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bors 666687a68c Auto merge of #45072 - nikomatsakis:issue-38714, r=arielb1 new rules for merging expected and supplied types in closure signatures As uncovered in #38714, we currently have some pretty bogus code for combining the "expected signature" of a closure with the "supplied signature". To set the scene, consider a case like this: ```rust fn foo<F>(f: F) where F: for<'a> FnOnce(&'a u32) -> &'a u32 // ^ expected signature comes from this where-clause { ... } fn main() { foo(\|x: &u32\| -> &u32 { .. } // ^^^^^^^^^^^^^^^^^ supplied signature // comes from here } ``` In this case, the supplied signature (a) includes all the parts and (b) is the same as the expected signature, modulo the names used for the regions. But often people supply only some parts of the signature. For example, one might write `foo(\|x\| ..)`, leaving everything to be inferred, or perhaps `foo(\|x: &u32\| ...)`, which leaves the return type to be inferred. In the current code, we use the expected type to supply the types that are not given, but otherwise use the type the user gave, except for one case: if the user writes `fn foo(\|x: _\| ..)` (i.e., an underscore at the outermost level), then we will take the expected type (rather than instantiating a fresh type variable). This can result in nonsensical situations, particularly with bound regions that link the types of parameters to one another or to the return type. Consider `foo(\|x: &u32\| ...)` -- if we literally splice the expected return type of `&'a u32` together with what the user gave, we wind up with a signature like `for<'a> fn(&u32) -> &'a u32`. This is not even permitted as a type, because bound regions like `'a` must appear also in the arguments somewhere, which is why #38714 leads to an ICE. This PR institutes some new rules. These are not meant to be the final set of rules, but they are a kind of "lower bar" for what kind of code we accept (i.e., we can extend these rules in the future to be smarter in some cases, but -- as we will see -- these rules do accept some things that we then would not be able to back off from). These rules are derived from a few premises: - First and foremost, anonymous regions in closure annotation are mostly requests for the code to "figure out the right lifetime" and shouldn't be read too closely. So for example when people write a closure signature like `\|x: &u32\|`, they are really intended for us to "figure out" the right region for `x`. - In contrast, the current code treats this supplied type as being more definitive. In particular, writing `\|x: &u32\|` would always result in the region of `x` being bound in the closure type. In other words, the signature would be something like `for<'a> fn(&'a u32)` -- this is derived from the fact that `fn(&u32)` expands to a type where the region is bound in the fn type. - This PR takes a different approach. The "binding level" for reference types appearing in closure signatures can be informed in some cases by the expected signature. So, for example, if the expected signature is something like `(&'f u32)`, where the region of the first argument appears free, then for `\|x: &u32\|`, the new code would infer `x` to also have the free region `'f`. - This inference has some limits. We don't do this for bindings that appear within the selected types themselves. So e.g. `\|x: fn(&u32)\|`, when combined with an expected type of `fn(fn(&'f u32))`, would still result in a closure that expects `for<'a> fn(&'a u32)`. Such an annotation will ultimately result in an error, as it happens, since `foo` is supplying a `fn(&'f u32)` to the closure, but the closure signature demands a `for<'a> fn(&'a u32)`. But still we choose to trust it and have the user change it. - I wanted to preserve the rough intuition that one can copy-and-paste a type out of the fn signature and into the fn body without dramatically changing its meaning. Interestingly, if one has `\|x: &u32\|`, then regardless of whether the region of `x` is bound or free in the closure signature, it is also free in the region body, and that is also true when one writes `let x: &u32`, so that intuition holds here. But the same would not be true for `fn(&u32)`, hence the different behavior. - Second, we must take either all the references to bound regions from the expected type or none. The current code, as we saw, will happily take a bound region in the return type but drop the other place where it is used, in the parameters. Since bound regions are all about linking multiple things together, I think it's important not to do that. (That said, we could conceivably be a bit less strict here, since the subtyping rules will get our back, but we definitely don't want any bound regions that appear only in the return type.) - Finally, we cannot take the bound region names from the supplied types and "intermix" them with the names from the expected types. - We could potentially do some alpha renaming, but I didn't do that. - Ultimately, if the types the user supplied do not match expectations in some way that we cannot recover from, we fallback to deriving the closure signature solely from those expected types. - For example, if the expected type is `u32` but the user wrote `i32`. - Or, more subtle, if the user wrote e.g. `&'x u32` for some named lifetime `'x`, but the expected type includes a bound lifetime (`for<'a> (&'a u32)`). In that case, preferring the type that the user explicitly wrote would hide an appearance of a bound name from the expected type, and we try to never do that. The detailed rules that I came up with are found in the code, but for ease of reading I've also [excerpted them into a gist](https://gist.github.com/nikomatsakis/e69252a2b57e6d97d044c2f254c177f1). I am not convinced they are correct and would welcome feedback for alternative approaches. (As an aside, the way I think I would ultimately prefer to think about this is that the conversion from HIR types to internal types could be parameterized by an "expected type" that it uses to guide itself. However, since that would be a pain, I opted in the code to first instantiate the supplied types as `Ty<'tcx>` and then "merge" those types with the `Ty<'tcx>` from the expected signature.) I think we should probably FCP this before landing. cc @rust-lang/lang r? @arielb1		2017-11-05 16:49:08 +00:00
src	Auto merge of #45072 - nikomatsakis:issue-38714, r=arielb1	2017-11-05 16:49:08 +00:00
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README.md

The Rust Programming Language

This is the main source code repository for Rust. It contains the compiler, standard library, and documentation.

Quick Start

Read "Installation" from The Book.

