Document unsafety checking (#1847)

Co-authored-by: Yuki Okushi <jtitor@2k36.org>

src/SUMMARY.md

@@ -142,6 +142,7 @@
         - [Return Position Impl Trait In Trait](./return-position-impl-trait-in-trait.md)
 - [Effect checking](./effects.md)
 - [Pattern and Exhaustiveness Checking](./pat-exhaustive-checking.md)
+- [Unsafety Checking](./unsafety-checking.md)
 - [MIR dataflow](./mir/dataflow.md)
 - [Drop elaboration](./mir/drop-elaboration.md)
 - [The borrow checker](./borrow_check.md)

src/thir.md

@@ -4,16 +4,13 @@
 
 The THIR ("Typed High-Level Intermediate Representation"), previously called HAIR for
 "High-Level Abstract IR", is another IR used by rustc that is generated after
-[type checking]. It is (as of <!-- date-check --> April 2022) only used for
-[MIR construction] and [exhaustiveness checking]. There is also
-[an experimental unsafety checker][thir-unsafeck] that operates on the THIR as a replacement for
-the current MIR unsafety checker, and can be used instead of the MIR unsafety checker by passing
-the `-Z thir-unsafeck` flag to `rustc`.
+[type checking]. It is (as of <!-- date-check --> January 2024) used for
+[MIR construction], [exhaustiveness checking], and [unsafety checking].
 
 [type checking]: ./type-checking.md
 [MIR construction]: ./mir/construction.md
 [exhaustiveness checking]: ./pat-exhaustive-checking.md
-[thir-unsafeck]: https://github.com/rust-lang/compiler-team/issues/402
+[unsafety checking]: ./unsafety-checking.md
 
 As the name might suggest, the THIR is a lowered version of the [HIR] where all
 the types have been filled in, which is possible after type checking has completed.

src/unsafety-checking.md

@@ -0,0 +1,79 @@
+# Unsafety Checking
+
+Certain expressions in Rust can violate memory safety and as such need to be
+inside an `unsafe` block or function. The compiler will also warn if an unsafe
+block is used without any corresponding unsafe operations.
+
+## Overview
+
+The unsafety check is located in the [`check_unsafety`] module. It performs a
+walk over the [THIR] of a function and all of its closures and inline constants.
+It keeps track of the unsafe context: whether it has entered an `unsafe` block.
+If an unsafe operation is used outside of an `unsafe` block, then an error is
+reported. If an unsafe operation is used in an unsafe block then that block is
+marked as used for [the unused_unsafe lint](#the-unused_unsafe-lint).
+
+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.
+
+Most unsafe operations can be identified by checking the `ExprKind` in THIR and
+checking the type of the argument. For example, dereferences of a raw pointer
+correspond to `ExprKind::Deref`s with an argument that has a raw pointer type.
+
+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.
+
+The other complicated safety check is for writes to fields of layout constrained
+structs (such as [`NonNull`]). These are found by looking for the borrow or
+assignment expression and then visiting the subexpression being borrowed or
+assigned with a separate visitor.
+
+[THIR]: ./thir.md
+[`check_unsafety`]: https://doc.rust-lang.org/nightly/nightly-rustc/rustc_mir_build/check_unsafety/index.html
+[`NonNull`]: https://doc.rust-lang.org/std/ptr/struct.NonNull.html
+
+## The unused_unsafe lint
+
+The unused_unsafe lint reports `unsafe` blocks that can be removed. The unsafety
+checker records whenever it finds an operation that requires unsafe. The lint is
+then reported if either:
+
+- An `unsafe` block contains no unsafe operations
+- An `unsafe` block is within another unsafe block, and the outer block
+  isn't considered unused
+
+```rust
+#![deny(unused_unsafe)]
+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;
+}
+```
+
+## Other checks involving `unsafe`
+
+[Unsafe traits] require an `unsafe impl` to be implemented, the check for this
+is done as part of [coherence]. The `unsafe_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.
+
+[Unsafe traits]: https://doc.rust-lang.org/reference/items/traits.html#unsafe-traits
+[coherence]: /home/matthew/rust/compiler/rustc_hir_analysis/src/coherence/unsafety.rs
+