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/deriving/debug.rs

@@ -78,7 +78,7 @@ fn show_substructure(cx: &mut ExtCtxt, span: Span,
 
     let fmt = substr.nonself_args[0].clone();
 
-    let stmts = match *substr.fields {
+    let mut stmts = match *substr.fields {
         Struct(_, ref fields) | EnumMatching(_, _, ref fields) => {
             let mut stmts = vec![];
             if !is_struct {
@@ -136,7 +136,8 @@ fn show_substructure(cx: &mut ExtCtxt, span: Span,
                                    token::str_to_ident("finish"),
                                    vec![]);
 
-    let block = cx.block(span, stmts, Some(expr));
+    stmts.push(cx.stmt_expr(expr));
+    let block = cx.block(span, stmts);
     cx.expr_block(block)
 }