/// }
+/// ```
+pub async fn exec_create_case_insensitive_collation(
+    manager: &SchemaManager<'_>,
+) -> Result<(), DbErr> {
+    let statement = create_case_insensitive_collation(manager);
+    if let Some(statement) = statement {
+        manager.get_connection().execute(statement).await?;
+    }
+
+    Ok(())
+}
+
+/// Create a case-insensitive collation.
+///
+/// See: <https://www.postgresql.org/docs/current/collation.html#COLLATION-NONDETERMINISTIC>
+///
+/// Note: Currently only supports Postgres. If another DB is used, will return [`None`].
+pub fn create_case_insensitive_collation(manager: &SchemaManager<'_>) -> Option<Statement> {
+    let backend = manager.get_database_backend();
+    create_case_insensitive_collation_for_db_backend(backend)
+}
+
+fn create_case_insensitive_collation_for_db_backend(backend: DbBackend) -> Option<Statement> {
+    if let DbBackend::Postgres = backend {
+        Some(Statement::from_string(
+            backend,
+            format!(
+                r#"CREATE COLLATION IF NOT EXISTS {} (
+provider = icu,
+locale = 'und-u-ks-level2',
+deterministic = false
+);
+"#,
+                Collation::CaseInsensitive.to_string()
+            ),
+        ))
+    } else {
+        None
+    }
+}
+
+/// Wrapper around [`drop_case_insensitive_collation`] to execute the returned [`Statement`], if
+/// present.
+///
+/// # Examples
+/// ```rust
+/// use roadster::migration::collation::exec_drop_case_insensitive_collation;
+/// use sea_orm_migration::prelude::*;
+///
+/// #[derive(DeriveMigrationName)]
+/// pub struct Migration;
+///
+/// #[async_trait::async_trait]
+/// impl MigrationTrait for Migration {
+/// #    async fn up(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+/// #        todo!()
+/// #    }
+/// #
+///     async fn down(&self, manager: &SchemaManager) -> Result<(), DbErr> {
+///         exec_drop_case_insensitive_collation(manager).await
+///     }
+/// }
+/// ```
+pub async fn exec_drop_case_insensitive_collation(
+    manager: &SchemaManager<'_>,
+) -> Result<(), DbErr> {
+    let statement = drop_case_insensitive_collation(manager);
+    if let Some(statement) = statement {
+        manager.get_connection().execute(statement).await?;
+    }
+
+    Ok(())
+}
+
+/// Drop the case-insensitive collation that was previously created by [`create_case_insensitive_collation`].
+///
+/// Note: Currently only supports Postgres. If another DB is used, will return [None].
+pub fn drop_case_insensitive_collation(manager: &SchemaManager<'_>) -> Option<Statement> {
+    let backend = manager.get_database_backend();
+    drop_case_insensitive_collation_for_db_backend(backend)
+}
+
+fn drop_case_insensitive_collation_for_db_backend(backend: DbBackend) -> Option<Statement> {
+    if let DbBackend::Postgres = backend {
+        Some(Statement::from_string(
+            backend,
+            format!(
+                "DROP COLLATION IF EXISTS {};",
+                Collation::CaseInsensitive.to_string()
+            ),
+        ))
+    } else {
+        None
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::testing::snapshot::TestCase;
+    use insta::assert_debug_snapshot;
+    use rstest::{fixture, rstest};
+    use sea_orm::DbBackend;
+
+    #[fixture]
+    fn case() -> TestCase {
+        Default::default()
+    }
+
+    #[rstest]
+    #[case(DbBackend::Postgres)]
+    #[case(DbBackend::MySql)]
+    #[case(DbBackend::Sqlite)]
+    #[cfg_attr(coverage_nightly, coverage(off))]
+    fn create_case_insensitive_collation_for_db_backend(
+        _case: TestCase,
+        #[case] backend: DbBackend,
+    ) {
+        let statement = super::create_case_insensitive_collation_for_db_backend(backend);
+
+        assert_debug_snapshot!(statement);
+    }
+
+    #[rstest]
+    #[case(DbBackend::Postgres)]
+    #[case(DbBackend::MySql)]
+    #[case(DbBackend::Sqlite)]
+    #[cfg_attr(coverage_nightly, coverage(off))]
+    fn drop_case_insensitive_collation_for_db_backend(_case: TestCase, #[case] backend: DbBackend) {
+        let statement = super::drop_case_insensitive_collation_for_db_backend(backend);
+
+        assert_debug_snapshot!(statement);
+    }
+}

src/migration/collation/snapshots/roadster__migration__collation__tests__create_case_insensitive_collation_for_db_backend@case_1.snap

@@ -0,0 +1,11 @@
+---
+source: src/migration/collation/mod.rs
+expression: statement
+---
+Some(
+    Statement {
+        sql: "CREATE COLLATION IF NOT EXISTS case_insensitive (\nprovider = icu,\nlocale = 'und-u-ks-level2',\ndeterministic = false\n);\n",
+        values: None,
+        db_backend: Postgres,
+    },
+)

src/migration/collation/snapshots/roadster__migration__collation__tests__create_case_insensitive_collation_for_db_backend@case_2.snap

@@ -0,0 +1,5 @@
+---
+source: src/migration/collation/mod.rs
+expression: statement
+---
+None

src/migration/collation/snapshots/roadster__migration__collation__tests__create_case_insensitive_collation_for_db_backend@case_3.snap

@@ -0,0 +1,5 @@
+---
+source: src/migration/collation/mod.rs
+expression: statement
+---
+None

src/migration/collation/snapshots/roadster__migration__collation__tests__drop_case_insensitive_collation_for_db_backend@case_1.snap

@@ -0,0 +1,11 @@
+---
+source: src/migration/collation/mod.rs
+expression: statement
+---
+Some(
+    Statement {
+        sql: "DROP COLLATION IF EXISTS case_insensitive;",
+        values: None,
+        db_backend: Postgres,
+    },
+)

src/migration/collation/snapshots/roadster__migration__collation__tests__drop_case_insensitive_collation_for_db_backend@case_2.snap

@@ -0,0 +1,5 @@
+---
+source: src/migration/collation/mod.rs
+expression: statement
+---
+None

src/migration/collation/snapshots/roadster__migration__collation__tests__drop_case_insensitive_collation_for_db_backend@case_3.snap

@@ -0,0 +1,5 @@
+---
+source: src/migration/collation/mod.rs
+expression: statement
+---
+None

src/migration/mod.rs

@@ -4,6 +4,7 @@
 //! Additionally, some utilities are provided to create some common column types.
 
 pub mod check;
+pub mod collation;
 pub mod schema;
 pub mod timestamp;
 pub mod user;

src/migration/timestamp/m20240723_201404_add_update_timestamp_function.rs

@@ -1,4 +1,4 @@
-//! Migration to create a SQL function to update the [Timestamps::UpdatedAt] column for a row
+//! Migration to create a SQL function to update the [`Timestamps::UpdatedAt`] column for a row
 //! with the current timestamp.
 //!
 //! Note: Currently only supports Postgres. If another DB is used, will do nothing.