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Frank Steffahn 8f8689fb31 Improve unused_unsafe lint

Main motivation: Fixes some issues with the current behavior. This PR is
more-or-less completely re-implementing the unused_unsafe lint; it’s also only
done in the MIR-version of the lint, the set of tests for the `-Zthir-unsafeck`
version no longer succeeds (and is thus disabled, see `lint-unused-unsafe.rs`).

On current nightly,
```rs
unsafe fn unsf() {}

fn inner_ignored() {
    unsafe {
        #[allow(unused_unsafe)]
        unsafe {
            unsf()
        }
    }
}
```

doesn’t create any warnings. This situation is not unrealistic to come by, the
inner `unsafe` block could e.g. come from a macro. Actually, this PR even
includes removal of one unused `unsafe` in the standard library that was missed
in a similar situation. (The inner `unsafe` coming from an external macro hides
    the warning, too.)

The reason behind this problem is how the check currently works:
* While generating MIR, it already skips nested unsafe blocks (i.e. unsafe
  nested in other unsafe) so that the inner one is always the one considered
  unused
* To differentiate the cases of no unsafe operations inside the `unsafe` vs.
  a surrounding `unsafe` block, there’s some ad-hoc magic walking up the HIR to
  look for surrounding used `unsafe` blocks.

There’s a lot of problems with this approach besides the one presented above.
E.g. the MIR-building uses checks for `unsafe_op_in_unsafe_fn` lint to decide
early whether or not `unsafe` blocks in an `unsafe fn` are redundant and ought
to be removed.
```rs
unsafe fn granular_disallow_op_in_unsafe_fn() {
    unsafe {
        #[deny(unsafe_op_in_unsafe_fn)]
        {
            unsf();
        }
    }
}
```
```
error: call to unsafe function is unsafe and requires unsafe block (error E0133)
  --> src/main.rs:13:13
   |
13 |             unsf();
   |             ^^^^^^ call to unsafe function
   |
note: the lint level is defined here
  --> src/main.rs:11:16
   |
11 |         #[deny(unsafe_op_in_unsafe_fn)]
   |                ^^^^^^^^^^^^^^^^^^^^^^
   = note: consult the function's documentation for information on how to avoid undefined behavior

warning: unnecessary `unsafe` block
  --> src/main.rs:10:5
   |
9  | unsafe fn granular_disallow_op_in_unsafe_fn() {
   | --------------------------------------------- because it's nested under this `unsafe` fn
10 |     unsafe {
   |     ^^^^^^ unnecessary `unsafe` block
   |
   = note: `#[warn(unused_unsafe)]` on by default

```
Here, the intermediate `unsafe` was ignored, even though it contains a unsafe
operation that is not allowed to happen in an `unsafe fn` without an additional `unsafe` block.

Also closures were problematic and the workaround/algorithms used on current
nightly didn’t work properly. (I skipped trying to fully understand what it was
supposed to do, because this PR uses a completely different approach.)
```rs
fn nested() {
    unsafe {
        unsafe { unsf() }
    }
}
```
```
warning: unnecessary `unsafe` block
  --> src/main.rs:10:9
   |
9  |     unsafe {
   |     ------ because it's nested under this `unsafe` block
10 |         unsafe { unsf() }
   |         ^^^^^^ unnecessary `unsafe` block
   |
   = note: `#[warn(unused_unsafe)]` on by default
```

vs

```rs
fn nested() {
    let _ = || unsafe {
        let _ = || unsafe { unsf() };
    };
}
```
```
warning: unnecessary `unsafe` block
 --> src/main.rs:9:16
  |
9 |     let _ = || unsafe {
  |                ^^^^^^ unnecessary `unsafe` block
  |
  = note: `#[warn(unused_unsafe)]` on by default

warning: unnecessary `unsafe` block
  --> src/main.rs:10:20
   |
10 |         let _ = || unsafe { unsf() };
   |                    ^^^^^^ unnecessary `unsafe` block
```

*note that this warning kind-of suggests that **both** unsafe blocks are redundant*

