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/mod.rs

@@ -142,6 +142,17 @@ pub struct Parser<'a> {
     /// If present, this `Parser` is not parsing Rust code but rather a macro call.
     subparser_name: Option<&'static str>,
     capture_state: CaptureState,
+    /// This allows us to recover when the user forget to add braces around
+    /// multiple statements in the closure body.
+    pub current_closure: Option<ClosureSpans>,
+}
+
+/// Stores span informations about a closure.
+#[derive(Clone)]
+pub struct ClosureSpans {
+    pub whole_closure: Span,
+    pub closing_pipe: Span,
+    pub body: Span,
 }
 
 /// Indicates a range of tokens that should be replaced by
@@ -440,6 +451,7 @@ impl<'a> Parser<'a> {
                 replace_ranges: Vec::new(),
                 inner_attr_ranges: Default::default(),
             },
+            current_closure: None,
         };
 
         // Make parser point to the first token.
@@ -761,8 +773,11 @@ impl<'a> Parser<'a> {
                     first = false;
                 } else {
                     match self.expect(t) {
-                        Ok(false) => {}
+                        Ok(false) => {
+                            self.current_closure.take();
+                        }
                         Ok(true) => {
+                            self.current_closure.take();
                             recovered = true;
                             break;
                         }
@@ -770,10 +785,29 @@ impl<'a> Parser<'a> {
                             let sp = self.prev_token.span.shrink_to_hi();
                             let token_str = pprust::token_kind_to_string(t);
 
-                            // Attempt to keep parsing if it was a similar separator.
-                            if let Some(ref tokens) = t.similar_tokens() {
-                                if tokens.contains(&self.token.kind) && !unclosed_delims {
-                                    self.bump();
+                            match self.current_closure.take() {
+                                Some(closure_spans) if self.token.kind == TokenKind::Semi => {
+                                    // Finding a semicolon instead of a comma
+                                    // after a closure body indicates that the
+                                    // closure body may be a block but the user
+                                    // forgot to put braces around its
+                                    // statements.
+
+                                    self.recover_missing_braces_around_closure_body(
+                                        closure_spans,
+                                        expect_err,
+                                    )?;
+
+                                    continue;
+                                }
+
+                                _ => {
+                                    // Attempt to keep parsing if it was a similar separator.
+                                    if let Some(ref tokens) = t.similar_tokens() {
+                                        if tokens.contains(&self.token.kind) && !unclosed_delims {
+                                            self.bump();
+                                        }
+                                    }
                                 }
                             }
 
@@ -839,6 +873,65 @@ impl<'a> Parser<'a> {
         Ok((v, trailing, recovered))
     }
 
+    fn recover_missing_braces_around_closure_body(
+        &mut self,
+        closure_spans: ClosureSpans,
+        mut expect_err: DiagnosticBuilder<'_>,
+    ) -> PResult<'a, ()> {
+        let initial_semicolon = self.token.span;
+
+        while self.eat(&TokenKind::Semi) {
+            let _ = self.parse_stmt(ForceCollect::Yes)?;
+        }
+
+        expect_err.set_primary_message(
+            "closure bodies that contain statements must be surrounded by braces",
+        );
+
+        let preceding_pipe_span = closure_spans.closing_pipe;
+        let following_token_span = self.token.span;
+
+        let mut first_note = MultiSpan::from(vec![initial_semicolon]);
+        first_note.push_span_label(
+            initial_semicolon,
+            "this `;` turns the preceding closure into a statement".to_string(),
+        );
+        first_note.push_span_label(
+            closure_spans.body,
+            "this expression is a statement because of the trailing semicolon".to_string(),
+        );
+        expect_err.span_note(first_note, "statement found outside of a block");
+
+        let mut second_note = MultiSpan::from(vec![closure_spans.whole_closure]);
+        second_note.push_span_label(
+            closure_spans.whole_closure,
+            "this is the parsed closure...".to_string(),
+        );
+        second_note.push_span_label(
+            following_token_span,
+            "...but likely you meant the closure to end here".to_string(),
+        );
+        expect_err.span_note(second_note, "the closure body may be incorrectly delimited");
+
+        expect_err.set_span(vec![preceding_pipe_span, following_token_span]);
+
+        let opening_suggestion_str = " {".to_string();
+        let closing_suggestion_str = "}".to_string();
+
+        expect_err.multipart_suggestion(
+            "try adding braces",
+            vec![
+                (preceding_pipe_span.shrink_to_hi(), opening_suggestion_str),
+                (following_token_span.shrink_to_lo(), closing_suggestion_str),
+            ],
+            Applicability::MaybeIncorrect,
+        );
+
+        expect_err.emit();
+
+        Ok(())
+    }
+
     /// Parses a sequence, not including the closing delimiter. The function
     /// `f` must consume tokens until reaching the next separator or
     /// closing bracket.