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