Don't copy-paste impl_froms into every crate

crates/stdx/src/macros.rs

@@ -17,3 +17,24 @@ macro_rules! format_to {
         { use ::std::fmt::Write as _; let _ = ::std::write!($buf, $lit $($arg)*); }
     };
 }
+
+// Generates `From` impls for `Enum E { Foo(Foo), Bar(Bar) }` enums
+#[macro_export]
+macro_rules! impl_from {
+    ($($variant:ident $(($($sub_variant:ident),*))?),* for $enum:ident) => {
+        $(
+            impl From<$variant> for $enum {
+                fn from(it: $variant) -> $enum {
+                    $enum::$variant(it)
+                }
+            }
+            $($(
+                impl From<$sub_variant> for $enum {
+                    fn from(it: $sub_variant) -> $enum {
+                        $enum::$variant($variant::$sub_variant(it))
+                    }
+                }
+            )*)?
+        )*
+    }
+}