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<h1 id="stability-attributes"><a class="header" href="#stability-attributes">Stability attributes</a></h1>
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<p>This section is about the stability attributes and schemes that allow stable
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APIs to use unstable APIs internally in the rustc standard library.</p>
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<p><strong>NOTE</strong>: this section is for <em>library</em> features, not <em>language</em> features. For instructions on
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stabilizing a language feature see <a href="./stabilization_guide.html">Stabilizing Features</a>.</p>
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<ul>
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<li><a href="#unstable">unstable</a></li>
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<li><a href="#stable">stable</a></li>
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<li><a href="#rustc_const_unstable">rustc_const_unstable</a></li>
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<li><a href="#rustc_const_stable">rustc_const_stable</a></li>
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<li><a href="#rustc_const_stable_indirect">rustc_const_stable_indirect</a></li>
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<li><a href="#rustc_intrinsic_const_stable_indirect">rustc_intrinsic_const_stable_indirect</a></li>
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<li><a href="#rustc_default_body_unstable">rustc_default_body_unstable</a></li>
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<li><a href="#stabilizing-a-library-feature">Stabilizing a library feature</a></li>
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<li><a href="#allow_internal_unstable">allow_internal_unstable</a></li>
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<li><a href="#rustc_allow_const_fn_unstable">rustc_allow_const_fn_unstable</a></li>
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<li><a href="#staged_api">staged_api</a></li>
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<li><a href="#deprecated">deprecated</a></li>
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</ul>
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<h2 id="unstable"><a class="header" href="#unstable">unstable</a></h2>
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<p>The <code>#[unstable(feature = "foo", issue = "1234", reason = "lorem ipsum")]</code>
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attribute explicitly marks an item as unstable. Items that are marked as
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"unstable" cannot be used without a corresponding <code>#![feature]</code> attribute on
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the crate, even on a nightly compiler. This restriction only applies across
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crate boundaries, unstable items may be used within the crate that defines
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them.</p>
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<p>The <code>issue</code> field specifies the associated GitHub <a href="https://github.com/rust-lang/rust/issues">issue number</a>. This field is
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required and all unstable features should have an associated tracking issue. In
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rare cases where there is no sensible value <code>issue = "none"</code> is used.</p>
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<p>The <code>unstable</code> attribute infects all sub-items, where the attribute doesn't
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have to be reapplied. So if you apply this to a module, all items in the module
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will be unstable.</p>
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<p>You can make specific sub-items stable by using the <code>#[stable]</code> attribute on
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them. The stability scheme works similarly to how <code>pub</code> works. You can have
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public functions of nonpublic modules and you can have stable functions in
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unstable modules or vice versa.</p>
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<p>Previously, due to a <a href="https://github.com/rust-lang/rust/issues/15702">rustc bug</a>, stable items inside unstable modules were
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available to stable code in that location.
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As of <!-- date-check --> September 2024, items with <a href="https://github.com/rust-lang/rust/issues/113387">accidentally stabilized
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paths</a> are marked with the <code>#[rustc_allowed_through_unstable_modules]</code> attribute
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to prevent code dependent on those paths from breaking. Do <em>not</em> add this attribute
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to any more items unless that is needed to avoid breaking changes.</p>
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<p>The <code>unstable</code> attribute may also have the <code>soft</code> value, which makes it a
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future-incompatible deny-by-default lint instead of a hard error. This is used
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by the <code>bench</code> attribute which was accidentally accepted in the past. This
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prevents breaking dependencies by leveraging Cargo's lint capping.</p>
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<h2 id="stable"><a class="header" href="#stable">stable</a></h2>
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<p>The <code>#[stable(feature = "foo", since = "1.420.69")]</code> attribute explicitly
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marks an item as stabilized. Note that stable functions may use unstable things in their body.</p>
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<h2 id="rustc_const_unstable"><a class="header" href="#rustc_const_unstable">rustc_const_unstable</a></h2>
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<p>The <code>#[rustc_const_unstable(feature = "foo", issue = "1234", reason = "lorem ipsum")]</code> has the same interface as the <code>unstable</code> attribute. It is used to mark
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<code>const fn</code> as having their constness be unstable. This is only needed in rare cases:</p>
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<ul>
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<li>If a <code>const fn</code> makes use of unstable language features or intrinsics.
