Suggest fix for ; within let-chains

compiler/rustc_parse/src/parser/expr.rs

@@ -2441,10 +2441,26 @@ fn parse_if_after_cond(&mut self, lo: Span, mut cond: P<Expr>) -> PResult<'a, P<
                     self.error_on_extra_if(&cond)?;
                     // Parse block, which will always fail, but we can add a nice note to the error
                     self.parse_block().map_err(|mut err| {
-                        err.span_note(
-                            cond_span,
-                            "the `if` expression is missing a block after this condition",
-                        );
+                        if self.prev_token == token::Semi
+                            && self.token == token::AndAnd
+                            && let maybe_let = self.look_ahead(1, |t| t.clone())
+                            && maybe_let.is_keyword(kw::Let)
+                        {
+                            err.span_suggestion(
+                                self.prev_token.span,
+                                "consider removing this semicolon to parse the `let` as part of the same chain",
+                                "",
+                                Applicability::MachineApplicable,
+                            ).span_note(
+                                self.token.span.to(maybe_let.span),
+                                "you likely meant to continue parsing the let-chain starting here",
+                            );
+                        } else {
+                            err.span_note(
+                                cond_span,
+                                "the `if` expression is missing a block after this condition",
+                            );
+                        }
                         err
                     })?
                 }

tests/ui/parser/semi-in-let-chain.rs

@@ -0,0 +1,27 @@
+// Issue #117720
+
+#![feature(let_chains)]
+
+fn main() {
+    if let () = ()
+        && let () = (); //~ERROR
+        && let () = ()
+    {
+    }
+}
+
+fn foo() {
+    if let () = ()
+        && () == (); //~ERROR
+        && 1 < 0
+    {
+    }
+}
+
+fn bar() {
+    if let () = ()
+        && () == (); //~ERROR
+        && let () = ()
+    {
+    }
+}

tests/ui/parser/semi-in-let-chain.stderr

@@ -0,0 +1,50 @@
+error: expected `{`, found `;`
+  --> $DIR/semi-in-let-chain.rs:7:23
+   |
+LL |         && let () = ();
+   |                       ^ expected `{`
+   |
+note: you likely meant to continue parsing the let-chain starting here
+  --> $DIR/semi-in-let-chain.rs:8:9
+   |
+LL |         && let () = ()
+   |         ^^^^^^
+help: consider removing this semicolon to parse the `let` as part of the same chain
+   |
+LL -         && let () = ();
+LL +         && let () = ()
+   |
+
+error: expected `{`, found `;`
+  --> $DIR/semi-in-let-chain.rs:15:20
+   |
+LL |         && () == ();
+   |                    ^ expected `{`
+   |
+note: the `if` expression is missing a block after this condition
+  --> $DIR/semi-in-let-chain.rs:14:8
+   |
+LL |       if let () = ()
+   |  ________^
+LL | |         && () == ();
+   | |___________________^
+
+error: expected `{`, found `;`
+  --> $DIR/semi-in-let-chain.rs:23:20
+   |
+LL |         && () == ();
+   |                    ^ expected `{`
+   |
+note: you likely meant to continue parsing the let-chain starting here
+  --> $DIR/semi-in-let-chain.rs:24:9
+   |
+LL |         && let () = ()
+   |         ^^^^^^
+help: consider removing this semicolon to parse the `let` as part of the same chain
+   |
+LL -         && () == ();
+LL +         && () == ()
+   |
+
+error: aborting due to 3 previous errors
+