Support long path names on all Windows versions

text/0000-long-paths-windows.md

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+- Feature Name: long_windows_paths
+- Start Date: 2017-10-24
+- RFC PR:
+- Rust Issue:
+
+# Summary
+[summary]: #summary
+
+Rust programs currently cannot access files on Windows 8 and below with paths longer than `MAX_PATH`.  This is a bug and should be fixed.
+
+# Motivation
+[motivation]: #motivation
+
+Windows 8 and below make it difficult but not impossible for a program to access a file with a name longer than `MAX_PATH`.  The kernel APIs have no problem with such paths at all, but the user-mode APIs do.  They require that the path be prefixed with `\\?\`, which disables all normalization.
+
+Fortunately, there is a solution that gets back normalization: if (and only if) the file name *does not* begin with `\??\` or `\\?\`, one can pass it to the Windows API function `GetFullPathNameW`, which supports long paths just fine and can easily be called from Rust.  The resulting path can safely have `\\?\` prepended and then be passed to the underlying Windows API calls.  `GetFullPathNameW`’s documentation claims that it does not support long paths, but it in fact does.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+This should require at most a brief mention in the Rust book.  All that is needed is a note that while many programs do not support long paths on Windows 8 and below (or Windows 10 without proper manifest/registry settings), Rust’s standard library has no such problems, and will work on any path that the underlying NT API supports.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Whenever Rust sends a path to an underlying NT API, it should use first check if the path length is less than 260 characters **or** begins with either `\\?\` or `\??\`, then pass it to the underlying Windows API function directly.  Otherwise, pass it to `GetFullPathNameW`.  If it fails, return; if not, pass the result to the underlying Windows API call.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+This adds a small amount of complexity to the standard library.  This complexity should be more than compensated by the ability to handle long paths generated by e.g. NPM.
+
+# Rationale and alternatives
+[alternatives]: #alternatives
+
+The alternatives are:
+
+- Do nothing.  This is bad, because tools like NPM already support and generate long paths.
+- Try to implement `GetFullPathNameW` in pure Rust.  This is not easy, and does not guarantee compatibility with the Windows implementation.
+- Use `GetFullPathNameW` to allow for long paths on Windows.  This is the approach chosen here.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None.  This is a bug fix, pure and simple.