internal: error on duplicate #[pin] attribute

adds the error and a compile_fail test to check for it.

Closes: #119

Signed-off-by: Luiz Georg <luizgngeorg@gmail.com>

Author: mentonin

internal/src/pin_data.rs

@@ -79,9 +79,13 @@ pub(crate) fn pin_data(
         .map(|field| {
             let len = field.attrs.len();
             field.attrs.retain(|a| !a.path().is_ident("pin"));
-            (len != field.attrs.len(), &*field)
+            let diff = len - field.attrs.len();
+            if diff > 1 {
+                return Err(dcx.error(field, "#[pin] attribute set twice"));
+            }
+            Ok((diff > 0, &*field))
         })
-        .collect();
+        .collect::<Result<_, _>>()?;
 
     for (pinned, field) in &fields {
         if !pinned && is_phantom_pinned(&field.ty) {

tests/ui/compile-fail/pin_data/twice_pin.rs

@@ -0,0 +1,10 @@
+use pin_init::*;
+
+#[pin_data]
+struct Foo {
+    #[pin]
+    #[pin]
+    a: usize,
+}
+
+fn main() {}

tests/ui/compile-fail/pin_data/twice_pin.stderr

@@ -0,0 +1,5 @@
+error: #[pin] attribute set twice
+ --> tests/ui/compile-fail/pin_data/twice_pin.rs:7:5
+  |
+7 |     a: usize,
+  |     ^^^^^^^^