new: surface cargo vendor failures and pin the vendoring toolchain

src/new/templates/mod.rs

@@ -51,6 +51,17 @@ impl OutputFile {
     }
 }
 
+/// What the template post-write hook did to the freshly written tree,
+/// threaded through [`super::scaffold`] so the flow can react (e.g. word
+/// the post-scaffold verification offer differently when vendoring failed).
+#[derive(Debug, Clone, Copy, Default)]
+pub struct ScaffoldOutcome {
+    /// The vendoring step did not complete (host `cargo` missing, `cargo
+    /// vendor` failed, or the offline config could not be written): the
+    /// package will not build until the dependencies are vendored manually.
+    pub vendoring_failed: bool,
+}
+
 /// Metadata extracted from an existing project by [`Template::probe`], used
 /// by the interactive wizard to pre-fill its answers (explicit flags always
 /// win). Every field is optional; probe failures are silent and the generic
@@ -126,13 +137,16 @@ pub trait Template: Sync {
     /// Hook run after the generated files have been written to `tree` and
     /// before the orig tarball is created, for templates that need to run
     /// host tooling over the freshly written tree (e.g. `cargo vendor`, so
-    /// the vendored sources land inside the tarball).
+    /// the vendored sources land inside the tarball). Returns the outcome
+    /// the flow should know about ([`ScaffoldOutcome`]); failures that leave
+    /// the tree in place but not buildable are reported through it instead
+    /// of failing the scaffold.
     fn post_write(
         &self,
         _opts: &NewOptions,
         _tree: &Path,
-    ) -> Result<(), Box<dyn std::error::Error>> {
-        Ok(())
+    ) -> Result<ScaffoldOutcome, Box<dyn std::error::Error>> {
+        Ok(ScaffoldOutcome::default())
     }
 }