Building from Source

Building on *nix

Make sure you have installed the dependencies:
- g++ 4.7 or later or clang++ 3.x or later
- python 2.7 (but not 3.x)
- GNU make 3.81 or later
- cmake 3.4.3 or later
- curl
- git

Clone the source with git:

$ git clone https://github.com/rust-lang/rust.git
$ cd rust

Build and install:
```
$ ./x.py build && sudo ./x.py install
```
Note: Install locations can be adjusted by copying the config file from ./config.toml.example to ./config.toml, and adjusting the prefix option under [install]. Various other options, such as enabling debug information, are also supported, and are documented in the config file.

When complete, sudo ./x.py install will place several programs into /usr/local/bin: rustc, the Rust compiler, and rustdoc, the API-documentation tool. This install does not include Cargo, Rust's package manager, which you may also want to build.

Building on Windows

There are two prominent ABIs in use on Windows: the native (MSVC) ABI used by Visual Studio, and the GNU ABI used by the GCC toolchain. Which version of Rust you need depends largely on what C/C++ libraries you want to interoperate with: for interop with software produced by Visual Studio use the MSVC build of Rust; for interop with GNU software built using the MinGW/MSYS2 toolchain use the GNU build.

MinGW

MSYS2 can be used to easily build Rust on Windows:

Grab the latest MSYS2 installer and go through the installer.
Run mingw32_shell.bat or mingw64_shell.bat from wherever you installed MSYS2 (i.e. C:\msys64), depending on whether you want 32-bit or 64-bit Rust. (As of the latest version of MSYS2 you have to run msys2_shell.cmd -mingw32 or msys2_shell.cmd -mingw64 from the command line instead)

From this terminal, install the required tools:

# Update package mirrors (may be needed if you have a fresh install of MSYS2)
$ pacman -Sy pacman-mirrors

# Install build tools needed for Rust. If you're building a 32-bit compiler,
# then replace "x86_64" below with "i686". If you've already got git, python,
# or CMake installed and in PATH you can remove them from this list. Note
# that it is important that you do **not** use the 'python2' and 'cmake'
# packages from the 'msys2' subsystem. The build has historically been known
# to fail with these packages.
$ pacman -S git \
            make \
            diffutils \
            tar \
            mingw-w64-x86_64-python2 \
            mingw-w64-x86_64-cmake \
            mingw-w64-x86_64-gcc

Navigate to Rust's source code (or clone it), then build it:
```
$ ./x.py build && ./x.py install
```

MSVC

MSVC builds of Rust additionally require an installation of Visual Studio 2013 (or later) so rustc can use its linker. Make sure to check the “C++ tools” option.

With these dependencies installed, you can build the compiler in a cmd.exe shell with:

> python x.py build

Currently building Rust only works with some known versions of Visual Studio. If you have a more recent version installed the build system doesn't understand then you may need to force rustbuild to use an older version. This can be done by manually calling the appropriate vcvars file before running the bootstrap.

CALL "C:\Program Files (x86)\Microsoft Visual Studio 14.0\VC\bin\amd64\vcvars64.bat"
python x.py build

Specifying an ABI

Each specific ABI can also be used from either environment (for example, using the GNU ABI in powershell) by using an explicit build triple. The available Windows build triples are:

GNU ABI (using GCC)
- i686-pc-windows-gnu
- x86_64-pc-windows-gnu
The MSVC ABI
- i686-pc-windows-msvc
- x86_64-pc-windows-msvc

The build triple can be specified by either specifying --build=<triple> when invoking x.py commands, or by copying the config.toml file (as described in Building From Source), and modifying the build option under the [build] section.

Configure and Make

While it's not the recommended build system, this project also provides a configure script and makefile (the latter of which just invokes x.py).

$ ./configure
$ make && sudo make install

When using the configure script, the generated config.mk file may override the config.toml file. To go back to the config.toml file, delete the generated config.mk file.

Building Documentation

If you’d like to build the documentation, it’s almost the same:

$ ./x.py doc

The generated documentation will appear under doc in the build directory for the ABI used. I.e., if the ABI was x86_64-pc-windows-msvc, the directory will be build\x86_64-pc-windows-msvc\doc.

Notes

Since the Rust compiler is written in Rust, it must be built by a precompiled "snapshot" version of itself (made in an earlier state of development). As such, source builds require a connection to the Internet, to fetch snapshots, and an OS that can execute the available snapshot binaries.

Snapshot binaries are currently built and tested on several platforms:

Platform / Architecture	x86	x86_64
Windows (7, 8, Server 2008 R2)	✓	✓
Linux (2.6.18 or later)	✓	✓
OSX (10.7 Lion or later)	✓	✓

You may find that other platforms work, but these are our officially supported build environments that are most likely to work.

Rust currently needs between 600MiB and 1.5GiB of RAM to build, depending on platform. If it hits swap, it will take a very long time to build.

There is more advice about hacking on Rust in CONTRIBUTING.md.

Getting Help

The Rust community congregates in a few places:

Stack Overflow - Direct questions about using the language.
users.rust-lang.org - General discussion and broader questions.
/r/rust - News and general discussion.

Contributing

To contribute to Rust, please see CONTRIBUTING.

Rust has an IRC culture and most real-time collaboration happens in a variety of channels on Mozilla's IRC network, irc.mozilla.org. The most popular channel is #rust, a venue for general discussion about Rust. And a good place to ask for help would be #rust-beginners.

License

Rust is primarily distributed under the terms of both the MIT license and the Apache License (Version 2.0), with portions covered by various BSD-like licenses.

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