Handle char* fields declared in libc structs (#156)

`char*` fields in libc structs are `*mut i8`. We represent `char*` as
`*mut u8`.

Author: lucic71

cpp2rust/converter/converter.cpp

@@ -2542,6 +2542,16 @@ bool Converter::VisitMemberExpr(clang::MemberExpr *expr) {
     return false;
   }
 
+  // char* fields in libc structs are *mut i8. We represent char* as *mut u8. Do
+  // the i8 -> u8 conversion here.
+  if (IsCharPointerFieldFromLibc(member)) {
+    StrCat(std::format("({} as {})", str,
+                       member->getType()->getPointeeType().isConstQualified()
+                           ? "*const u8"
+                           : "*mut u8"));
+    return false;
+  }
+
   StrCat(str);
   return false;
 }

cpp2rust/converter/converter_lib.cpp

@@ -129,6 +129,16 @@ bool IsInMainFile(const clang::Decl *decl) {
   return src_mgr.isInMainFile(src_mgr.getExpansionLoc(loc));
 }
 
+bool IsCharPointerFieldFromLibc(const clang::ValueDecl *decl) {
+  auto field = clang::dyn_cast<clang::FieldDecl>(decl);
+  if (!field || !field->getType()->isPointerType() ||
+      !field->getType()->getPointeeType()->isCharType()) {
+    return false;
+  }
+  return field->getASTContext().getSourceManager().isInSystemHeader(
+      field->getParent()->getLocation());
+}
+
 bool IsUserDefinedDecl(const clang::Decl *decl) {
   const auto &ctx = decl->getASTContext();
   const auto &src_mgr = ctx.getSourceManager();

cpp2rust/converter/converter_lib.h

@@ -39,6 +39,8 @@ bool IsComparisonWithNullOp(const clang::BinaryOperator *expr);
 
 bool IsInMainFile(const clang::Decl *decl);
 
+bool IsCharPointerFieldFromLibc(const clang::ValueDecl *decl);
+
 bool IsUserDefinedDecl(const clang::Decl *decl);
 
 bool RefersToUserDefinedDecl(const clang::Expr *expr);

tests/unit/libc_char_ptr_field.c

@@ -0,0 +1,12 @@
+// no-compile: refcount
+#include <pwd.h>
+#include <unistd.h>
+
+int main(void) {
+  struct passwd *pw = getpwuid(geteuid());
+  if (!pw) {
+    return 0;
+  }
+  char *home = pw->pw_dir;
+  return home == 0;
+}

tests/unit/out/unsafe/libc_char_ptr_field.rs

@@ -0,0 +1,21 @@
+extern crate libc;
+use libc::*;
+extern crate libcc2rs;
+use libcc2rs::*;
+use std::collections::BTreeMap;
+use std::io::{Read, Seek, Write};
+use std::os::fd::{AsFd, FromRawFd, IntoRawFd};
+use std::rc::Rc;
+pub fn main() {
+    unsafe {
+        std::process::exit(main_0() as i32);
+    }
+}
+unsafe fn main_0() -> i32 {
+    let mut pw: *mut passwd = libc::getpwuid(libc::geteuid());
+    if !!(pw).is_null() {
+        return 0;
+    }
+    let mut home: *mut u8 = ((*pw).pw_dir as *mut u8);
+    return (((home).is_null()) as i32);
+}