Rollup merge of #40518 - michaelwoerister:hir-id, r=eddyb

Introduce HirId, a replacement for ast::NodeId after lowering to HIR

This is the first step towards implementing #40303. This PR introduces the `HirId` type and generates a `HirId` for everything that would be assigned one (i.e. stuff in the HIR), but the HIR data types still use `NodeId` for now. Changing that is a big refactoring that I want to do in a separate PR.

A `HirId` uniquely identifies a node in the HIR of the current crate. It is composed of the `owner`, which is the `DefIndex` of the directly enclosing `hir::Item`, `hir::TraitItem`, or `hir::ImplItem` (i.e. the closest "item-like"), and the `local_id` which is unique within the given owner.

This PR is also running a number of consistency checks for the generated `HirId`s:
- Does `NodeId` in the HIR have a corresponding `HirId`?
- Is the `owner` part of each `HirId` consistent with its position in the HIR?
- Do the numerical values of the `local_id` part all lie within a dense range of integers?

cc @rust-lang/compiler

r? @eddyb or @nikomatsakis

2 files changed, 14 insertions, 11 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index 4347046b6b8..3dd4bdbd14d 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -23,6 +23,7 @@ use abi::Abi;
 use ext::hygiene::SyntaxContext;
 use print::pprust;
 use ptr::P;
+use rustc_data_structures::indexed_vec;
 use symbol::{Symbol, keywords};
 use tokenstream::{ThinTokenStream, TokenStream};
 
@@ -275,6 +276,16 @@ impl serialize::UseSpecializedDecodable for NodeId {
     }
 }
 
+impl indexed_vec::Idx for NodeId {
+    fn new(idx: usize) -> Self {
+        NodeId::new(idx)
+    }
+
+    fn index(self) -> usize {
+        self.as_usize()
+    }
+}
+
 /// Node id used to represent the root of the crate.
 pub const CRATE_NODE_ID: NodeId = NodeId(0);
 
diff --git a/src/libsyntax/ext/placeholders.rs b/src/libsyntax/ext/placeholders.rs
index f60b1d17a5e..f0e328a551d 100644
--- a/src/libsyntax/ext/placeholders.rs
+++ b/src/libsyntax/ext/placeholders.rs
@@ -178,17 +178,9 @@ impl<'a, 'b> Folder for PlaceholderExpander<'a, 'b> {
             block.stmts = block.stmts.move_flat_map(|mut stmt| {
                 remaining_stmts -= 1;
 
-                match stmt.node {
-                    // Avoid wasting a node id on a trailing expression statement,
-                    // which shares a HIR node with the expression itself.
-                    ast::StmtKind::Expr(ref expr) if remaining_stmts == 0 => stmt.id = expr.id,
-
-                    _ if self.monotonic => {
-                        assert_eq!(stmt.id, ast::DUMMY_NODE_ID);
-                        stmt.id = self.cx.resolver.next_node_id();
-                    }
-
-                    _ => {}
+                if self.monotonic {
+                    assert_eq!(stmt.id, ast::DUMMY_NODE_ID);
+                    stmt.id = self.cx.resolver.next_node_id();
                 }
 
                 Some(stmt)