Summary

Deprecate std::fs::soft_link in favor of platform-specific versions: std::os::unix::fs::symlink, std::os::windows::fs::symlink_file, and std::os::windows::fs::symlink_dir.

Motivation

Windows Vista introduced the ability to create symbolic links, in order to provide compatibility with applications ported from Unix:

Symbolic links are designed to aid in migration and application compatibility with UNIX operating systems. Microsoft has implemented its symbolic links to function just like UNIX links.

However, symbolic links on Windows behave differently enough than symbolic links on Unix family operating systems that you can't, in general, assume that code that works on one will work on the other. On Unix family operating systems, a symbolic link may refer to either a directory or a file, and which one is determined when it is resolved to an actual file. On Windows, you must specify at the time of creation whether a symbolic link refers to a file or directory.

In addition, an arbitrary process on Windows is not allowed to create a symlink; you need to have particular privileges in order to be able to do so; while on Unix, ordinary users can create symlinks, and any additional security policy (such as Grsecurity) generally restricts whether applications follow symlinks, not whether a user can create them.

Thus, there needs to be a way to distinguish between the two operations on Windows, but that distinction is meaningless on Unix, and any code that deals with symlinks on Windows will need to depend on having appropriate privilege or have some way of obtaining appropriate privilege, which is all quite platform specific.

These two facts mean that it is unlikely that arbitrary code dealing with symbolic links will be portable between Windows and Unix. Rather than trying to support both under one API, it would be better to provide platform specific APIs, making it much more clear upon inspection where portability issues may arise.

In addition, the current name soft_link is fairly non-standard. At some point in the split up version of rust-lang/rfcs#517, std::fs::symlink was renamed to sym_link and then to soft_link.

The new name is somewhat surprising and can be difficult to find. After a poll of a number of different platforms and languages, every one appears to contain symlink, symbolic_link, or some camel case variant of those for their equivalent API. Every piece of formal documentation found, for both Windows and various Unix like platforms, used "symbolic link" exclusively in prose.

Here are the names I found for this functionality on various platforms, libraries, and languages:

POSIX/Single Unix Specification: symlink
Windows: CreateSymbolicLink
Objective-C/Swift: createSymbolicLinkAtPath:withDestinationPath:error:
Java: createSymbolicLink
C++ (Boost/draft standard): create_symlink
Ruby: symlink
Python: symlink
Perl: symlink
PHP: symlink
Delphi: FileCreateSymLink
PowerShell has no official version, but several community cmdlets (one example, another example) are named New-SymLink

The term "soft link", probably as a contrast with "hard link", is found frequently in informal descriptions, but almost always in the form of a parenthetical of an alternate phrase, such as "a symbolic link (or soft link)". I could not find it used in any formal documentation or APIs outside of Rust.

The name soft_link was chosen to be shorter than symbolic_link, but without using Unix specific jargon like symlink, to not give undue weight to one platform over the other. However, based on the evidence above it doesn't have any precedent as a formal name for the concept or API.

Furthermore, even on Windows, the name for the reparse point tag used to represent symbolic links is IO_REPARSE_TAG_SYMLINK.

If you do a Google search for "windows symbolic link" or "windows soft link", many of the documents you find start using "symlink" after introducing the concept, so it seems to be a fairly common abbreviation for the full name even among Windows developers and users.

Detailed design

Move std::fs::soft_link to std::os::unix::fs::symlink, and create std::os::windows::fs::symlink_file and std::os::windows::fs::symlink_dir that call CreateSymbolicLink with the appropriate arguments.

Keep a deprecated compatibility wrapper std::fs::soft_link which wraps std::os::unix::fs::symlink or std::os::windows::fs::symlink_file, depending on the platform (as that is the current behavior of std::fs::soft_link, to create a file symbolic link).

Drawbacks

This deprecates a stable API during the 1.0.0 beta, leaving an extra wrapper around.

Alternatives

Have a cross platform symlink and symlink_dir, that do the same thing on Unix but differ on Windows. This has the drawback of invisible compatibility hazards; code that works on Unix using symlink may fail silently on Windows, as creating the wrong type of symlink may succeed but it may not be interpreted properly once a destination file of the other type is created.
Have a cross platform symlink that detects the type of the destination on Windows. This is not always possible as it's valid to create dangling symbolic links.
Have symlink, symlink_dir, and symlink_file all cross-platform, where the first dispatches based on the destination file type, and the latter two panic if called with the wrong destination file type. Again, this is not always possible as it's valid to create dangling symbolic links.
Rather than having two separate functions on Windows, you could have a separate parameter on Windows to specify the type of link to create; symlink("a", "b", FILE_SYMLINK) vs symlink("a", "b", DIR_SYMLINK). However, having a symlink that had different arity on Unix and Windows would likely be confusing, and since there are only the two possible choices, simply having two functions seems like a much simpler solution.

Other choices for the naming convention would be:

The status quo, soft_link
The original proposal from rust-lang/rfcs#517, sym_link
The full name, symbolic_link

The first choice is non-obvious, for people coming from either Windows or Unix. It is a classic compromise, that makes everyone unhappy.

sym_link is slightly more consistent with the complementary hard_link function, and treating "sym link" as two separate words has some precedent in two of the Windows-targeted APIs, Delphi and some of the PowerShell cmdlets observed. However, I have not found any other snake case API that uses that, and only a couple of Windows-specific APIs that use it in camel case; most usage prefers the single word "symlink" to the two word "sym link" as the abbreviation.

The full name symbolic_link, is a bit long and cumbersome compared to most of the rest of the API, but is explicit and is the term used in prose to describe the concept everywhere, so shouldn't emphasize any one platform over the other. However, unlike all other operations for creating a file or directory (open, create, create_dir, etc), it is a noun, not a verb. When used as a verb, it would be called "symbolically link", but that sounds quite odd in the context of an API: symbolically_link("a", "b"). "symlink", on the other hand, can act as either a noun or a verb.

It would be possible to prefix any of the forms above that read as a noun with create_, such as create_symlink, create_sym_link, create_symbolic_link. This adds further to the verbosity, though it is consisted with create_dir; you would probably need to also rename hard_link to create_hard_link for consistency, and this seems like a lot of churn and extra verbosity for not much benefit, as symlink and hard_link already act as verbs on their own. If you picked this, then the Windows versions would need to be named create_file_symlink and create_dir_symlink (or the variations with sym_link or symbolic_link).

Unresolved questions

If we deprecate soft_link now, early in the beta cycle, would it be acceptable to remove it rather than deprecate it before 1.0.0, thus avoiding a permanently stable but deprecated API right out the gate?

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