Rollup merge of #34436 - jseyfried:no_block_expr, r=eddyb

To allow these braced macro invocation, this PR removes the optional expression from `ast::Block` and instead uses a `StmtKind::Expr` at the end of the statement list.

Currently, braced macro invocations in blocks can expand into statements (and items) except when they are last in a block, in which case they can only expand into expressions.

For example,
```rust
macro_rules! make_stmt {
    () => { let x = 0; }
}

fn f() {
    make_stmt! {} //< This is OK...
    let x = 0; //< ... unless this line is commented out.
}
```

Fixes #34418.

src/libsyntax/ext/expand.rs

@@ -611,23 +611,14 @@ pub fn expand_block(blk: P<Block>, fld: &mut MacroExpander) -> P<Block> {
 
 // expand the elements of a block.
 pub fn expand_block_elts(b: P<Block>, fld: &mut MacroExpander) -> P<Block> {
-    b.map(|Block {id, stmts, expr, rules, span}| {
+    b.map(|Block {id, stmts, rules, span}| {
         let new_stmts = stmts.into_iter().flat_map(|x| {
             // perform pending renames and expand macros in the statement
             fld.fold_stmt(x).into_iter()
         }).collect();
-        let new_expr = expr.map(|x| {
-            let expr = {
-                let pending_renames = &mut fld.cx.syntax_env.info().pending_renames;
-                let mut rename_fld = IdentRenamer{renames:pending_renames};
-                rename_fld.fold_expr(x)
-            };
-            fld.fold_expr(expr)
-        });
         Block {
             id: fld.new_id(id),
             stmts: new_stmts,
-            expr: new_expr,
             rules: rules,
             span: span
         }