Rollup merge of #70209 - Centril:recover-quant-closure, r=petrochenkov

parser: recover on `for<'a> |...| body` closures

When encountering `for` and `<` is 1 token ahead, interpret this as an explicitly quantified generic closure and recover, rather than attempting to parse a `for` loop. This provides both improved diagnostics as well as an insurance policy for the ability to use this as the syntax for generic closures in the future.

As requested by r? @eddyb

src/test/ui/parser/recover-quantified-closure.rs

@@ -0,0 +1,10 @@
+fn main() {
+    for<'a> |x: &'a u8| *x + 1;
+    //~^ ERROR cannot introduce explicit parameters for a closure
+}
+
+enum Foo { Bar }
+fn foo(x: impl Iterator<Item = Foo>) {
+    for <Foo>::Bar in x {}
+    //~^ ERROR expected one of `move`, `static`, `|`
+}

src/test/ui/parser/recover-quantified-closure.stderr

@@ -0,0 +1,16 @@
+error: cannot introduce explicit parameters for a closure
+  --> $DIR/recover-quantified-closure.rs:2:5
+   |
+LL |     for<'a> |x: &'a u8| *x + 1;
+   |     ^^^^^^^ ------------------ the parameters are attached to this closure
+   |     |
+   |     help: remove the parameters
+
+error: expected one of `move`, `static`, `|`, or `||`, found `::`
+  --> $DIR/recover-quantified-closure.rs:8:14
+   |
+LL |     for <Foo>::Bar in x {}
+   |              ^^ expected one of `move`, `static`, `|`, or `||`
+
+error: aborting due to 2 previous errors
+