Add regression test for #66706

src/test/ui/issues/issue-66706.rs

@@ -0,0 +1,13 @@
+fn a() {
+    [0; [|_: _ &_| ()].len()]
+    //~^ ERROR expected `,`, found `&`
+    //~| ERROR type annotations needed
+    //~| ERROR mismatched types
+}
+
+fn b() {
+    [0; [|f @ &ref _| {} ; 0 ].len() ];
+    //~^ ERROR expected identifier, found reserved identifier `_`
+}
+
+fn main() {}

src/test/ui/issues/issue-66706.stderr

@@ -0,0 +1,32 @@
+error: expected `,`, found `&`
+  --> $DIR/issue-66706.rs:2:16
+   |
+LL |     [0; [|_: _ &_| ()].len()]
+   |               -^ expected `,`
+   |               |
+   |               help: missing `,`
+
+error: expected identifier, found reserved identifier `_`
+  --> $DIR/issue-66706.rs:9:20
+   |
+LL |     [0; [|f @ &ref _| {} ; 0 ].len() ];
+   |                    ^ expected identifier, found reserved identifier
+
+error[E0282]: type annotations needed
+  --> $DIR/issue-66706.rs:2:11
+   |
+LL |     [0; [|_: _ &_| ()].len()]
+   |           ^ consider giving this closure parameter a type
+
+error[E0308]: mismatched types
+  --> $DIR/issue-66706.rs:2:5
+   |
+LL | fn a() {
+   |        - help: try adding a return type: `-> [{integer}; _]`
+LL |     [0; [|_: _ &_| ()].len()]
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found array `[{integer}; _]`
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0282, E0308.
+For more information about an error, try `rustc --explain E0282`.