use vendor sources by default on dist tarballs

config.example.toml

@@ -311,9 +311,8 @@
 # Indicate whether the vendored sources are used for Rust dependencies or not.
 #
 # Vendoring requires additional setup. We recommend using the pre-generated source tarballs if you
-# want to use vendoring. See
-# https://forge.rust-lang.org/infra/other-installation-methods.html#source-code.
-#vendor = false
+# want to use vendoring. See https://forge.rust-lang.org/infra/other-installation-methods.html#source-code.
+#vendor = if "is a tarball source" && "vendor" dir exists && ".cargo/config.toml" file exists { true } else { false }
 
 # Typically the build system will build the Rust compiler twice. The second
 # compiler, however, will simply use its own libraries to link against. If you

src/bootstrap/src/core/config/config.rs

@@ -1632,7 +1632,6 @@ impl Config {
         set(&mut config.docs_minification, docs_minification);
         set(&mut config.docs, docs);
         set(&mut config.locked_deps, locked_deps);
-        set(&mut config.vendor, vendor);
         set(&mut config.full_bootstrap, full_bootstrap);
         set(&mut config.extended, extended);
         config.tools = tools;
@@ -1711,6 +1710,12 @@ impl Config {
         config.in_tree_llvm_info = GitInfo::new(false, &config.src.join("src/llvm-project"));
         config.in_tree_gcc_info = GitInfo::new(false, &config.src.join("src/gcc"));
 
+        config.vendor = vendor.unwrap_or(
+            config.rust_info.is_from_tarball()
+                && config.src.join("vendor").exists()
+                && config.src.join(".cargo/config.toml").exists(),
+        );
+
         if let Some(rust) = toml.rust {
             let Rust {
                 optimize: optimize_toml,

src/bootstrap/src/utils/change_tracker.rs

@@ -310,4 +310,9 @@ pub const CONFIG_CHANGE_HISTORY: &[ChangeInfo] = &[
         severity: ChangeSeverity::Warning,
         summary: "Revert `rust.download-rustc` global default to `false` and only use `rust.download-rustc = \"if-unchanged\"` default for library and tools profile. As alt CI rustc is built without debug assertions, `rust.debug-assertions = true` will now inhibit downloading CI rustc.",
     },
+    ChangeInfo {
+        change_id: 133853,
+        severity: ChangeSeverity::Info,
+        summary: "`build.vendor` is now enabled by default for dist/tarball sources when 'vendor' directory and '.cargo/config.toml' file are present.",
+    },
 ];