Move symlink to OS specific modules

As described in the RFC, the differences in how creating symlinks works
differs sufficiently between Unix and Windows that trying to have a
cross-platform API is a compability hazard.

Author: lambda

text/0000-rename-soft-link-to-symlink.md

@@ -5,13 +5,51 @@
 
 # Summary
 
-Rename `std::fs::soft_link` to `std::fs::symlink` and provide a deprecated
-`std::fs::soft_link` alias.
+Rename `std::fs::soft_link` into platform-specific versions:
+`std::os::unix::fs::symlink`, `std::os::windows::fs::symlink_file`, and
+`std::os::windows::fs::symlink_dir`.
 
 # Motivation
 
-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`.
+Windows Vista introduced the ability to create symbolic links, in order to
+[provide compatibility with applications ported from Unix](https://msdn.microsoft.com/en-us/library/windows/desktop/aa365680%28v=vs.85%29.aspx):
+
+> 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][1] in order to be able to do
+so; while on Unix, ordinary users can create symlinks, and any additional
+security policy (such as [Grsecurity][2]) generally restricts
+whether applications follow symlinks, not whether a user can create them.
+
+[1]: (https://technet.microsoft.com/en-us/library/cc766301%28WS.10%29.aspx) in order to be able to do
+[2]: https://en.wikibooks.org/wiki/Grsecurity/Appendix/Grsecurity_and_PaX_Configuration_Options#Linking_restrictions
+
+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
@@ -44,14 +82,12 @@ 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,
-symbolic links themselves are a conept that were only relatively recently
-(2007, with Windows Vista) introduced to Windows, with the [main motivator
-being Unix compatibility](https://msdn.microsoft.com/en-us/library/windows/desktop/aa365680%28v=vs.85%29.aspx):
+have any precedent as a formal name for the concept or API.
 
-> 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.
+Furthermore, even on Windows, the name for the [reparse point tag used][3] to
+represent symbolic links is `IO_REPARSE_TAG_SYMLINK`.
+
+[3]: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365511%28v=vs.85%29.aspx
 
 If you do a Google search for "[windows symbolic link](https://www.google.com/search?q=windows+symbolic+link&ie=utf-8&oe=utf-8)" or "[windows soft link](https://www.google.com/search?q=windows+soft+link&ie=utf-8&oe=utf-8)",
 many of the documents you find start using "symlink" after introducing the
@@ -60,9 +96,14 @@ among Windows developers and users.
 
 # Detailed design
 
-Rename `std::fs::soft_link` to `std::fs::symlink`, and provide a deprecated
-`std::fs::soft_link` wrapper for backwards compatibility.  Update the
-documentaiton to use "symbolic link" in prose, rather than "soft link".
+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::softlink`, to create a file symbolic link).
 
 # Drawbacks
 
@@ -71,7 +112,27 @@ around.
 
 # Alternatives
 
-Other choices for the name would be:
+* 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`
@@ -83,12 +144,29 @@ 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-targetted APIs, Delphi and some of the PowerShell cmdlets
-observed.
+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.
+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