--------------------------------------------------------------------------------

I also dislike the fact that it always suggests keeping the outermost `unsafe`.
E.g. for
```rs
fn granularity() {
    unsafe {
        unsafe { unsf() }
        unsafe { unsf() }
        unsafe { unsf() }
    }
}
```
I prefer if `rustc` suggests removing the more-course outer-level `unsafe`
instead of the fine-grained inner `unsafe` blocks, which it currently does on nightly:
```
warning: unnecessary `unsafe` block
  --> src/main.rs:10:9
   |
9  |     unsafe {
   |     ------ because it's nested under this `unsafe` block
10 |         unsafe { unsf() }
   |         ^^^^^^ unnecessary `unsafe` block
   |
   = note: `#[warn(unused_unsafe)]` on by default

warning: unnecessary `unsafe` block
  --> src/main.rs:11:9
   |
9  |     unsafe {
   |     ------ because it's nested under this `unsafe` block
10 |         unsafe { unsf() }
11 |         unsafe { unsf() }
   |         ^^^^^^ unnecessary `unsafe` block

warning: unnecessary `unsafe` block
  --> src/main.rs:12:9
   |
9  |     unsafe {
   |     ------ because it's nested under this `unsafe` block
...
12 |         unsafe { unsf() }
   |         ^^^^^^ unnecessary `unsafe` block
```

--------------------------------------------------------------------------------

Needless to say, this PR addresses all these points. For context, as far as my
understanding goes, the main advantage of skipping inner unsafe blocks was that
a test case like
```rs
fn top_level_used() {
    unsafe {
        unsf();
        unsafe { unsf() }
        unsafe { unsf() }
        unsafe { unsf() }
    }
}
```
should generate some warning because there’s redundant nested `unsafe`, however
every single `unsafe` block _does_ contain some statement that uses it. Of course
this PR doesn’t aim change the warnings on this kind of code example, because
the current behavior, warning on all the inner `unsafe` blocks, makes sense in this case.

As mentioned, during MIR building all the unsafe blocks *are* kept now, and usage
is attributed to them. The way to still generate a warning like
```
warning: unnecessary `unsafe` block
  --> src/main.rs:11:9
   |
9  |     unsafe {
   |     ------ because it's nested under this `unsafe` block
10 |         unsf();
11 |         unsafe { unsf() }
   |         ^^^^^^ unnecessary `unsafe` block
   |
   = note: `#[warn(unused_unsafe)]` on by default

warning: unnecessary `unsafe` block
  --> src/main.rs:12:9
   |
9  |     unsafe {
   |     ------ because it's nested under this `unsafe` block
...
12 |         unsafe { unsf() }
   |         ^^^^^^ unnecessary `unsafe` block

warning: unnecessary `unsafe` block
  --> src/main.rs:13:9
   |
9  |     unsafe {
   |     ------ because it's nested under this `unsafe` block
...
13 |         unsafe { unsf() }
   |         ^^^^^^ unnecessary `unsafe` block
```

in this case is by emitting a `unused_unsafe` warning for all of the `unsafe`
blocks that are _within a **used** unsafe block_.

The previous code had a little HIR traversal already anyways to collect a set of
all the unsafe blocks (in order to afterwards determine which ones are unused
afterwards). This PR uses such a traversal to do additional things including logic
like _always_ warn for an `unsafe` block that’s inside of another **used**
unsafe block. The traversal is expanded to include nested closures in the same go,
this simplifies a lot of things.

The whole logic around `unsafe_op_in_unsafe_fn` is a little complicated, there’s
some test cases of corner-cases in this PR. (The implementation involves
differentiating between whether a used unsafe block was used exclusively by
operations where `allow(unsafe_op_in_unsafe_fn)` was active.) The main goal was
to make sure that code should compile successfully if all the `unused_unsafe`-warnings
are addressed _simultaneously_ (by removing the respective `unsafe` blocks)
no matter how complicated the patterns of `unsafe_op_in_unsafe_fn` being
disallowed and allowed throughout the function are.

