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.

10 files changed, 72 insertions, 96 deletions

diff --git a/src/libsyntax/ast.rs b/src/libsyntax/ast.rs
index dcdc1e60a99..a352715b20b 100644
--- a/src/libsyntax/ast.rs
+++ b/src/libsyntax/ast.rs
@@ -557,9 +557,6 @@ impl PartialEq for MetaItemKind {
 pub struct Block {
     /// Statements in a block
     pub stmts: Vec<Stmt>,
-    /// An expression at the end of the block
-    /// without a semicolon, if any
-    pub expr: Option<P<Expr>>,
     pub id: NodeId,
     /// Distinguishes between `unsafe { ... }` and `{ ... }`
     pub rules: BlockCheckMode,
@@ -832,7 +829,7 @@ pub enum StmtKind {
     /// An item definition.
     Item(P<Item>),
 
-    /// Expr without trailing semi-colon (must have unit type).
+    /// Expr without trailing semi-colon.
     Expr(P<Expr>),
 
     Semi(P<Expr>),
diff --git a/src/libsyntax/ext/build.rs b/src/libsyntax/ext/build.rs
index f4ae23ed8be..435241f426e 100644
--- a/src/libsyntax/ext/build.rs
+++ b/src/libsyntax/ext/build.rs
@@ -88,6 +88,7 @@ pub trait AstBuilder {
 
     // statements
     fn stmt_expr(&self, expr: P<ast::Expr>) -> ast::Stmt;
+    fn stmt_semi(&self, expr: P<ast::Expr>) -> ast::Stmt;
     fn stmt_let(&self, sp: Span, mutbl: bool, ident: ast::Ident, ex: P<ast::Expr>) -> ast::Stmt;
     fn stmt_let_typed(&self,
                       sp: Span,
@@ -99,12 +100,8 @@ pub trait AstBuilder {
     fn stmt_item(&self, sp: Span, item: P<ast::Item>) -> ast::Stmt;
 
     // blocks
-    fn block(&self, span: Span, stmts: Vec<ast::Stmt>,
-             expr: Option<P<ast::Expr>>) -> P<ast::Block>;
+    fn block(&self, span: Span, stmts: Vec<ast::Stmt>) -> P<ast::Block>;
     fn block_expr(&self, expr: P<ast::Expr>) -> P<ast::Block>;
-    fn block_all(&self, span: Span,
-                 stmts: Vec<ast::Stmt>,
-                 expr: Option<P<ast::Expr>>) -> P<ast::Block>;
 
     // expressions
     fn expr(&self, span: Span, node: ast::ExprKind) -> P<ast::Expr>;
@@ -512,6 +509,14 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
         ast::Stmt {
             id: ast::DUMMY_NODE_ID,
             span: expr.span,
+            node: ast::StmtKind::Expr(expr),
+        }
+    }
+
+    fn stmt_semi(&self, expr: P<ast::Expr>) -> ast::Stmt {
+        ast::Stmt {
+            id: ast::DUMMY_NODE_ID,
+            span: expr.span,
             node: ast::StmtKind::Semi(expr),
         }
     }
@@ -567,11 +572,6 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
         })
     }
 
-    fn block(&self, span: Span, stmts: Vec<ast::Stmt>,
-             expr: Option<P<Expr>>) -> P<ast::Block> {
-        self.block_all(span, stmts, expr)
-    }
-
     fn stmt_item(&self, sp: Span, item: P<ast::Item>) -> ast::Stmt {
         ast::Stmt {
             id: ast::DUMMY_NODE_ID,
@@ -581,19 +581,19 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
     }
 
     fn block_expr(&self, expr: P<ast::Expr>) -> P<ast::Block> {
-        self.block_all(expr.span, Vec::new(), Some(expr))
-    }
-    fn block_all(&self,
-                 span: Span,
-                 stmts: Vec<ast::Stmt>,
-                 expr: Option<P<ast::Expr>>) -> P<ast::Block> {
-            P(ast::Block {
-               stmts: stmts,
-               expr: expr,
-               id: ast::DUMMY_NODE_ID,
-               rules: BlockCheckMode::Default,
-               span: span,
-            })
+        self.block(expr.span, vec![ast::Stmt {
+            id: ast::DUMMY_NODE_ID,
+            span: expr.span,
+            node: ast::StmtKind::Expr(expr),
+        }])
+    }
+    fn block(&self, span: Span, stmts: Vec<ast::Stmt>) -> P<ast::Block> {
+        P(ast::Block {
+           stmts: stmts,
+           id: ast::DUMMY_NODE_ID,
+           rules: BlockCheckMode::Default,
+           span: span,
+        })
     }
 
