Auto merge of #18192 - darichey:read-buildfile-into-vfs, r=Veykril

Include buildfiles in VFS

We subscribe to `textDocument/didSave` for `filesToWatch`, but the VFS doesn't contain those files. Before https://github.com/rust-lang/rust-analyzer/pull/18105, this would bring down the server. Now, it's only a benign error logged:
```
ERROR notification handler failed handler=textDocument/didSave error=file not found: /foo/bar/TARGETS
```
It's benign, because we will also receive a `workspace/didChangeWatchedFiles` for the file which will invalidate and load it.

Explicitly include the buildfiles in the VFS to prevent the handler from erroring.

crates/load-cargo/src/lib.rs

@@ -265,6 +265,11 @@ impl ProjectFolders {
                 entries.push(manifest.to_owned());
             }
 
+            for buildfile in ws.buildfiles() {
+                file_set_roots.push(VfsPath::from(buildfile.to_owned()));
+                entries.push(buildfile.to_owned());
+            }
+
             // In case of detached files we do **not** look for a rust-analyzer.toml.
             if !matches!(ws.kind, ProjectWorkspaceKind::DetachedFile { .. }) {
                 let ws_root = ws.workspace_root();

crates/project-model/src/workspace.rs

@@ -539,6 +539,17 @@ impl ProjectWorkspace {
         }
     }
 
+    pub fn buildfiles(&self) -> Vec<AbsPathBuf> {
+        match &self.kind {
+            ProjectWorkspaceKind::Json(project) => project
+                .crates()
+                .filter_map(|(_, krate)| krate.build.as_ref().map(|build| build.build_file.clone()))
+                .map(AbsPathBuf::assert)
+                .collect(),
+            _ => vec![],
+        }
+    }
+
     pub fn find_sysroot_proc_macro_srv(&self) -> anyhow::Result<AbsPathBuf> {
         self.sysroot.discover_proc_macro_srv()
     }