Use heuristics to recover parsing of missing `;`

- Detect `,` and `:` typos where `;` was intended.
- When the next token could have been the start of a new statement,
  detect a missing semicolon.

1 files changed, 54 insertions, 37 deletions

diff --git a/src/libsyntax/parse/parser/diagnostics.rs b/src/libsyntax/parse/parser/diagnostics.rs
index 06982c789db..677d16a40d9 100644
--- a/src/libsyntax/parse/parser/diagnostics.rs
+++ b/src/libsyntax/parse/parser/diagnostics.rs
@@ -6,7 +6,7 @@ use crate::ast::{
     self, Param, BinOpKind, BindingMode, BlockCheckMode, Expr, ExprKind, Ident, Item, ItemKind,
     Mutability, Pat, PatKind, PathSegment, QSelf, Ty, TyKind,
 };
-use crate::parse::token::{self, TokenKind};
+use crate::parse::token::{self, TokenKind, token_can_begin_expr};
 use crate::print::pprust;
 use crate::ptr::P;
 use crate::symbol::{kw, sym};
@@ -326,34 +326,8 @@ impl<'a> Parser<'a> {
             }
         }
 
-        let is_semi_suggestable = expected.iter().any(|t| match t {
-            TokenType::Token(token::Semi) => true, // We expect a `;` here.
-            _ => false,
-        }) && ( // A `;` would be expected before the current keyword.
-            self.token.is_keyword(kw::Break) ||
-            self.token.is_keyword(kw::Continue) ||
-            self.token.is_keyword(kw::For) ||
-            self.token.is_keyword(kw::If) ||
-            self.token.is_keyword(kw::Let) ||
-            self.token.is_keyword(kw::Loop) ||
-            self.token.is_keyword(kw::Match) ||
-            self.token.is_keyword(kw::Return) ||
-            self.token.is_keyword(kw::While)
-        );
         let sm = self.sess.source_map();
         match (sm.lookup_line(self.token.span.lo()), sm.lookup_line(sp.lo())) {
-            (Ok(ref a), Ok(ref b)) if a.line != b.line && is_semi_suggestable => {
-                // The spans are in different lines, expected `;` and found `let` or `return`.
-                // High likelihood that it is only a missing `;`.
-                err.span_suggestion_short(
-                    label_sp,
-                    "a semicolon may be missing here",
-                    ";".to_string(),
-                    Applicability::MaybeIncorrect,
-                );
-                err.emit();
-                return Ok(true);
-            }
             (Ok(ref a), Ok(ref b)) if a.line == b.line => {
                 // When the spans are in the same line, it means that the only content between
                 // them is whitespace, point at the found token in that case:
@@ -902,18 +876,61 @@ impl<'a> Parser<'a> {
             }
         }
         let sm = self.sess.source_map();
-        match (sm.lookup_line(prev_sp.lo()), sm.lookup_line(sp.lo())) {
-            (Ok(ref a), Ok(ref b)) if a.line == b.line => {
-                // When the spans are in the same line, it means that the only content
-                // between them is whitespace, point only at the found token.
-                err.span_label(sp, label_exp);
+        if !sm.is_multiline(prev_sp.until(sp)) {
+            // When the spans are in the same line, it means that the only content
+            // between them is whitespace, point only at the found token.
+            err.span_label(sp, label_exp);
+        } else {
+            err.span_label(prev_sp, label_exp);
+            err.span_label(sp, "unexpected token");
+        }
+        Err(err)
+    }
+
+    pub(super) fn expect_semi(&mut self) -> PResult<'a, ()> {
+        if self.eat(&token::Semi) {
+            return Ok(());
+        }
+        let sm = self.sess.source_map();
+        let msg = format!("expected `;`, found `{}`", self.this_token_descr());
+        let appl = Applicability::MachineApplicable;
+        if self.look_ahead(1, |t| t == &token::CloseDelim(token::Brace)
+            || token_can_begin_expr(t) && t.kind != token::Colon
+        ) && [token::Comma, token::Colon].contains(&self.token.kind) {
+            // Likely typo: `,` → `;` or `:` → `;`. This is triggered if the current token is
+            // either `,` or `:`, and the next token could either start a new statement or is a
+            // block close. For example:
+            //
+            //   let x = 32:
+            //   let y = 42;
+            if sm.is_multiline(self.prev_span.until(self.token.span)) {
+                self.bump();
+                let sp = self.prev_span;
+                self.struct_span_err(sp, &msg)
+                    .span_suggestion(sp, "change this to `;`", ";".to_string(), appl)
+                    .emit();
+                return Ok(())
             }
-            _ => {
-                err.span_label(prev_sp, label_exp);
-                err.span_label(sp, "unexpected token");
+        } else if self.look_ahead(0, |t| t == &token::CloseDelim(token::Brace) || (
+                token_can_begin_expr(t)
+                && t != &token::Semi
+                && t != &token::Pound // Avoid triggering with too many trailing `#` in raw string.
+        )) {
+            // Missing semicolon typo. This is triggered if the next token could either start a
+            // new statement or is a block close. For example:
+            //
+            //   let x = 32
+            //   let y = 42;
+            if sm.is_multiline(self.prev_span.until(self.token.span)) {
+                let sp = self.prev_span.shrink_to_hi();
+                self.struct_span_err(sp, &msg)
+                    .span_label(self.token.span, "unexpected token")
+                    .span_suggestion_short(sp, "add `;` here", ";".to_string(), appl)
+                    .emit();
+                return Ok(())
             }
         }
-        Err(err)
+        self.expect(&token::Semi).map(|_| ()) // Error unconditionally
     }
 
     pub(super) fn parse_semi_or_incorrect_foreign_fn_body(
@@ -943,7 +960,7 @@ impl<'a> Parser<'a> {
                 Err(mut err) => {
                     err.cancel();
                     mem::replace(self, parser_snapshot);
-                    self.expect(&token::Semi)?;
+                    self.expect_semi()?;
                 }
             }
         } else {