     fn expr(&self, span: Span, node: ast::ExprKind) -> P<ast::Expr> {
@@ -962,14 +962,14 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
                     ids: Vec<ast::Ident>,
                     stmts: Vec<ast::Stmt>)
                     -> P<ast::Expr> {
-        self.lambda(span, ids, self.block(span, stmts, None))
+        self.lambda(span, ids, self.block(span, stmts))
     }
     fn lambda_stmts_0(&self, span: Span, stmts: Vec<ast::Stmt>) -> P<ast::Expr> {
-        self.lambda0(span, self.block(span, stmts, None))
+        self.lambda0(span, self.block(span, stmts))
     }
     fn lambda_stmts_1(&self, span: Span, stmts: Vec<ast::Stmt>,
                       ident: ast::Ident) -> P<ast::Expr> {
-        self.lambda1(span, self.block(span, stmts, None), ident)
+        self.lambda1(span, self.block(span, stmts), ident)
     }
 
     fn arg(&self, span: Span, ident: ast::Ident, ty: P<ast::Ty>) -> ast::Arg {
diff --git a/src/libsyntax/ext/expand.rs b/src/libsyntax/ext/expand.rs
index 32635f5cdd2..3036a88430a 100644
--- a/src/libsyntax/ext/expand.rs
+++ b/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
         }
diff --git a/src/libsyntax/ext/quote.rs b/src/libsyntax/ext/quote.rs
index 3dc7c92f016..68527b0797d 100644
--- a/src/libsyntax/ext/quote.rs
+++ b/src/libsyntax/ext/quote.rs
@@ -513,10 +513,8 @@ pub fn expand_quote_matcher(cx: &mut ExtCtxt,
     let (cx_expr, tts) = parse_arguments_to_quote(cx, tts);
     let mut vector = mk_stmts_let(cx, sp);
     vector.extend(statements_mk_tts(cx, &tts[..], true));
-    let block = cx.expr_block(
-        cx.block_all(sp,
-                     vector,
-                     Some(cx.expr_ident(sp, id_ext("tt")))));
+    vector.push(cx.stmt_expr(cx.expr_ident(sp, id_ext("tt"))));
+    let block = cx.expr_block(cx.block(sp, vector));
 
     let expanded = expand_wrapper(cx, sp, cx_expr, block, &[&["syntax", "ext", "quote", "rt"]]);
     base::MacEager::expr(expanded)
@@ -766,8 +764,9 @@ fn statements_mk_tt(cx: &ExtCtxt, tt: &TokenTree, matcher: bool) -> Vec<ast::Stm
             let stmt_let_tt = cx.stmt_let(sp, true, id_ext("tt"), cx.expr_vec_ng(sp));
             let mut tts_stmts = vec![stmt_let_tt];
             tts_stmts.extend(statements_mk_tts(cx, &seq.tts[..], matcher));
-            let e_tts = cx.expr_block(cx.block(sp, tts_stmts,
-                                                   Some(cx.expr_ident(sp, id_ext("tt")))));
+            tts_stmts.push(cx.stmt_expr(cx.expr_ident(sp, id_ext("tt"))));
+            let e_tts = cx.expr_block(cx.block(sp, tts_stmts));
+
             let e_separator = match seq.separator {
                 Some(ref sep) => cx.expr_some(sp, expr_mk_token(cx, sp, sep)),
                 None => cx.expr_none(sp),
@@ -882,10 +881,8 @@ fn expand_tts(cx: &ExtCtxt, sp: Span, tts: &[TokenTree])
 
     let mut vector = mk_stmts_let(cx, sp);
     vector.extend(statements_mk_tts(cx, &tts[..], false));
-    let block = cx.expr_block(
-        cx.block_all(sp,
-                     vector,
-                     Some(cx.expr_ident(sp, id_ext("tt")))));
+    vector.push(cx.stmt_expr(cx.expr_ident(sp, id_ext("tt"))));
+    let block = cx.expr_block(cx.block(sp, vector));
 
