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<h1 id="queries-demand-driven-compilation"><a class="header" href="#queries-demand-driven-compilation">Queries: demand-driven compilation</a></h1>
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<ul>
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<li><a href="#invoking-queries">Invoking queries</a></li>
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<li><a href="#how-the-compiler-executes-a-query">How the compiler executes a query</a>
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<ul>
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<li><a href="#providers">Providers</a></li>
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<li><a href="#how-providers-are-set-up">How providers are set up</a>
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<ul>
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<li><a href="#how-are-providers-registered">How are providers registered?</a></li>
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<li><a href="#adding-a-new-provider">Adding a new provider</a></li>
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</ul>
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</li>
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</ul>
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</li>
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<li><a href="#adding-a-new-query">Adding a new query</a></li>
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<li><a href="#external-links">External links</a></li>
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</ul>
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<p>As described in <a href="overview.html#queries">Overview of the compiler</a>, the Rust compiler
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is still (as of <!-- date-check --> July 2021) transitioning from a
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traditional "pass-based" setup to a "demand-driven" system. The compiler query
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system is the key to rustc's demand-driven organization.
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The idea is pretty simple. Instead of entirely independent passes
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(parsing, type-checking, etc.), a set of function-like <em>queries</em>
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compute information about the input source. For example,
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there is a query called <code>type_of</code> that, given the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/def_id/struct.DefId.html"><code>DefId</code></a> of
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some item, will compute the type of that item and return it to you.</p>
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<p>Query execution is <em>memoized</em>. The first time you invoke a
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query, it will go do the computation, but the next time, the result is
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returned from a hashtable. Moreover, query execution fits nicely into
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<em>incremental computation</em>; the idea is roughly that, when you invoke a
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query, the result <em>may</em> be returned to you by loading stored data
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from disk.<sup class="footnote-reference" id="fr-incr-comp-detail-1"><a href="#footnote-incr-comp-detail">1</a></sup></p>
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<p>Eventually, we want the entire compiler
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control-flow to be query driven. There will effectively be one
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top-level query (<code>compile</code>) that will run compilation on a crate; this
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will in turn demand information about that crate, starting from the
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<em>end</em>. For example:</p>
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<ul>
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<li>The <code>compile</code> query might demand to get a list of codegen-units
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(i.e. modules that need to be compiled by LLVM).</li>
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<li>But computing the list of codegen-units would invoke some subquery
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that returns the list of all modules defined in the Rust source.</li>
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<li>That query in turn would invoke something asking for the HIR.</li>
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<li>This keeps going further and further back until we wind up doing the
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actual parsing.</li>
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</ul>
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<p>Although this vision is not fully realized, large sections of the
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compiler (for example, generating <a href="mir/index.html">MIR</a>) currently work exactly like this.</p>
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<h2 id="invoking-queries"><a class="header" href="#invoking-queries">Invoking queries</a></h2>
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<p>Invoking a query is simple. The <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html"><code>TyCtxt</code></a> ("type context") struct offers a method
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for each defined query. For example, to invoke the <code>type_of</code>
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query, you would just do this:</p>
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<pre><code class="language-rust ignore">let ty = tcx.type_of(some_def_id);</code></pre>
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<h2 id="how-the-compiler-executes-a-query"><a class="header" href="#how-the-compiler-executes-a-query">How the compiler executes a query</a></h2>
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<p>So you may be wondering what happens when you invoke a query
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method. The answer is that, for each query, the compiler maintains a
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cache – if your query has already been executed, then, the answer is
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simple: we clone the return value out of the cache and return it
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(therefore, you should try to ensure that the return types of queries
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are cheaply cloneable; insert an <code>Rc</code> if necessary).</p>
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<h3 id="providers"><a class="header" href="#providers">Providers</a></h3>
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<p>If, however, the query is <em>not</em> in the cache, then the compiler will
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call the corresponding <strong>provider</strong> function. A provider is a function
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implemented in a specific module and <strong>manually registered</strong> into the
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<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/struct.Providers.html"><code>Providers</code></a> struct during compiler initialization.
