add code blocks to [try_][pin_]init! macros

Allow writing `_: { /* any number of statements */ }` in initializers to
run arbitrary code during initialization. It also allows declaring
values that can be used later in the initializer:

    init!(MyStruct {
        _: {
            let val = if check_something() {
                42
            } else {
                0
            };
        },
        foo: Foo::new(val),
    })

Signed-off-by: Benno Lossin <[email protected]>

Author: BennoLossin

src/macros.rs

@@ -1202,6 +1202,21 @@ macro_rules! __init_internal {
         // have been initialized. Therefore we can now dismiss the guards by forgetting them.
         $(::core::mem::forget($guards);)*
     };
+    (init_slot($($use_data:ident)?):
+        @data($data:ident),
+        @slot($slot:ident),
+        @guards($($guards:ident,)*),
+        // arbitrary code block
+        @munch_fields(_: { $($code:tt)* }, $($rest:tt)*),
+    ) => {
+        $($code)*
+        $crate::__init_internal!(init_slot($($use_data)?):
+            @data($data),
+            @slot($slot),
+            @guards($($guards,)*),
+            @munch_fields($($rest)*),
+        );
+    };
     (init_slot($use_data:ident): // `use_data` is present, so we use the `data` to init fields.
         @data($data:ident),
         @slot($slot:ident),
@@ -1351,6 +1366,19 @@ macro_rules! __init_internal {
             );
         }
     };
+    (make_initializer:
+        @slot($slot:ident),
+        @type_name($t:path),
+        @munch_fields(_: { $($code:tt)* }, $($rest:tt)*),
+        @acc($($acc:tt)*),
+    ) => {
+        $crate::__init_internal!(make_initializer:
+            @slot($slot),
+            @type_name($t),
+            @munch_fields($($rest)*),
+            @acc($($acc)*),
+        );
+    };
     (make_initializer:
         @slot($slot:ident),
         @type_name($t:path),

tests/ui/compile-fail/init/wrong_generics2.stderr

@@ -12,7 +12,7 @@ help: you might have forgotten to add the struct literal inside the block
  --> src/macros.rs
   |
   ~                 ::core::ptr::write($slot, $t { SomeStruct {
-  |9                    $($acc)*
+  |4                    $($acc)*
   ~                 } });
   |