     (cx_expr, block)
 }
@@ -899,13 +896,14 @@ fn expand_wrapper(cx: &ExtCtxt,
     let cx_expr_borrow = cx.expr_addr_of(sp, cx.expr_deref(sp, cx_expr));
     let stmt_let_ext_cx = cx.stmt_let(sp, false, id_ext("ext_cx"), cx_expr_borrow);
 
-    let stmts = imports.iter().map(|path| {
+    let mut stmts = imports.iter().map(|path| {
         // make item: `use ...;`
         let path = path.iter().map(|s| s.to_string()).collect();
         cx.stmt_item(sp, cx.item_use_glob(sp, ast::Visibility::Inherited, ids_ext(path)))
-    }).chain(Some(stmt_let_ext_cx)).collect();
+    }).chain(Some(stmt_let_ext_cx)).collect::<Vec<_>>();
+    stmts.push(cx.stmt_expr(expr));
 
-    cx.expr_block(cx.block_all(sp, stmts, Some(expr)))
+    cx.expr_block(cx.block(sp, stmts))
 }
 
 fn expand_parse_call(cx: &ExtCtxt,
diff --git a/src/libsyntax/fold.rs b/src/libsyntax/fold.rs
index b2b286c9b14..6789e7be058 100644
--- a/src/libsyntax/fold.rs
+++ b/src/libsyntax/fold.rs
@@ -818,10 +818,9 @@ fn noop_fold_bounds<T: Folder>(bounds: TyParamBounds, folder: &mut T)
 }
 
 pub fn noop_fold_block<T: Folder>(b: P<Block>, folder: &mut T) -> P<Block> {
-    b.map(|Block {id, stmts, expr, rules, span}| Block {
+    b.map(|Block {id, stmts, rules, span}| Block {
         id: folder.new_id(id),
         stmts: stmts.move_flat_map(|s| folder.fold_stmt(s).into_iter()),
-        expr: expr.and_then(|x| folder.fold_opt_expr(x)),
         rules: rules,
         span: folder.new_span(span),
     })
diff --git a/src/libsyntax/parse/mod.rs b/src/libsyntax/parse/mod.rs
index 0c5a672dfbc..bbcc044d43c 100644
--- a/src/libsyntax/parse/mod.rs
+++ b/src/libsyntax/parse/mod.rs
@@ -957,7 +957,6 @@ mod tests {
                                                 attrs: ThinVec::new()})),
                                             id: ast::DUMMY_NODE_ID,
                                             span: sp(17,19)}),
-                                        expr: None,
                                         id: ast::DUMMY_NODE_ID,
                                         rules: ast::BlockCheckMode::Default, // no idea
                                         span: sp(15,21),
diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 83401011ed4..813d90103b8 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -3236,9 +3236,12 @@ impl<'a> Parser<'a> {
                 let body_expr = self.parse_expr()?;
                 P(ast::Block {
                     id: ast::DUMMY_NODE_ID,
-                    stmts: vec![],
                     span: body_expr.span,
-                    expr: Some(body_expr),
+                    stmts: vec![Stmt {
+                        span: body_expr.span,
+                        node: StmtKind::Expr(body_expr),
+                        id: ast::DUMMY_NODE_ID,
+                    }],
                     rules: BlockCheckMode::Default,
                 })
             }
@@ -4098,7 +4101,6 @@ impl<'a> Parser<'a> {
     /// Precondition: already parsed the '{'.
     fn parse_block_tail(&mut self, lo: BytePos, s: BlockCheckMode) -> PResult<'a, P<Block>> {
         let mut stmts = vec![];
-        let mut expr = None;
 
         while !self.eat(&token::CloseDelim(token::Brace)) {
             let Stmt {node, span, ..} = if let Some(s) = self.parse_stmt_() {
@@ -4112,10 +4114,10 @@ impl<'a> Parser<'a> {
 
             match node {
                 StmtKind::Expr(e) => {
-                    self.handle_expression_like_statement(e, span, &mut stmts, &mut expr)?;
+                    self.handle_expression_like_statement(e, span, &mut stmts)?;
                 }
                 StmtKind::Mac(mac) => {
-                    self.handle_macro_in_block(mac.unwrap(), span, &mut stmts, &mut expr)?;
+                    self.handle_macro_in_block(mac.unwrap(), span, &mut stmts)?;
                 }
                 _ => { // all other kinds of statements:
                     let mut hi = span.hi;
@@ -4135,7 +4137,6 @@ impl<'a> Parser<'a> {
 
