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| <h1 id="unsafety-checking"><a class="header" href="#unsafety-checking">Unsafety checking</a></h1> |
| <p>Certain expressions in Rust can violate memory safety and as such need to be |
| inside an <code>unsafe</code> block or function. The compiler will also warn if an unsafe |
| block is used without any corresponding unsafe operations.</p> |
| <h2 id="overview"><a class="header" href="#overview">Overview</a></h2> |
| <p>The unsafety check is located in the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_build/check_unsafety/index.html"><code>check_unsafety</code></a> module. It performs a |
| walk over the <a href="./thir.html">THIR</a> of a function and all of its closures and inline constants. |
| It keeps track of the unsafe context: whether it has entered an <code>unsafe</code> block. |
| If an unsafe operation is used outside of an <code>unsafe</code> block, then an error is |
| reported. If an unsafe operation is used in an unsafe block then that block is |
| marked as used for <a href="#the-unused_unsafe-lint">the unused_unsafe lint</a>.</p> |
| <p>The unsafety check needs type information so could potentially be done on the |
| HIR, making use of typeck results, THIR or MIR. THIR is chosen because there are |
| fewer cases to consider than in HIR, for example unsafe function calls and |
| unsafe method calls have the same representation in THIR. The check is not done |
| on MIR because safety checks do not depend on control flow so MIR is not |
| necessary to use and MIR doesn’t have as precise spans for some expressions.</p> |
| <p>Most unsafe operations can be identified by checking the <code>ExprKind</code> in THIR and |
| checking the type of the argument. For example, dereferences of a raw pointer |
| correspond to <code>ExprKind::Deref</code>s with an argument that has a raw pointer type.</p> |
| <p>Looking for unsafe Union field accesses is a bit more complex because writing to |
| a field of a union is safe. The checker tracks when it’s visiting the left-hand |
| side of an assignment expression and allows union fields to directly appear |
| there, while erroring in all other cases. Union field accesses can also occur |
| in patterns, so those have to be walked as well.</p> |
| <p>The other complicated safety check is for writes to fields of layout constrained |
| structs (such as <a href="https://doc.rust-lang.org/std/ptr/struct.NonNull.html"><code>NonNull</code></a>). These are found by looking for the borrow or |
| assignment expression and then visiting the subexpression being borrowed or |
| assigned with a separate visitor.</p> |
| <h2 id="the-unused_unsafe-lint"><a class="header" href="#the-unused_unsafe-lint">The unused_unsafe lint</a></h2> |
| <p>The unused_unsafe lint reports <code>unsafe</code> blocks that can be removed. The unsafety |
| checker records whenever it finds an operation that requires unsafe. The lint is |
| then reported if either:</p> |
| <ul> |
| <li>An <code>unsafe</code> block contains no unsafe operations</li> |
| <li>An <code>unsafe</code> block is within another unsafe block, and the outer block |
| isn’t considered unused</li> |
| </ul> |
| <pre class="playground"><code class="language-rust"><span class="boring">#![allow(unused)] |
| </span>#![deny(unused_unsafe)] |
| <span class="boring">fn main() { |
| </span>let y = 0; |
| let x: *const u8 = core::ptr::addr_of!(y); |
| unsafe { // lint reported for this block |
| unsafe { |
| let z = *x; |
| } |
| let safe_expr = 123; |
| } |
| unsafe { |
| unsafe { // lint reported for this block |
| let z = *x; |
| } |
| let unsafe_expr = *x; |
| } |
| <span class="boring">}</span></code></pre> |
| <h2 id="other-checks-involving-unsafe"><a class="header" href="#other-checks-involving-unsafe">Other checks involving <code>unsafe</code></a></h2> |
| <p><a href="https://doc.rust-lang.org/reference/items/traits.html#unsafe-traits">Unsafe traits</a> require an <code>unsafe impl</code> to be implemented, the check for this |
| is done as part of <a href="https://github.com/rust-lang/rust/blob/HEAD/compiler/rustc_hir_analysis/src/coherence/unsafety.rs">coherence</a>. The <code>unsafe_code</code> lint is run as a lint pass on |
| the ast that searches for unsafe blocks, functions and implementations, as well |
| as certain unsafe attributes.</p> |
| |
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