Introduce ra_db::fixture fixture module

The goal here is to share more testing infrastructure between crates.

crates/ra_db/src/fixture.rs

@@ -0,0 +1,40 @@
+//! FIXME: write short doc here
+
+use std::sync::Arc;
+
+use ra_cfg::CfgOptions;
+
+use crate::{
+    CrateGraph, Edition, FileId, RelativePathBuf, SourceDatabaseExt, SourceRoot, SourceRootId,
+};
+
+pub const WORKSPACE: SourceRootId = SourceRootId(0);
+
+pub trait WithFixture: Default + SourceDatabaseExt + 'static {
+    fn with_single_file(text: &str) -> (Self, FileId) {
+        let mut db = Self::default();
+        let file_id = with_single_file(&mut db, text);
+        (db, file_id)
+    }
+}
+
+impl<DB: SourceDatabaseExt + Default + 'static> WithFixture for DB {}
+
+fn with_single_file(db: &mut dyn SourceDatabaseExt, text: &str) -> FileId {
+    let file_id = FileId(0);
+    let rel_path: RelativePathBuf = "/main.rs".into();
+
+    let mut source_root = SourceRoot::default();
+    source_root.insert_file(rel_path.clone(), file_id);
+
+    let mut crate_graph = CrateGraph::default();
+    crate_graph.add_crate_root(file_id, Edition::Edition2018, CfgOptions::default());
+
+    db.set_file_text(file_id, Arc::new(text.to_string()));
+    db.set_file_relative_path(file_id, rel_path);
+    db.set_file_source_root(file_id, WORKSPACE);
+    db.set_source_root(WORKSPACE, Arc::new(source_root));
+    db.set_crate_graph(Arc::new(crate_graph));
+
+    file_id
+}