Make stock strings rusty

This converts the stock strings API to be more safe-rust style.  The
API is kept roughly the same for now but moved to methods on the
context.

src/test_utils.rs

@@ -0,0 +1,50 @@
+//! Utilities to help writing tests.
+//!
+//! This module is only compiled for test runs.
+
+use tempfile::{tempdir, TempDir};
+
+use crate::context::{dc_context_new, dc_open, Context};
+use crate::types::dc_callback_t;
+
+use crate::dc_tools::OsStrExt;
+
+/// A Context and temporary directory.
+///
+/// The temporary directory can be used to store the SQLite database,
+/// see e.g. [test_context] which does this.
+pub struct TestContext {
+    pub ctx: Context,
+    pub dir: TempDir,
+}
+
+/// Create a new, opened [TestContext] using given callback.
+///
+/// The [Context] will be opened with the SQLite database named
+/// "db.sqlite" in the [TestContext.dir] directory.
+///
+/// [Context]: crate::context::Context
+pub fn test_context(cb: Option<dc_callback_t>) -> TestContext {
+    unsafe {
+        let mut ctx = dc_context_new(cb, std::ptr::null_mut(), std::ptr::null_mut());
+        let dir = tempdir().unwrap();
+        let dbfile = dir.path().join("db.sqlite");
+        let dbfile_c = dbfile.to_c_string().unwrap();
+        assert_eq!(
+            dc_open(&mut ctx, dbfile_c.as_ptr(), std::ptr::null()),
+            1,
+            "Failed to open {}",
+            dbfile.display(),
+        );
+        TestContext { ctx: ctx, dir: dir }
+    }
+}
+
+/// Return a dummy [TestContext].
+///
+/// The context will be opened and use the SQLite database as
+/// specified in [test_context] but there is no callback hooked up,
+/// i.e. [Context::call_cb] will always return `0`.
+pub fn dummy_context() -> TestContext {
+    test_context(None)
+}