2 files changed, 31 insertions, 2 deletions

diff --git a/editors/code/src/debug.ts b/editors/code/src/debug.ts
index 3889a2773..fe8ec1be4 100644
--- a/editors/code/src/debug.ts
+++ b/editors/code/src/debug.ts
@@ -3,7 +3,7 @@ import * as vscode from 'vscode';
 import * as path from 'path';
 import * as ra from './lsp_ext';
 
-import { Cargo } from './toolchain';
+import { Cargo, sysrootForDir as getSysroot } from './toolchain';
 import { Ctx } from "./ctx";
 import { prepareEnv } from "./run";
 
@@ -104,7 +104,15 @@ async function getDebugConfiguration(ctx: Ctx, runnable: ra.Runnable): Promise<v
 
     const executable = await getDebugExecutable(runnable);
     const env = prepareEnv(runnable, ctx.config.runnableEnv);
-    const debugConfig = knownEngines[debugEngine.id](runnable, simplifyPath(executable), env, debugOptions.sourceFileMap);
+    let sourceFileMap = debugOptions.sourceFileMap;
+    if ( !sourceFileMap || Object.keys(sourceFileMap).length === 0 ) {
+        // let's try to use the default toolchain
+        const sysroot = await getSysroot(wsFolder);
+        const rustlib_src = path.normalize(sysroot + "/lib/rustlib/src/rust");
+        sourceFileMap = { "/rustc/*": rustlib_src };
+    }
+
+    const debugConfig = knownEngines[debugEngine.id](runnable, simplifyPath(executable), env, sourceFileMap);
     if (debugConfig.type in debugOptions.engineSettings) {
         const settingsMap = (debugOptions.engineSettings as any)[debugConfig.type];
         for (var key in settingsMap) {
diff --git a/editors/code/src/toolchain.ts b/editors/code/src/toolchain.ts
index a5dc3cf0c..b746da1d9 100644
--- a/editors/code/src/toolchain.ts
+++ b/editors/code/src/toolchain.ts
@@ -121,6 +121,27 @@ export class Cargo {
     }
 }
 
+/** Mirrors `project_model::sysroot::discover_sysroot_dir()` implementation*/
+export function sysrootForDir(dir: string): Promise<string> {
+    const rustc_path = getPathForExecutable("rustc");
+
+    return new Promise((resolve, reject) => {
+        cp.exec(`${rustc_path} --print sysroot`, { cwd: dir }, (err, stdout, stderr) => {
+            if (err) {
+                reject(err);
+                return;
+            }
+
+            if (stderr) {
+                reject(new Error(stderr));
+                return;
+            }
+
+            resolve(stdout.trimEnd());
+        });
+    });
+}
+
 /** Mirrors `toolchain::cargo()` implementation */
 export function cargoPath(): string {
     return getPathForExecutable("cargo");