         Ok(P(ast::Block {
             stmts: stmts,
-            expr: expr,
             id: ast::DUMMY_NODE_ID,
             rules: s,
             span: mk_sp(lo, self.last_span.hi),
@@ -4145,8 +4146,7 @@ impl<'a> Parser<'a> {
     fn handle_macro_in_block(&mut self,
                              (mac, style, attrs): (ast::Mac, MacStmtStyle, ThinVec<Attribute>),
                              span: Span,
-                             stmts: &mut Vec<Stmt>,
-                             last_block_expr: &mut Option<P<Expr>>)
+                             stmts: &mut Vec<Stmt>)
                              -> PResult<'a, ()> {
         if style == MacStmtStyle::NoBraces {
             // statement macro without braces; might be an
@@ -4165,7 +4165,7 @@ impl<'a> Parser<'a> {
                     let lo = e.span.lo;
                     let e = self.parse_dot_or_call_expr_with(e, lo, attrs)?;
                     let e = self.parse_assoc_expr_with(0, LhsExpr::AlreadyParsed(e))?;
-                    self.handle_expression_like_statement(e, span, stmts, last_block_expr)?;
+                    self.handle_expression_like_statement(e, span, stmts)?;
                 }
             }
         } else {
@@ -4179,11 +4179,6 @@ impl<'a> Parser<'a> {
                     });
                     self.bump();
                 }
-                token::CloseDelim(token::Brace) => {
-                    // if a block ends in `m!(arg)` without
-                    // a `;`, it must be an expr
-                    *last_block_expr = Some(self.mk_mac_expr(span.lo, span.hi, mac.node, attrs));
-                }
                 _ => {
                     stmts.push(Stmt {
                         id: ast::DUMMY_NODE_ID,
@@ -4199,8 +4194,7 @@ impl<'a> Parser<'a> {
     fn handle_expression_like_statement(&mut self,
                                         e: P<Expr>,
                                         span: Span,
-                                        stmts: &mut Vec<Stmt>,
-                                        last_block_expr: &mut Option<P<Expr>>)
+                                        stmts: &mut Vec<Stmt>)
                                         -> PResult<'a, ()> {
         // expression without semicolon
         if classify::expr_requires_semi_to_be_stmt(&e) {
@@ -4227,7 +4221,6 @@ impl<'a> Parser<'a> {
                     span: span_with_semi,
                 });
             }
-            token::CloseDelim(token::Brace) => *last_block_expr = Some(e),
             _ => {
                 stmts.push(Stmt {
                     id: ast::DUMMY_NODE_ID,
diff --git a/src/libsyntax/print/pprust.rs b/src/libsyntax/print/pprust.rs
index d399f538004..b56cec72a95 100644
--- a/src/libsyntax/print/pprust.rs
+++ b/src/libsyntax/print/pprust.rs
@@ -1619,12 +1619,16 @@ impl<'a> State<'a> {
                     try!(self.word_space("="));
                     try!(self.print_expr(&init));
                 }
+                try!(word(&mut self.s, ";"));
                 self.end()?;
             }
             ast::StmtKind::Item(ref item) => self.print_item(&item)?,
             ast::StmtKind::Expr(ref expr) => {
                 try!(self.space_if_not_bol());
                 try!(self.print_expr_outer_attr_style(&expr, false));
+                if parse::classify::expr_requires_semi_to_be_stmt(expr) {
+                    try!(word(&mut self.s, ";"));
+                }
             }
             ast::StmtKind::Semi(ref expr) => {
                 try!(self.space_if_not_bol());
@@ -1646,9 +1650,6 @@ impl<'a> State<'a> {
                 }
             }
         }
-        if parse::classify::stmt_ends_with_semi(&st.node) {
-            try!(word(&mut self.s, ";"));
-        }
         self.maybe_print_trailing_comment(st.span, None)
     }
 
