fix(diagnostics): map solc offsets against the live buffer, not disk

docs/pages/reference/diagnostics.md

@@ -43,6 +43,22 @@ Diagnostic fields:
 - `code`: numeric solc error code (e.g. `2072`)
 - `severity`: mapped from solc severity string (`"error"` → `ERROR`, `"warning"` → `WARNING`, `"info"` → `INFORMATION`)
 
+#### Which text offsets are resolved against
+
+solc reports locations as byte offsets into the source it was given, so each
+offset must be converted using that same text — otherwise the reported range
+drifts.
+
+- **The file being compiled**: its offsets are resolved against the buffer text
+  passed to `solc_ast()`, which is what the editor currently shows. Resolving
+  them against the file on disk instead puts every range in the file at the
+  wrong position whenever the buffer has unsaved edits, which happens routinely
+  because `run_did_save()` falls back to the text cache when a client omits the
+  optional `text` field on `didSave`.
+- **Its imports**: those reach solc as `"urls"` rather than inlined content, so
+  solc reads them from disk itself and `build::cross_file_error_diagnostics()`
+  correctly resolves their offsets against disk.
+
 ### Source 2: forge lint
 
 Lint diagnostics come from `forge lint --json`.

src/lsp.rs

@@ -804,14 +804,13 @@ impl ForgeLsp {
                 );
                 match solc {
                     Ok(data) => {
-                        // Extract diagnostics from the same solc output
-                        let content = tokio::fs::read_to_string(&file_path)
-                            .await
-                            .unwrap_or_default();
+                        // Extract diagnostics from the same live buffer text
+                        // passed to solc so byte offsets and LSP ranges stay
+                        // aligned with the editor, not a stale on-disk file.
                         let build_diags = crate::build::build_output_to_diagnostics(
                             &data,
                             &file_path,
-                            &content,
+                            &params.text,
                             &foundry_cfg.ignored_error_codes,
                         );
                         (Some(lint), Ok(build_diags), Ok(data))
@@ -839,13 +838,10 @@ impl ForgeLsp {
                     .await;
                 match solc_future.await {
                     Ok(data) => {
-                        let content = tokio::fs::read_to_string(&file_path)
-                            .await
-                            .unwrap_or_default();
                         let build_diags = crate::build::build_output_to_diagnostics(
                             &data,
                             &file_path,
-                            &content,
+                            &params.text,
                             &foundry_cfg.ignored_error_codes,
                         );
                         (None, Ok(build_diags), Ok(data))