Include better context for "already exists" error in compiletest

Cargo.lock

@@ -723,6 +723,7 @@ dependencies = [
 name = "compiletest"
 version = "0.0.0"
 dependencies = [
+ "anyhow",
  "build_helper",
  "colored",
  "diff",

src/tools/compiletest/Cargo.toml

@@ -20,6 +20,7 @@ once_cell = "1.16.0"
 walkdir = "2"
 glob = "0.3.0"
 lazycell = "1.3.0"
+anyhow = "1"
 
 [target.'cfg(unix)'.dependencies]
 libc = "0.2"

src/tools/compiletest/src/runtest.rs

@@ -32,6 +32,7 @@ use std::process::{Child, Command, ExitStatus, Output, Stdio};
 use std::str;
 use std::sync::Arc;
 
+use anyhow::Context;
 use glob::glob;
 use once_cell::sync::Lazy;
 use tracing::*;
@@ -131,7 +132,11 @@ pub fn run(config: Arc<Config>, testpaths: &TestPaths, revision: Option<&str>) {
     }
 
     let cx = TestCx { config: &config, props: &props, testpaths, revision };
-    create_dir_all(&cx.output_base_dir()).unwrap();
+    create_dir_all(&cx.output_base_dir())
+        .with_context(|| {
+            format!("failed to create output base directory {}", cx.output_base_dir().display())
+        })
+        .unwrap();
     if props.incremental {
         cx.init_incremental_test();
     }