Represent ownership transfer in RustdocVisitor::visit

Previously visit could be called multiple times, but this is inaccurate,
as it deconstructs Visitor state.

src/librustdoc/clean/mod.rs

@@ -49,7 +49,6 @@ use parking_lot::ReentrantMutex;
 
 use crate::core::{self, DocContext};
 use crate::doctree;
-use crate::visit_ast;
 use crate::html::render::{cache, ExternalLocation};
 use crate::html::item_type::ItemType;
 
@@ -138,7 +137,10 @@ pub struct Crate {
     pub masked_crates: FxHashSet<CrateNum>,
 }
 
-impl<'a, 'tcx> Clean<Crate> for (visit_ast::RustdocVisitor<'a, 'tcx>, doctree::Module<'tcx>) {
+// The `()` here is rather ugly and would be great to remove. Unfortunately, we
+// already have a different Clean impl for `doctree::Module` which makes this
+// the only way to easily disambiguate.
+impl<'tcx> Clean<Crate> for ((), doctree::Module<'tcx>) {
     fn clean(&self, cx: &DocContext<'_>) -> Crate {
         use crate::visit_lib::LibEmbargoVisitor;
 

src/librustdoc/core.rs

@@ -392,9 +392,9 @@ pub fn run_core(options: RustdocOptions) -> (clean::Crate, RenderInfo, RenderOpt
             debug!("crate: {:?}", tcx.hir().krate());
 
             let mut krate = {
-                let mut v = RustdocVisitor::new(&ctxt);
+                let v = RustdocVisitor::new(&ctxt);
                 let module = v.visit(tcx.hir().krate());
-                (v, module).clean(&ctxt)
+                ((), module).clean(&ctxt)
             };
 
             fn report_deprecated_attr(name: &str, diag: &errors::Handler) {

src/librustdoc/visit_ast.rs

@@ -68,7 +68,7 @@ impl<'a, 'tcx> RustdocVisitor<'a, 'tcx> {
             .and_then(|def_id| self.cx.tcx.lookup_deprecation(def_id))
     }
 
-    pub fn visit(&mut self, krate: &'tcx hir::Crate) -> Module<'tcx> {
+    pub fn visit(mut self, krate: &'tcx hir::Crate) -> Module<'tcx> {
         let mut module = self.visit_mod_contents(krate.span,
                                               &krate.attrs,
                                               &Spanned { span: syntax_pos::DUMMY_SP,