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(The compiler will tell you to add the attribute if you run into this.)</li>
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<li>If a <code>const fn</code> is <code>#[stable]</code> but not yet intended to be const-stable.</li>
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<li>To change the feature gate that is required to call a const-unstable intrinsic.</li>
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</ul>
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<p>Const-stability differs from regular stability in that it is <em>recursive</em>: a
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<code>#[rustc_const_unstable(...)]</code> function cannot even be indirectly called from stable code. This is
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to avoid accidentally leaking unstable compiler implementation artifacts to stable code or locking
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us into the accidental quirks of an incomplete implementation. See the rustc_const_stable_indirect
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and rustc_allow_const_fn_unstable attributes below for how to fine-tune this check.</p>
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<h2 id="rustc_const_stable"><a class="header" href="#rustc_const_stable">rustc_const_stable</a></h2>
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<p>The <code>#[rustc_const_stable(feature = "foo", since = "1.420.69")]</code> attribute explicitly marks
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a <code>const fn</code> as having its constness be <code>stable</code>.</p>
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<h2 id="rustc_const_stable_indirect"><a class="header" href="#rustc_const_stable_indirect">rustc_const_stable_indirect</a></h2>
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<p>The <code>#[rustc_const_stable_indirect]</code> attribute can be added to a <code>#[rustc_const_unstable(...)]</code>
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function to make it callable from <code>#[rustc_const_stable(...)]</code> functions. This indicates that the
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function is ready for stable in terms of its implementation (i.e., it doesn't use any unstable
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compiler features); the only reason it is not const-stable yet are API concerns.</p>
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<p>This should also be added to lang items for which const-calls are synthesized in the compiler, to
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ensure those calls do not bypass recursive const stability rules.</p>
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<h2 id="rustc_intrinsic_const_stable_indirect"><a class="header" href="#rustc_intrinsic_const_stable_indirect">rustc_intrinsic_const_stable_indirect</a></h2>
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<p>On an intrinsic, this attribute marks the intrinsic as "ready to be used by public stable functions".
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If the intrinsic has a <code>rustc_const_unstable</code> attribute, it should be removed.
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<strong>Adding this attribute to an intrinsic requires t-lang and wg-const-eval approval!</strong></p>
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<h2 id="rustc_default_body_unstable"><a class="header" href="#rustc_default_body_unstable">rustc_default_body_unstable</a></h2>
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<p>The <code>#[rustc_default_body_unstable(feature = "foo", issue = "1234", reason = "lorem ipsum")]</code> attribute has the same interface as the <code>unstable</code> attribute.
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It is used to mark the default implementation for an item within a trait as
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unstable.
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A trait with a default-body-unstable item can be implemented stably by providing
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an explicit body for any such item, or the default body can be used by enabling
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its corresponding <code>#![feature]</code>.</p>
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<h2 id="stabilizing-a-library-feature"><a class="header" href="#stabilizing-a-library-feature">Stabilizing a library feature</a></h2>
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<p>To stabilize a feature, follow these steps:</p>
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<ol>
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<li>Ask a <strong>@T-libs-api</strong> member to start an FCP on the tracking issue and wait for
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the FCP to complete (with <code>disposition-merge</code>).</li>
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<li>Change <code>#[unstable(...)]</code> to <code>#[stable(since = "CURRENT_RUSTC_VERSION")]</code>.</li>
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<li>Remove <code>#![feature(...)]</code> from any test or doc-test for this API. If the feature is used in the
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compiler or tools, remove it from there as well.</li>
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<li>If this is a <code>const fn</code>, add <code>#[rustc_const_stable(since = "CURRENT_RUSTC_VERSION")]</code>.
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Alternatively, if this is not supposed to be const-stabilized yet,
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add <code>#[rustc_const_unstable(...)]</code> for some new feature gate (with a new tracking issue).</li>
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<li>Open a PR against <code>rust-lang/rust</code>.
