Emit proper errors on missing closure braces

This commit focuses on emitting clean errors for the following syntax
error:

```
Some(42).map(|a|
    dbg!(a);
    a
);
```

Previous implementation tried to recover after parsing the closure body
(the `dbg` expression) by replacing the next `;` with a `,`, which made
the next expression belong to the next function argument. As such, the
following errors were emitted (among others):
  - the semicolon token was not expected,
  - a is not in scope,
  - Option::map is supposed to take one argument, not two.

This commit allows us to gracefully handle this situation by adding
giving the parser the ability to remember when it has just parsed a
closure body inside a function call. When this happens, we can treat the
unexpected `;` specifically and try to parse as much statements as
possible in order to eat the whole block. When we can't parse statements
anymore, we generate a clean error indicating that the braces are
missing, and return an ExprKind::Err.

compiler/rustc_parse/src/parser/expr.rs

@@ -1,9 +1,12 @@
 use super::pat::{RecoverColon, RecoverComma, PARAM_EXPECTED};
 use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
-use super::{AttrWrapper, BlockMode, ForceCollect, Parser, PathStyle, Restrictions, TokenType};
+use super::{
+    AttrWrapper, BlockMode, ClosureSpans, ForceCollect, Parser, PathStyle, Restrictions, TokenType,
+};
 use super::{SemiColonMode, SeqSep, TokenExpectType, TrailingToken};
 use crate::maybe_recover_from_interpolated_ty_qpath;
 
+use ast::token::DelimToken;
 use rustc_ast::ptr::P;
 use rustc_ast::token::{self, Token, TokenKind};
 use rustc_ast::tokenstream::Spacing;
@@ -91,6 +94,8 @@ impl<'a> Parser<'a> {
     /// Parses an expression.
     #[inline]
     pub fn parse_expr(&mut self) -> PResult<'a, P<Expr>> {
+        self.current_closure.take();
+
         self.parse_expr_res(Restrictions::empty(), None)
     }
 
@@ -1736,7 +1741,7 @@ impl<'a> Parser<'a> {
         let capture_clause = self.parse_capture_clause()?;
         let decl = self.parse_fn_block_decl()?;
         let decl_hi = self.prev_token.span;
-        let body = match decl.output {
+        let mut body = match decl.output {
             FnRetTy::Default(_) => {
                 let restrictions = self.restrictions - Restrictions::STMT_EXPR;
                 self.parse_expr_res(restrictions, None)?
@@ -1753,11 +1758,28 @@ impl<'a> Parser<'a> {
             self.sess.gated_spans.gate(sym::async_closure, span);
         }
 
-        Ok(self.mk_expr(
+        if self.token.kind == TokenKind::Semi && self.token_cursor.frame.delim == DelimToken::Paren
+        {
+            // It is likely that the closure body is a block but where the
+            // braces have been removed. We will recover and eat the next
+            // statements later in the parsing process.
+            body = self.mk_expr_err(body.span);
+        }
+
+        let body_span = body.span;
+
+        let closure = self.mk_expr(
             lo.to(body.span),
             ExprKind::Closure(capture_clause, asyncness, movability, decl, body, lo.to(decl_hi)),
             attrs,
-        ))
+        );
+
+        // Disable recovery for closure body
+        let spans =
+            ClosureSpans { whole_closure: closure.span, closing_pipe: decl_hi, body: body_span };
+        self.current_closure = Some(spans);
+
+        Ok(closure)
     }
 
     /// Parses an optional `move` prefix to a closure-like construct.