--------------------------------------------------------------------------------

One noteworthy design decision I took here: An `unsafe` block
with `allow(unused_unsafe)` **is considered used** for the purposes of
linting about redundant contained unsafe blocks. So while
```rs

fn granularity() {
    unsafe { //~ ERROR: unnecessary `unsafe` block
        unsafe { unsf() }
        unsafe { unsf() }
        unsafe { unsf() }
    }
}
```
warns for the outer `unsafe` block,
```rs

fn top_level_ignored() {
    #[allow(unused_unsafe)]
    unsafe {
        #[deny(unused_unsafe)]
        {
            unsafe { unsf() } //~ ERROR: unnecessary `unsafe` block
            unsafe { unsf() } //~ ERROR: unnecessary `unsafe` block
            unsafe { unsf() } //~ ERROR: unnecessary `unsafe` block
        }
    }
}
```
warns on the inner ones.

2022-02-20 21:00:12 +01:00

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Improve unused_unsafe lint

2022-02-20 21:00:12 +01:00

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Improve unused_unsafe lint

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Release notes for 1.59

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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.

Note: this README is for users rather than contributors. If you wish to contribute to the compiler, you should read the Getting Started section of the rustc-dev-guide instead. You can ask for help in the #new members Zulip stream.

Quick Start

Read "Installation" from The Book.

Installing from Source

The Rust build system uses a Python script called x.py to build the compiler, which manages the bootstrapping process. It lives in the root of the project.

The x.py command can be run directly on most systems in the following format:

./x.py <subcommand> [flags]

This is how the documentation and examples assume you are running x.py.

Systems such as Ubuntu 20.04 LTS do not create the necessary python command by default when Python is installed that allows x.py to be run directly. In that case you can either create a symlink for python (Ubuntu provides the python-is-python3 package for this), or run x.py using Python itself:

# Python 3
python3 x.py <subcommand> [flags]

# Python 2.7
python2.7 x.py <subcommand> [flags]

More information about x.py can be found by running it with the --help flag or reading the rustc dev guide.

Building on a Unix-like system

Make sure you have installed the dependencies:
- g++ 5.1 or later or clang++ 3.5 or later
- python 3 or 2.7
- GNU make 3.81 or later
- cmake 3.13.4 or later
- ninja
- curl
- git
- ssl which comes in libssl-dev or openssl-devel
- pkg-config if you are compiling on Linux and targeting Linux

Clone the source with git:

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

Configure the build settings:

The Rust build system uses a file named config.toml in the root of the source tree to determine various configuration settings for the build. Copy the default config.toml.example to config.toml to get started.
```
cp config.toml.example config.toml
```
If you plan to use x.py install to create an installation, it is recommended that you set the prefix value in the [install] section to a directory.

Create install directory if you are not installing in default directory
Build and install:
```
./x.py build && ./x.py install
```
When complete, ./x.py install will place several programs into $PREFIX/bin: rustc, the Rust compiler, and rustdoc, the API-documentation tool. This install does not include Cargo, Rust's package manager. To build and install Cargo, you may run ./x.py install cargo or set the build.extended key in config.toml to true to build and install all tools.

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', 'cmake' and 'ninja'
# 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-python \
            mingw-w64-x86_64-cmake \
            mingw-w64-x86_64-gcc \
            mingw-w64-x86_64-ninja

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 2017 (or later) so rustc can use its linker. The simplest way is to get the Visual Studio, check the “C++ build tools” and “Windows 10 SDK” workload.

(If you're installing cmake yourself, be careful that “C++ CMake tools for Windows” doesn't get included under “Individual components”.)

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 and the build system doesn't understand, 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\2019\Community\VC\Auxiliary\Build\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 Installing 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 stage 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, 10, ...)	✓	✓
Linux (kernel 2.6.32, glibc 2.11 or later)	✓	✓
macOS (10.7 Lion or later)	(*)	✓

(*): Apple dropped support for running 32-bit binaries starting from macOS 10.15 and iOS 11. Due to this decision from Apple, the targets are no longer useful to our users. Please read our blog post for more info.

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

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

If you are interested in contributing to the Rust project, please take a look at the Getting Started guide in the rustc-dev-guide.

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.

See LICENSE-APACHE, LICENSE-MIT, and COPYRIGHT for details.

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README.md Unescape Escape

The Rust Programming Language

Quick Start

Installing from Source

Building on a Unix-like system

Building on Windows

MinGW

MSVC

Specifying an ABI

Configure and Make

Building Documentation

Notes

Getting Help

Contributing

License

Trademark

README.md