Allow SQLite to be linked against dynamically (#7057)

Keeping the default behaviour of SQLite being built statically,
so as not to break anyone's workflow, but allowing for downstream
packagers to link dynamically against SQLite (where it's fine because
that's the point of package managers).

Note that SQLite is still *not* enabled by default, thanks to the `?` operator.

Co-authored-by: Daniel García <dani-garcia@users.noreply.github.com>

Cargo.toml

@@ -23,15 +23,17 @@ publish.workspace = true
 
 [features]
 default = [
-    # "sqlite",
+    # "sqlite" or "sqlite_system",
     # "mysql",
     # "postgresql",
 ]
 # Empty to keep compatibility, prefer to set USE_SYSLOG=true
 enable_syslog = []
+# Please enable at least one of these DB backends.
 mysql = ["diesel/mysql", "diesel_migrations/mysql"]
 postgresql = ["diesel/postgres", "diesel_migrations/postgres"]
-sqlite = ["diesel/sqlite", "diesel_migrations/sqlite", "dep:libsqlite3-sys"]
+sqlite_system = ["diesel/sqlite", "diesel_migrations/sqlite"]
+sqlite = ["sqlite_system", "libsqlite3-sys/bundled"] # Alternative to the above, statically linked SQLite into the binary instead of dynamically.
 # Enable to use a vendored and statically linked openssl
 vendored_openssl = ["openssl/vendored"]
 # Enable MiMalloc memory allocator to replace the default malloc
@@ -94,8 +96,8 @@ diesel_migrations = "2.3.2"
 derive_more = { version = "2.1.1", features = ["from", "into", "as_ref", "deref", "display"] }
 diesel-derive-newtype = "2.1.2"
 
-# Bundled/Static SQLite
-libsqlite3-sys = { version = "0.37.0", features = ["bundled"], optional = true }
+# SQLite, statically bundled unless the `sqlite_system` feature is enabled
+libsqlite3-sys = { version = "0.37.0", optional = true }
 
 # Crypto-related libraries
 rand = "0.10.1"

build.rs

@@ -2,21 +2,21 @@ use std::env;
 use std::process::Command;
 
 fn main() {
-    // This allow using #[cfg(sqlite)] instead of #[cfg(feature = "sqlite")], which helps when trying to add them through macros
-    #[cfg(feature = "sqlite")]
+    // These allow using e.g. #[cfg(mysql)] instead of #[cfg(feature = "mysql")], which helps when trying to add them through macros
+    #[cfg(feature = "sqlite_system")] // The `sqlite` feature implies this one.
     println!("cargo:rustc-cfg=sqlite");
     #[cfg(feature = "mysql")]
     println!("cargo:rustc-cfg=mysql");
     #[cfg(feature = "postgresql")]
     println!("cargo:rustc-cfg=postgresql");
-    #[cfg(feature = "s3")]
-    println!("cargo:rustc-cfg=s3");
-
-    #[cfg(not(any(feature = "sqlite", feature = "mysql", feature = "postgresql")))]
+    #[cfg(not(any(feature = "sqlite_system", feature = "mysql", feature = "postgresql")))]
     compile_error!(
         "You need to enable one DB backend. To build with previous defaults do: cargo build --features sqlite"
     );
 
+    #[cfg(feature = "s3")]
+    println!("cargo:rustc-cfg=s3");
+
     // Use check-cfg to let cargo know which cfg's we define,
     // and avoid warnings when they are used in the code.
     println!("cargo::rustc-check-cfg=cfg(sqlite)");