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<main>
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<h1 id="constant-evaluation"><a class="header" href="#constant-evaluation">Constant Evaluation</a></h1>
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<p>Constant evaluation is the process of computing values at compile time. For a
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specific item (constant/static/array length) this happens after the MIR for the
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item is borrow-checked and optimized. In many cases trying to const evaluate an
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item will trigger the computation of its MIR for the first time.</p>
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<p>Prominent examples are:</p>
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<ul>
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<li>The initializer of a <code>static</code></li>
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<li>Array length
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<ul>
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<li>needs to be known to reserve stack or heap space</li>
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</ul>
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</li>
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<li>Enum variant discriminants
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<ul>
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<li>needs to be known to prevent two variants from having the same
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discriminant</li>
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</ul>
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</li>
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<li>Patterns
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<ul>
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<li>need to be known to check for overlapping patterns</li>
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</ul>
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</li>
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</ul>
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<p>Additionally constant evaluation can be used to reduce the workload or binary
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size at runtime by precomputing complex operations at compile time and only
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storing the result.</p>
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<p>All uses of constant evaluation can either be categorized as "influencing the type system"
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(array lengths, enum variant discriminants, const generic parameters), or as solely being
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done to precompute expressions to be used at runtime.</p>
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<p>Constant evaluation can be done by calling the <code>const_eval_*</code> functions of <code>TyCtxt</code>.
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They're the wrappers of the <code>const_eval</code> query.</p>
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<ul>
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<li><code>const_eval_global_id_for_typeck</code> evaluates a constant to a valtree,
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so the result value can be further inspected by the compiler.</li>
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<li><code>const_eval_global_id</code> evaluate a constant to an "opaque blob" containing its final value;
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this is only useful for codegen backends and the CTFE evaluator engine itself.</li>
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<li><code>eval_static_initializer</code> specifically computes the initial values of a static.
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Statics are special; all other functions do not represent statics correctly
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and have thus assertions preventing their use on statics.</li>
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</ul>
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<p>The <code>const_eval_*</code> functions use a <a href="./typing_parameter_envs.html"><code>ParamEnv</code></a> of environment
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in which the constant is evaluated (e.g. the function within which the constant is used)
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and a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/struct.GlobalId.html"><code>GlobalId</code></a>. The <code>GlobalId</code> is made up of an <code>Instance</code> referring to a constant
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or static or of an <code>Instance</code> of a function and an index into the function's <code>Promoted</code> table.</p>
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<p>Constant evaluation returns an <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/error/type.EvalToValTreeResult.html"><code>EvalToValTreeResult</code></a> for type system constants
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or <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/interpret/error/type.EvalToConstValueResult.html"><code>EvalToConstValueResult</code></a> with either the error, or a representation of the
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evaluated constant: a <a href="mir/index.html#valtrees">valtree</a> or a <a href="mir/index.html#mir-constant-values">MIR constant
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value</a>, respectively.</p>
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