rename rustc_attr to rustc_attr_parsing and create rustc_attr_data_structures

compiler/rustc_attr_parsing/src/attributes/transparency.rs

@@ -0,0 +1,36 @@
+use rustc_ast::attr::AttributeExt;
+use rustc_attr_data_structures::TransparencyError;
+use rustc_span::hygiene::Transparency;
+use rustc_span::sym;
+
+pub fn find_transparency(
+    attrs: &[impl AttributeExt],
+    macro_rules: bool,
+) -> (Transparency, Option<TransparencyError>) {
+    let mut transparency = None;
+    let mut error = None;
+    for attr in attrs {
+        if attr.has_name(sym::rustc_macro_transparency) {
+            if let Some((_, old_span)) = transparency {
+                error = Some(TransparencyError::MultipleTransparencyAttrs(old_span, attr.span()));
+                break;
+            } else if let Some(value) = attr.value_str() {
+                transparency = Some((
+                    match value {
+                        sym::transparent => Transparency::Transparent,
+                        sym::semitransparent => Transparency::SemiTransparent,
+                        sym::opaque => Transparency::Opaque,
+                        _ => {
+                            error =
+                                Some(TransparencyError::UnknownTransparency(value, attr.span()));
+                            continue;
+                        }
+                    },
+                    attr.span(),
+                ));
+            }
+        }
+    }
+    let fallback = if macro_rules { Transparency::SemiTransparent } else { Transparency::Opaque };
+    (transparency.map_or(fallback, |t| t.0), error)
+}