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diff --git a/crates/vfs/src/lib.rs b/crates/vfs/src/lib.rs
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+//! # Virtual File System
+//!
+//! VFS stores all files read by rust-analyzer. Reading file contents from VFS
+//! always returns the same contents, unless VFS was explicitly modified with
+//! `set_file_contents`. All changes to VFS are logged, and can be retrieved via
+//! `take_changes` method. The pack of changes is then pushed to `salsa` and
+//! triggers incremental recomputation.
+//!
+//! Files in VFS are identified with `FileId`s -- interned paths. The notion of
+//! the path, `VfsPath` is somewhat abstract: at the moment, it is represented
+//! as an `std::path::PathBuf` internally, but this is an implementation detail.
+//!
+//! VFS doesn't do IO or file watching itself. For that, see the `loader`
+//! module. `loader::Handle` is an object-safe trait which abstracts both file
+//! loading and file watching. `Handle` is dynamically configured with a set of
+//! directory entries which should be scanned and watched. `Handle` then
+//! asynchronously pushes file changes. Directory entries are configured in
+//! free-form via list of globs, it's up to the `Handle` to interpret the globs
+//! in any specific way.
+//!
+//! A simple `WalkdirLoaderHandle` is provided, which doesn't implement watching
+//! and just scans the directory using walkdir.
+//!
+//! VFS stores a flat list of files. `FileSet` can partition this list of files
+//! into disjoint sets of files. Traversal-like operations (including getting
+//! the neighbor file by the relative path) are handled by the `FileSet`.
+//! `FileSet`s are also pushed to salsa and cause it to re-check `mod foo;`
+//! declarations when files are created or deleted.
+//!
+//! `file_set::FileSet` and `loader::Entry` play similar, but different roles.
+//! Both specify the "set of paths/files", one is geared towards file watching,
+//! the other towards salsa changes. In particular, single `file_set::FileSet`
+//! may correspond to several `loader::Entry`. For example, a crate from
+//! crates.io which uses code generation would have two `Entries` -- for sources
+//! in `~/.cargo`, and for generated code in `./target/debug/build`. It will
+//! have a single `FileSet` which unions the two sources.
+mod vfs_path;
+mod path_interner;
+pub mod file_set;
+pub mod loader;
+pub mod walkdir_loader;
+
+use std::{fmt, mem};
+
+use crate::path_interner::PathInterner;
+
+pub use crate::vfs_path::VfsPath;
+pub use paths::{AbsPath, AbsPathBuf};
+
+#[derive(Copy, Clone, Debug, Ord, PartialOrd, Eq, PartialEq, Hash)]
+pub struct FileId(pub u32);
+
+#[derive(Default)]
+pub struct Vfs {
+    interner: PathInterner,
+    data: Vec<Option<Vec<u8>>>,
+    changes: Vec<ChangedFile>,
+}
+
+pub struct ChangedFile {
+    pub file_id: FileId,
+    pub change_kind: ChangeKind,
+}
+
+impl ChangedFile {
+    pub fn exists(&self) -> bool {
+        self.change_kind != ChangeKind::Delete
+    }
+    pub fn is_created_or_deleted(&self) -> bool {
+        matches!(self.change_kind, ChangeKind::Create | ChangeKind::Delete)
+    }
+}
+
+#[derive(Eq, PartialEq)]
+pub enum ChangeKind {
+    Create,
+    Modify,
+    Delete,
+}
+
+impl Vfs {
+    pub fn len(&self) -> usize {
+        self.data.len()
+    }
+    pub fn file_id(&self, path: &VfsPath) -> Option<FileId> {
+        self.interner.get(path).filter(|&it| self.get(it).is_some())
+    }
+    pub fn file_path(&self, file_id: FileId) -> VfsPath {
+        self.interner.lookup(file_id).clone()
+    }
+    pub fn file_contents(&self, file_id: FileId) -> &[u8] {
+        self.get(file_id).as_deref().unwrap()
+    }
+    pub fn iter(&self) -> impl Iterator<Item = (FileId, VfsPath)> + '_ {
+        (0..self.data.len())
+            .map(|it| FileId(it as u32))
+            .filter(move |&file_id| self.get(file_id).is_some())
+            .map(move |file_id| {
+                let path = self.interner.lookup(file_id).clone();
+                (file_id, path)
+            })
+    }
+    pub fn set_file_contents(&mut self, path: VfsPath, contents: Option<Vec<u8>>) {
+        let file_id = self.alloc_file_id(path);
+        let change_kind = match (&self.get(file_id), &contents) {
+            (None, None) => return,
+            (None, Some(_)) => ChangeKind::Create,
+            (Some(_), None) => ChangeKind::Delete,
+            (Some(old), Some(new)) if old == new => return,
+            (Some(_), Some(_)) => ChangeKind::Modify,
+        };
+
+        *self.get_mut(file_id) = contents;
+        self.changes.push(ChangedFile { file_id, change_kind })
+    }
+    pub fn has_changes(&self) -> bool {
+        !self.changes.is_empty()
+    }
+    pub fn take_changes(&mut self) -> Vec<ChangedFile> {
+        mem::take(&mut self.changes)
+    }
+    fn alloc_file_id(&mut self, path: VfsPath) -> FileId {
+        let file_id = self.interner.intern(path);
+        let idx = file_id.0 as usize;
+        let len = self.data.len().max(idx + 1);
+        self.data.resize_with(len, || None);
+        file_id
+    }
+    fn get(&self, file_id: FileId) -> &Option<Vec<u8>> {
+        &self.data[file_id.0 as usize]
+    }
+    fn get_mut(&mut self, file_id: FileId) -> &mut Option<Vec<u8>> {
+        &mut self.data[file_id.0 as usize]
+    }
+}
+
+impl fmt::Debug for Vfs {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Vfs").field("n_files", &self.data.len()).finish()
+    }
+}