@@ -1692,17 +1693,17 @@ impl<'a> State<'a> {
 
         try!(self.print_inner_attributes(attrs));
 
-        for st in &blk.stmts {
-            try!(self.print_stmt(st));
-        }
-        match blk.expr {
-            Some(ref expr) => {
-                try!(self.space_if_not_bol());
-                try!(self.print_expr_outer_attr_style(&expr, false));
-                try!(self.maybe_print_trailing_comment(expr.span, Some(blk.span.hi)));
+        for (i, st) in blk.stmts.iter().enumerate() {
+            match st.node {
+                ast::StmtKind::Expr(ref expr) if i == blk.stmts.len() - 1 => {
+                    try!(self.space_if_not_bol());
+                    try!(self.print_expr_outer_attr_style(&expr, false));
+                    try!(self.maybe_print_trailing_comment(expr.span, Some(blk.span.hi)));
+                }
+                _ => try!(self.print_stmt(st)),
             }
-            _ => ()
         }
+
         try!(self.bclose_maybe_open(blk.span, indented, close_box));
         self.ann.post(self, NodeBlock(blk))
     }
@@ -2111,22 +2112,21 @@ impl<'a> State<'a> {
                     _ => false
                 };
 
-                if !default_return || !body.stmts.is_empty() || body.expr.is_none() {
-                    try!(self.print_block_unclosed(&body));
-                } else {
-                    // we extract the block, so as not to create another set of boxes
-                    let i_expr = body.expr.as_ref().unwrap();
-                    match i_expr.node {
-                        ast::ExprKind::Block(ref blk) => {
+                match body.stmts.last().map(|stmt| &stmt.node) {
+                    Some(&ast::StmtKind::Expr(ref i_expr)) if default_return &&
+                                                              body.stmts.len() == 1 => {
+                        // we extract the block, so as not to create another set of boxes
+                        if let ast::ExprKind::Block(ref blk) = i_expr.node {
                             try!(self.print_block_unclosed_with_attrs(&blk, &i_expr.attrs));
-                        }
-                        _ => {
+                        } else {
                             // this is a bare expression
                             try!(self.print_expr(&i_expr));
                             try!(self.end()); // need to close a box
                         }
                     }
+                    _ => try!(self.print_block_unclosed(&body)),
                 }
+
                 // a box will be closed by print_expr, but we didn't want an overall
                 // wrapper so we closed the corresponding opening. so create an
                 // empty box to satisfy the close.
diff --git a/src/libsyntax/test.rs b/src/libsyntax/test.rs
index 078103f834a..0a60b7fd430 100644
--- a/src/libsyntax/test.rs
+++ b/src/libsyntax/test.rs
@@ -478,7 +478,7 @@ fn mk_main(cx: &mut TestCtxt) -> P<ast::Item> {
     let main_attr = ecx.attribute(sp, main_meta);
     // pub fn main() { ... }
     let main_ret_ty = ecx.ty(sp, ast::TyKind::Tup(vec![]));
-    let main_body = ecx.block_all(sp, vec![call_test_main], None);
+    let main_body = ecx.block(sp, vec![call_test_main]);
     let main = ast::ItemKind::Fn(ecx.fn_decl(vec![], main_ret_ty),
                            ast::Unsafety::Normal,
                            ast::Constness::NotConst,
diff --git a/src/libsyntax/visit.rs b/src/libsyntax/visit.rs
index 571965ef872..1fc4e54d218 100644
--- a/src/libsyntax/visit.rs
+++ b/src/libsyntax/visit.rs
@@ -590,7 +590,6 @@ pub fn walk_struct_field<V: Visitor>(visitor: &mut V, struct_field: &StructField
 
 pub fn walk_block<V: Visitor>(visitor: &mut V, block: &Block) {
     walk_list!(visitor, visit_stmt, &block.stmts);
-    walk_list!(visitor, visit_expr, &block.expr);
 }
 
 pub fn walk_stmt<V: Visitor>(visitor: &mut V, statement: &Stmt) {