Add error checks for duplicate attributes

compatibility-tests/compile-fail/tests/ui/attribute-duplication.rs

@@ -0,0 +1,51 @@
+mod enum_duplication {
+    use snafu::Snafu;
+
+    #[derive(Debug, Snafu)]
+    #[snafu(visibility(pub))]
+    #[snafu(visibility(pub))]
+    enum EnumError {
+        #[snafu(display("an error variant"))]
+        #[snafu(display("should not allow duplicate display"))]
+        #[snafu(visibility(pub))]
+        #[snafu(visibility(pub))]
+        AVariant {
+            #[snafu(source(from(EnumError, Box::new)))]
+            #[snafu(backtrace(delegate))]
+            source: Box<EnumError>,
+            #[snafu(source(from(EnumError, Box::new)))]
+            #[snafu(backtrace(delegate))]
+            source2: Box<EnumError>,
+            #[snafu(source)]
+            #[snafu(backtrace(delegate))]
+            source3: String,
+            #[snafu(source)]
+            #[snafu(source)]
+            #[snafu(backtrace(delegate))]
+            source4: String,
+
+            #[snafu(backtrace)]
+            backtrace1: String,
+            #[snafu(backtrace)]
+            backtrace2: String,
+            #[snafu(backtrace)]
+            #[snafu(backtrace)]
+            #[snafu(backtrace)]
+            backtrace3: String,
+        },
+    }
+}
+
+mod struct_duplication {
+    use snafu::Snafu;
+
+    #[derive(Debug, Snafu)]
+    enum UsableError {}
+
+    #[derive(Debug, Snafu)]
+    #[snafu(source(from(UsableError, Box::new)))]
+    #[snafu(source(from(UsableError, Box::new)))]
+    struct StructError(Box<UsableError>);
+}
+
+fn main() {}

compatibility-tests/compile-fail/tests/ui/attribute-duplication.stderr

@@ -0,0 +1,83 @@
+error: Multiple 'visibility' attributes are not supported on an error enum; found a duplicate here:
+ --> $DIR/attribute-duplication.rs:7:10
+  |
+7 |     enum EnumError {
+  |          ^^^^^^^^^
+
+error: Multiple 'source' attributes are not supported on one field; found a duplicate here:
+  --> $DIR/attribute-duplication.rs:25:13
+   |
+25 |             source4: String,
+   |             ^^^^^^^
+
+error: Multiple 'backtrace' attributes are not supported on one field; found a duplicate here:
+  --> $DIR/attribute-duplication.rs:34:13
+   |
+34 |             backtrace3: String,
+   |             ^^^^^^^^^^
+
+error: Multiple 'source' attributes are not supported within an error variant; found a duplicate here:
+  --> $DIR/attribute-duplication.rs:21:13
+   |
+21 |             source3: String,
+   |             ^^^^^^^
+
+error: Multiple 'source' attributes are not supported within an error variant; found a duplicate here:
+  --> $DIR/attribute-duplication.rs:25:13
+   |
+25 |             source4: String,
+   |             ^^^^^^^
+
+error: Multiple 'backtrace' attributes are not supported within an error variant; found a duplicate here:
+  --> $DIR/attribute-duplication.rs:30:13
+   |
+30 |             backtrace2: String,
+   |             ^^^^^^^^^^
+
+error: Multiple 'backtrace' attributes are not supported within an error variant; found a duplicate here:
+  --> $DIR/attribute-duplication.rs:34:13
+   |
+34 |             backtrace3: String,
+   |             ^^^^^^^^^^
+
+error: Multiple 'backtrace(delegate)' attributes are not supported within an error variant; found a duplicate here:
+  --> $DIR/attribute-duplication.rs:21:13
+   |
+21 |             source3: String,
+   |             ^^^^^^^
+
+error: Multiple 'backtrace(delegate)' attributes are not supported within an error variant; found a duplicate here:
+  --> $DIR/attribute-duplication.rs:25:13
+   |
+25 |             source4: String,
+   |             ^^^^^^^
+
+error: Cannot have 'backtrace' and 'backtrace(delegate)' fields in the same error variant
+  --> $DIR/attribute-duplication.rs:28:13
+   |
+28 |             backtrace1: String,
+   |             ^^^^^^^^^^
+
+error: Cannot have 'backtrace' and 'backtrace(delegate)' fields in the same error variant
+  --> $DIR/attribute-duplication.rs:15:13
+   |
+15 |             source: Box<EnumError>,
+   |             ^^^^^^
+
+error: Multiple 'display' attributes are not supported on an error variant; found a duplicate here:
+  --> $DIR/attribute-duplication.rs:12:9
+   |
+12 |         AVariant {
+   |         ^^^^^^^^
+
+error: Multiple 'visibility' attributes are not supported on an error variant; found a duplicate here:
+  --> $DIR/attribute-duplication.rs:12:9
+   |
+12 |         AVariant {
+   |         ^^^^^^^^
+
+error: Multiple 'source(from)' attributes are not supported on an error struct; found a duplicate here:
+  --> $DIR/attribute-duplication.rs:47:25
+   |
+47 |     #[snafu(source(from(UsableError, Box::new)))]
+   |                         ^^^^^^^^^^^