Deny warnings (#47)

* Add another example to the mem::replace idiom

This fixes #38

* New anti-pattern: deny-warnings

* added deprecated link

* fix link title, add explanation why not deprecated

* incorporated hints from lfairy

Author: llogiq

anti_patterns/deny-warnings.md

@@ -0,0 +1,110 @@
+# `#![deny(warnings)]`
+
+## Description
+
+A well-intentioned crate author wants to ensure their code builds without
+warnings. So they annotate their crate root with the following:
+
+## Example
+
+```rust
+#![deny(warnings)]
+
+// All is well.
+```
+
+## Advantages
+
+It is short and will stop the build if anything is amiss.
+
+## Drawbacks
+
+By disallowing the compiler to build with warnings, a crate author opts out of
+Rust's famed stability. Sometimes new features or old misfeatures need a change
+in how things are done, thus lints are written that `warn` for a certain grace
+period before being turned to `deny`.
+
+For example, it was discovered that a type could have two `impl`s with the same
+method. This was deemed a bad idea, but in order to make the transition smooth,
+the `overlapping-inherent-impls` lint was introduced to give a warning to those
+stumbling on this fact, before it becomes a hard error in a future release.
+
+Also sometimes APIs get deprecated, so their use will emit a warning where
+before there was none.
+
+All this conspires to potentially break the build whenever something changes.
+
+Furthermore, crates that supply additional lints (e.g. [rust-clippy]) can no
+longer be used unless the annotation is removed.
+
+## Alternatives
+
+There are two ways of tackling this problem: First, we can decouple the build
+setting from the code, and second, we can name the lints we want to deny
+explicitly.
+
+The following command line will build with all warnings set to `deny`:
+ 
+```RUSTFLAGS="-D warnings" cargo build"```
+
+This can be done by any individual developer (or be set in a CI tool like
+Travis, but remember that this may break the build when something changes)
+without requiring a change to the code.
+
+Alternatively, we can specify the lints that we want to `deny` in the code.
+Here is a list of warning lints that is (hopefully) safe to deny:
+
+```rust
+#[deny(bad-style,
+       const-err,
+       dead-code,
+       extra-requirement-in-impl,
+       improper-ctypes,
+       legacy-directory-ownership,
+       non-shorthand-field-patterns,
+       no-mangle-generic-items,
+       overflowing-literals,
+       path-statements ,
+       patterns-in-fns-without-body,
+       plugin-as-library,
+       private-in-public,
+       private-no-mangle-fns,
+       private-no-mangle-statics,
+       raw-pointer-derive,
+       safe-extern-statics,
+       unconditional-recursion,
+       unions-with-drop-fields,
+       unused,
+       unused-allocation,
+       unused-comparisons,
+       unused-parens,
+       while-true)]
+```
+
+In addition, the following `allow`ed lints may be a good idea to `deny`:
+
+```rust
+#[deny(missing-debug-implementations,
+       missing-docs,
+       trivial-casts,
+       trivial-numeric-casts,
+       unused-extern-crates,
+       unused-import-braces,
+       unused-qualifications,
+       unused-results)]
+```
+
+Some may also want to add `missing-copy-implementations` to their list.
+
+Note that we explicitly did not add the `deprecated` lint, as it is fairly
+certain that there will be more deprecated APIs in the future.
+
+## See also
+
+- [deprecate attribute] documentation
+- Type `rustc -W help` for a list of lints on your system. Also type
+`rustc --help` for a general list of options
+- [rust-clippy] is a collection of lints for better Rust code
+
+[rust-clippy]: https://github.com/Manishearth/rust-clippy
+[deprecate attribute]: https://doc.rust-lang.org/reference.html#miscellaneous-attributes