Deprecate type aliases and structs in std::os::$platform::raw in favor of trait-based accessors which return Rust types rather than the equivalent C type aliases.


RFC 517 set forth a vision for the raw modules in the standard library to perform lowering operations on various Rust types to their platform equivalents. For example the fs::Metadata structure can be lowered to the underlying sys::stat structure. The rationale for this was to enable building abstractions externally from the standard library by exposing all of the underlying data that is obtained from the OS.

This strategy, however, runs into a few problems:

  • For some libc structures, such as stat, there's not actually one canonical definition. For example on 32-bit Linux the definition of stat will change depending on whether LFS is enabled (via the -D_FILE_OFFSET_BITS macro). This means that if std is advertises these raw types as being "FFI compatible with libc", it's not actually correct in all circumstances!
  • Intricately exporting raw underlying interfaces (such as &stat from &fs::Metadata) makes it difficult to change the implementation over time. Today the 32-bit Linux standard library doesn't use LFS functions, so files over 4GB cannot be opened. Changing this, however, would involve changing the stat structure and may be difficult to do.
  • Trait extensions in the raw module attempt to return the libc aliased type on all platforms, for example DirEntryExt::ino returns a type of ino_t. The ino_t type is billed as being FFI compatible with the libc ino_t type, but not all platforms store the d_ino field in dirent with the ino_t type. For example on Android the definition of ino_t is u32 but the actual stored value is u64. This means that on Android we're actually silently truncating the return value!

Over time it's basically turned out that exporting the somewhat-messy details of libc has gotten a little messy in the standard library as well. Exporting this functionality (e.g. being able to access all of the fields), is quite useful however! This RFC proposes tweaking the design of the extensions in std::os::*::raw to allow the same level of information exposure that happens today but also cut some of the tie from libc to std to give us more freedom to change these implementation details and work around weird platforms.

Detailed design

First, the types and type aliases in std::os::*::raw will all be deprecated. For example stat, ino_t, dev_t, mode_t, etc, will all be deprecated (in favor of their definitions in the libc crate). Note that the C integer types, c_int and friends, will not be deprecated.

Next, all existing extension traits will cease to return platform specific type aliases (such as the DirEntryExt::ino function). Instead they will return u64 across the board unless it's 100% known for sure that fewer bits will suffice. This will improve consistency across platforms as well as avoid truncation problems such as those Android is experiencing. Furthermore this frees std from dealing with any odd FFI compatibility issues, punting that to the libc crate itself it the values are handed back into C.

The std::os::*::fs::MetadataExt will have its as_raw_stat method deprecated, and it will instead grow functions to access all the associated fields of the underlying stat structure. This means that there will now be a trait-per-platform to expose all this information. Also note that all the methods will likely return u64 in accordance with the above modification.

With these modifications to what std::os::*::raw includes and how it's defined, it should be easy to tweak existing implementations and ensure values are transmitted in a lossless fashion. The changes, however, are both breaking changes and don't immediately enable fixing bugs like using LFS on Linux:

  • Code such as let a: ino_t = entry.ino() would break as the ino() function will return u64, but the definition of ino_t may not be u64 for all platforms.
  • The stat structure itself on 32-bit Linux still uses 32-bit fields (e.g. it doesn't mirror stat64 in libc).

To help with these issues, more extensive modifications can be made to the platform specific modules. All type aliases can be switched over to u64 and the stat structure could simply be redefined to stat64 on Linux (minus keeping the same name). This would, however, explicitly mean that std::os::raw is no longer FFI compatible with C.

This breakage can be clearly indicated in the deprecation messages, however. Additionally, this fits within std's breaking changes policy as a local as cast should be all that's needed to patch code that breaks to straddle versions of Rust.


As mentioned above, this RFC is strictly-speaking a breaking change. It is expected that not much code will break, but currently there is no data supporting this.

Returning u64 across the board could be confusing in some circumstances as it may wildly differ both in terms of signedness as well as size from the underlying C type. Converting it back to the appropriate type runs the risk of being onerous, but accessing these raw fields in theory happens quite rarely as std should primarily be exporting cross-platform accessors for the various fields here and there.


  • The documentation of the raw modules in std could be modified to indicate that the types contained within are intentionally not FFI compatible, and the same structure could be preserved today with the types all being rewritten to what they would be anyway if this RFC were implemented. For example ino_t on Android would change to u64 and stat on 32-bit Linux would change to stat64. In doing this, however, it's not clear why we'd keep around all the C namings and structure.

  • Instead of breaking existing functionality, new accessors and types could be added to acquire the "lossless" version of a type. For example we could add a ino64 function on DirEntryExt which returns a u64, and for stat we could add as_raw_stat64. This would, however, force Metadata to store two different stat structures, and the breakage in practice this will cause may be small enough to not warrant these great lengths.

Unresolved questions

  • Is the policy of almost always returning u64 too strict? Should types like mode_t be allowed as i32 explicitly? Should the sign at least attempt to always be preserved?