operate on HirId in hir::Pat::each_binding, and consequences of that

Changing the `each_binding` utility method to take the `HirId` of a
binding pattern rather than its `NodeId` seems like a modest first step
in support of the `HirId`ification initiative rust-lang#50928. (The inspiration
for choosing this in particular came from the present author's previous
work on diagnostics issued during liveness analysis, which is the most
greatly affected module in this change.)

src/librustc/hir/pat_util.rs

@@ -10,7 +10,7 @@
 
 use hir::def::Def;
 use hir::def_id::DefId;
-use hir::{self, PatKind};
+use hir::{self, HirId, PatKind};
 use syntax::ast;
 use syntax::codemap::Spanned;
 use syntax_pos::Span;
@@ -91,11 +91,11 @@ impl hir::Pat {
     /// Call `f` on every "binding" in a pattern, e.g., on `a` in
     /// `match foo() { Some(a) => (), None => () }`
     pub fn each_binding<F>(&self, mut f: F)
-        where F: FnMut(hir::BindingAnnotation, ast::NodeId, Span, &Spanned<ast::Name>),
+        where F: FnMut(hir::BindingAnnotation, HirId, Span, &Spanned<ast::Name>),
     {
         self.walk(|p| {
             if let PatKind::Binding(binding_mode, _, ref pth, _) = p.node {
-                f(binding_mode, p.id, p.span, pth);
+                f(binding_mode, p.hir_id, p.span, pth);
             }
             true
         });

src/librustc/middle/expr_use_visitor.rs

@@ -608,9 +608,10 @@ impl<'a, 'gcx, 'tcx> ExprUseVisitor<'a, 'gcx, 'tcx> {
     fn walk_local(&mut self, local: &hir::Local) {
         match local.init {
             None => {
-                let delegate = &mut self.delegate;
-                local.pat.each_binding(|_, id, span, _| {
-                    delegate.decl_without_init(id, span);
+                local.pat.each_binding(|_, hir_id, span, _| {
+                    // FIXME: converting HirId → NodeId is said to be relatively expensive
+                    let node_id = self.mc.tcx.hir.definitions().find_node_for_hir_id(hir_id);
+                    self.delegate.decl_without_init(node_id, span);
                 })
             }