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<ul>
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<li>Add the appropriate labels: <code>@rustbot modify labels: +T-libs-api</code>.</li>
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<li>Link to the tracking issue and say "Closes #XXXXX".</li>
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</ul>
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</li>
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</ol>
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<p>You can see an example of stabilizing a feature with
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<a href="https://github.com/rust-lang/rust/issues/81656">tracking issue #81656 with FCP</a>
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and the associated
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<a href="https://github.com/rust-lang/rust/pull/84642">implementation PR #84642</a>.</p>
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<h2 id="allow_internal_unstable"><a class="header" href="#allow_internal_unstable">allow_internal_unstable</a></h2>
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<p>Macros and compiler desugarings expose their bodies to the call
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site. To work around not being able to use unstable things in the standard
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library's macros, there's the <code>#[allow_internal_unstable(feature1, feature2)]</code>
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attribute that allows the given features to be used in stable macros.</p>
|
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<p>Note that if a macro is used in const context and generates a call to a
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<code>#[rustc_const_unstable(...)]</code> function, that will <em>still</em> be rejected even with
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<code>allow_internal_unstable</code>. Add <code>#[rustc_const_stable_indirect]</code> to the function to ensure the macro
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cannot accidentally bypass the recursive const stability checks.</p>
|
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<h2 id="rustc_allow_const_fn_unstable"><a class="header" href="#rustc_allow_const_fn_unstable">rustc_allow_const_fn_unstable</a></h2>
|
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<p>As explained above, no unstable const features are allowed inside stable <code>const fn</code>, not even
|
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indirectly.</p>
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<p>However, sometimes we do know that a feature will get stabilized, just not when, or there is a
|
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stable (but e.g. runtime-slow) workaround, so we could always fall back to some stable version if we
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scrapped the unstable feature. In those cases, the <code>[rustc_allow_const_fn_unstable(feature1, feature2)]</code> attribute can be used to allow some unstable features in the body of a stable (or
|
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indirectly stable) <code>const fn</code>.</p>
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<p>You also need to take care to uphold the <code>const fn</code> invariant that calling it at runtime and
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compile-time needs to behave the same (see also <a href="https://www.ralfj.de/blog/2018/07/19/const.html">this blog post</a>). This means that you
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may not create a <code>const fn</code> that e.g. transmutes a memory address to an integer,
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|
because the addresses of things are nondeterministic and often unknown at
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|
compile-time.</p>
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<p><strong>Always ping @rust-lang/wg-const-eval if you are adding more
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<code>rustc_allow_const_fn_unstable</code> attributes to any <code>const fn</code>.</strong></p>
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<h2 id="staged_api"><a class="header" href="#staged_api">staged_api</a></h2>
|
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<p>Any crate that uses the <code>stable</code> or <code>unstable</code> attributes must include the
|
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<code>#![feature(staged_api)]</code> attribute on the crate.</p>
|
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<h2 id="deprecated"><a class="header" href="#deprecated">deprecated</a></h2>
|
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<p>Deprecations in the standard library are nearly identical to deprecations in
|
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user code. When <code>#[deprecated]</code> is used on an item, it must also have a <code>stable</code>
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or <code>unstable </code>attribute.</p>
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<p><code>deprecated</code> has the following form:</p>
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<pre><code class="language-rust ignore">#[deprecated(
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since = "1.38.0",
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note = "explanation for deprecation",
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suggestion = "other_function"
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)]</code></pre>
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<p>The <code>suggestion</code> field is optional. If given, it should be a string that can be
|
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used as a machine-applicable suggestion to correct the warning. This is
|
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typically used when the identifier is renamed, but no other significant changes
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are necessary. When the <code>suggestion</code> field is used, you need to have
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<code>#![feature(deprecated_suggestion)]</code> at the crate root.</p>
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<p>Another difference from user code is that the <code>since</code> field is actually checked
|
|
against the current version of <code>rustc</code>. If <code>since</code> is in a future version, then
|
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the <code>deprecated_in_future</code> lint is triggered which is default <code>allow</code>, but most
|
|
of the standard library raises it to a warning with
|
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<code>#![warn(deprecated_in_future)]</code>.</p>
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