2 files changed, 99 insertions, 0 deletions

diff --git a/crates/base_db/src/change.rs b/crates/base_db/src/change.rs
new file mode 100644
index 000000000..043e03bba
--- /dev/null
+++ b/crates/base_db/src/change.rs
@@ -0,0 +1,97 @@
+//! Defines a unit of change that can applied to the database to get the next
+//! state. Changes are transactional.
+
+use std::{fmt, sync::Arc};
+
+use rustc_hash::FxHashSet;
+use salsa::Durability;
+use vfs::FileId;
+
+use crate::{CrateGraph, SourceDatabaseExt, SourceRoot, SourceRootId};
+
+/// Encapsulate a bunch of raw `.set` calls on the database.
+#[derive(Default)]
+pub struct Change {
+    pub roots: Option<Vec<SourceRoot>>,
+    pub files_changed: Vec<(FileId, Option<Arc<String>>)>,
+    pub crate_graph: Option<CrateGraph>,
+}
+
+impl fmt::Debug for Change {
+    fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
+        let mut d = fmt.debug_struct("AnalysisChange");
+        if let Some(roots) = &self.roots {
+            d.field("roots", roots);
+        }
+        if !self.files_changed.is_empty() {
+            d.field("files_changed", &self.files_changed.len());
+        }
+        if self.crate_graph.is_some() {
+            d.field("crate_graph", &self.crate_graph);
+        }
+        d.finish()
+    }
+}
+
+impl Change {
+    pub fn new() -> Change {
+        Change::default()
+    }
+
+    pub fn set_roots(&mut self, roots: Vec<SourceRoot>) {
+        self.roots = Some(roots);
+    }
+
+    pub fn change_file(&mut self, file_id: FileId, new_text: Option<Arc<String>>) {
+        self.files_changed.push((file_id, new_text))
+    }
+
+    pub fn set_crate_graph(&mut self, graph: CrateGraph) {
+        self.crate_graph = Some(graph);
+    }
+
+    pub fn apply(self, db: &mut dyn SourceDatabaseExt) {
+        let _p = profile::span("RootDatabase::apply_change");
+        // db.request_cancellation();
+        // log::info!("apply_change {:?}", change);
+        if let Some(roots) = self.roots {
+            let mut local_roots = FxHashSet::default();
+            let mut library_roots = FxHashSet::default();
+            for (idx, root) in roots.into_iter().enumerate() {
+                let root_id = SourceRootId(idx as u32);
+                let durability = durability(&root);
+                if root.is_library {
+                    library_roots.insert(root_id);
+                } else {
+                    local_roots.insert(root_id);
+                }
+                for file_id in root.iter() {
+                    db.set_file_source_root_with_durability(file_id, root_id, durability);
+                }
+                db.set_source_root_with_durability(root_id, Arc::new(root), durability);
+            }
+            // db.set_local_roots_with_durability(Arc::new(local_roots), Durability::HIGH);
+            // db.set_library_roots_with_durability(Arc::new(library_roots), Durability::HIGH);
+        }
+
+        for (file_id, text) in self.files_changed {
+            let source_root_id = db.file_source_root(file_id);
+            let source_root = db.source_root(source_root_id);
+            let durability = durability(&source_root);
+            // XXX: can't actually remove the file, just reset the text
+            let text = text.unwrap_or_default();
+            db.set_file_text_with_durability(file_id, text, durability)
+        }
+        if let Some(crate_graph) = self.crate_graph {
+            db.set_crate_graph_with_durability(Arc::new(crate_graph), Durability::HIGH)
+        }
+    }
+}
+
+fn durability(source_root: &SourceRoot) -> Durability {
+    if source_root.is_library {
+        Durability::HIGH
+    } else {
+        Durability::LOW
+    }
+}
diff --git a/crates/base_db/src/lib.rs b/crates/base_db/src/lib.rs
index ee3415850..e38aa7257 100644
--- a/crates/base_db/src/lib.rs
+++ b/crates/base_db/src/lib.rs
@@ -1,6 +1,7 @@
 //! base_db defines basic database traits. The concrete DB is defined by ide.
 mod cancellation;
 mod input;
+mod change;
 pub mod fixture;
 
 use std::{panic, sync::Arc};
@@ -10,6 +11,7 @@ use syntax::{ast, Parse, SourceFile, TextRange, TextSize};
 
 pub use crate::{
     cancellation::Canceled,
+    change::Change,
     input::{
         CrateData, CrateGraph, CrateId, CrateName, Dependency, Edition, Env, FileId, ProcMacroId,
         SourceRoot, SourceRootId,