internal: Skip rustfmt test if nightly toolchain is missing

Currently the rustfmt slow test fails if you don't have a nightly
toolchain installed. We install a nightly toolchain on CI, but it's a
little confusing that tests can fail on a fresh checkout on a new
laptop.

Instead, skip this test if there's no nightly toolchain.

Author: Wilfred

crates/rust-analyzer/tests/slow-tests/main.rs

@@ -22,6 +22,7 @@ mod testdir;
 
 use std::{collections::HashMap, path::PathBuf, time::Instant};
 
+use ide_db::FxHashMap;
 use lsp_types::{
     CodeActionContext, CodeActionParams, CompletionParams, DidOpenTextDocumentParams,
     DocumentFormattingParams, DocumentRangeFormattingParams, FileRename, FormattingOptions,
@@ -673,6 +674,17 @@ fn test_format_document_range() {
         return;
     }
 
+    // This test requires a nightly toolchain, so skip if it's not available.
+    let cwd = std::env::current_dir().unwrap_or_default();
+    let has_nightly_rustfmt = toolchain::command("rustfmt", cwd, &FxHashMap::default())
+        .args(["+nightly", "--version"])
+        .output()
+        .is_ok_and(|out| out.status.success());
+    if !has_nightly_rustfmt {
+        tracing::warn!("skipping test_format_document_range: nightly rustfmt not available");
+        return;
+    }
+
     let server = Project::with_fixture(
         r#"
 //- /Cargo.toml