ast-exporter: prefer non-implicit declarations when exporting function declarations

implicit declarations for functions like malloc do not know about the typedefs that may be used in the declaration present in system headers. because we want to reify these typedefs into portable Rust types, we want to prefer the actual written decl when exporting function parameter declarations and function types

Author: fw-immunant

c2rust-ast-exporter/src/AstExporter.cpp

@@ -1838,6 +1838,15 @@ class TranslateASTVisitor final
         auto body =
             FD->getBody(paramsFD); // replaces its argument if body exists
 
+        // Avoid getting params from an implicit decl if a subsequent non-implicit decl exists.
+        // Implicit decls will not have names for params, but more importantly, they will never
+        // reference header-declared typedefs, so we would miss the fact that e.g. malloc is
+        // declared to accept `size_t` in its stdlib.h declaration, while its implicit declaration
+        // accepts the built-in `unsigned long`.
+        if (FD->isImplicit()) {
+            paramsFD = FD->getMostRecentDecl();
+        }
+
         std::vector<void *> childIds;
         for (auto x : paramsFD->parameters()) {
             auto cd = x->getCanonicalDecl();
@@ -1847,7 +1856,8 @@ class TranslateASTVisitor final
 
         childIds.push_back(body);
 
-        auto functionType = FD->getType();
+        // We prefer non-implicit decls for their type information.
+        auto functionType = paramsFD->getType();
         auto span = paramsFD->getSourceRange();
         encode_entry(
             FD, TagFunctionDecl, span, childIds, functionType,