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-rw-r--r--Documentation/rust/coding-guidelines.rst110
1 files changed, 110 insertions, 0 deletions
diff --git a/Documentation/rust/coding-guidelines.rst b/Documentation/rust/coding-guidelines.rst
index 6db0420f0cea..f7194f7124b0 100644
--- a/Documentation/rust/coding-guidelines.rst
+++ b/Documentation/rust/coding-guidelines.rst
@@ -262,6 +262,116 @@ default (i.e. outside ``W=`` levels). In particular, those that may have some
false positives but that are otherwise quite useful to keep enabled to catch
potential mistakes.
+On top of that, Rust provides the ``expect`` attribute which takes this further.
+It makes the compiler warn if the warning was not produced. For instance, the
+following will ensure that, when ``f()`` is called somewhere, we will have to
+remove the attribute:
+
+.. code-block:: rust
+
+ #[expect(dead_code)]
+ fn f() {}
+
+If we do not, we get a warning from the compiler::
+
+ warning: this lint expectation is unfulfilled
+ --> x.rs:3:10
+ |
+ 3 | #[expect(dead_code)]
+ | ^^^^^^^^^
+ |
+ = note: `#[warn(unfulfilled_lint_expectations)]` on by default
+
+This means that ``expect``\ s do not get forgotten when they are not needed, which
+may happen in several situations, e.g.:
+
+- Temporary attributes added while developing.
+
+- Improvements in lints in the compiler, Clippy or custom tools which may
+ remove a false positive.
+
+- When the lint is not needed anymore because it was expected that it would be
+ removed at some point, such as the ``dead_code`` example above.
+
+It also increases the visibility of the remaining ``allow``\ s and reduces the
+chance of misapplying one.
+
+Thus prefer ``except`` over ``allow`` unless:
+
+- The lint attribute is intended to be temporary, e.g. while developing.
+
+- Conditional compilation triggers the warning in some cases but not others.
+
+ If there are only a few cases where the warning triggers (or does not
+ trigger) compared to the total number of cases, then one may consider using
+ a conditional ``expect`` (i.e. ``cfg_attr(..., expect(...))``). Otherwise,
+ it is likely simpler to just use ``allow``.
+
+- Inside macros, when the different invocations may create expanded code that
+ triggers the warning in some cases but not in others.
+
+- When code may trigger a warning for some architectures but not others, such
+ as an ``as`` cast to a C FFI type.
+
+As a more developed example, consider for instance this program:
+
+.. code-block:: rust
+
+ fn g() {}
+
+ fn main() {
+ #[cfg(CONFIG_X)]
+ g();
+ }
+
+Here, function ``g()`` is dead code if ``CONFIG_X`` is not set. Can we use
+``expect`` here?
+
+.. code-block:: rust
+
+ #[expect(dead_code)]
+ fn g() {}
+
+ fn main() {
+ #[cfg(CONFIG_X)]
+ g();
+ }
+
+This would emit a lint if ``CONFIG_X`` is set, since it is not dead code in that
+configuration. Therefore, in cases like this, we cannot use ``expect`` as-is.
+
+A simple possibility is using ``allow``:
+
+.. code-block:: rust
+
+ #[allow(dead_code)]
+ fn g() {}
+
+ fn main() {
+ #[cfg(CONFIG_X)]
+ g();
+ }
+
+An alternative would be using a conditional ``expect``:
+
+.. code-block:: rust
+
+ #[cfg_attr(not(CONFIG_X), expect(dead_code))]
+ fn g() {}
+
+ fn main() {
+ #[cfg(CONFIG_X)]
+ g();
+ }
+
+This would ensure that, if someone introduces another call to ``g()`` somewhere
+(e.g. unconditionally), then it would be spotted that it is not dead code
+anymore. However, the ``cfg_attr`` is more complex than a simple ``allow``.
+
+Therefore, it is likely that it is not worth using conditional ``expect``\ s when
+more than one or two configurations are involved or when the lint may be
+triggered due to non-local changes (such as ``dead_code``).
+
For more information about diagnostics in Rust, please see:
https://doc.rust-lang.org/stable/reference/attributes/diagnostics.html