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The macro system generates the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/struct.Providers.html"><code>Providers</code></a> struct,
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which acts as a function table for all query implementations, where each
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field is a function pointer to the actual provider.</p>
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<p><strong>Note:</strong> The <code>Providers</code> struct is generated by macros and acts as a function table for all query implementations.
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It is <strong>not</strong> a Rust trait, but a plain struct with function pointer fields.</p>
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<p><strong>Providers are defined per-crate.</strong> The compiler maintains,
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internally, a table of providers for every crate, at least
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conceptually. Right now, there are really two sets: the providers for
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queries about the <strong>local crate</strong> (that is, the one being compiled)
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and providers for queries about <strong>external crates</strong> (that is,
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dependencies of the local crate). Note that what determines the crate
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that a query is targeting is not the <em>kind</em> of query, but the <em>key</em>.
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For example, when you invoke <code>tcx.type_of(def_id)</code>, that could be a
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local query or an external query, depending on what crate the <code>def_id</code>
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is referring to (see the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/keys/trait.Key.html"><code>self::keys::Key</code></a> trait for more
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information on how that works).</p>
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<p>Providers always have the same signature:</p>
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<pre><code class="language-rust ignore">fn provider<'tcx>(
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tcx: TyCtxt<'tcx>,
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key: QUERY_KEY,
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) -> QUERY_RESULT {
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...
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}</code></pre>
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<p>Providers take two arguments: the <code>tcx</code> and the query key.
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They return the result of the query.</p>
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<p>N.B. Most of the <code>rustc_*</code> crates only provide <strong>local
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providers</strong>. Almost all <strong>extern providers</strong> wind up going through the
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<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_metadata/index.html"><code>rustc_metadata</code> crate</a>, which loads the information
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from the crate metadata. But in some cases there are crates that
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provide queries for <em>both</em> local and external crates, in which case
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they define both a <code>provide</code> and a <code>provide_extern</code> function, through
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<a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_codegen_ssa/back/symbol_export/fn.wasm_import_module_map.html"><code>wasm_import_module_map</code></a>, that <code>rustc_driver</code> can invoke.</p>
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<h3 id="how-providers-are-set-up"><a class="header" href="#how-providers-are-set-up">How providers are set up</a></h3>
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<p>When the tcx is created, it is given the providers by its creator using
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the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/struct.Providers.html"><code>Providers</code></a> struct. This struct is generated by
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the macros here, but it is basically a big list of function pointers:</p>
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<pre><code class="language-rust ignore">struct Providers {
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type_of: for<'tcx> fn(TyCtxt<'tcx>, DefId) -> Ty<'tcx>,
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// ... one field for each query
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}</code></pre>
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<h4 id="how-are-providers-registered"><a class="header" href="#how-are-providers-registered">How are providers registered?</a></h4>
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<p>The <code>Providers</code> struct is filled in during compiler initialization, mainly by the <code>rustc_driver</code> crate.<br />
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But the actual provider functions are implemented in various <code>rustc_*</code> crates (like <code>rustc_middle</code>, <code>rustc_hir_analysis</code>, etc).</p>
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<p>To register providers, each crate exposes a <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/hir/fn.provide.html"><code>provide</code></a> function that looks like this:</p>
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<pre><code class="language-rust ignore">pub fn provide(providers: &mut Providers) {
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*providers = Providers {
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type_of,
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// ... add more providers here
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..*providers
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};
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}</code></pre>
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<ul>
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<li>This function takes a mutable reference to the <code>Providers</code> struct and sets the fields to point to the correct provider functions.</li>
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<li>You can also assign fields individually, e.g. <code>providers.type_of = type_of;</code>.</li>
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</ul>
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<h4 id="adding-a-new-provider"><a class="header" href="#adding-a-new-provider">Adding a new provider</a></h4>
|
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<p>Suppose you want to add a new query called <code>fubar</code>. You would:</p>
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<ol>
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<li>Implement the provider function:
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<pre><code class="language-rust ignore">fn fubar<'tcx>(tcx: TyCtxt<'tcx>, key: DefId) -> Fubar<'tcx> { ... }</code></pre>
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</li>
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<li>Register it in the <code>provide</code> function:
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<pre><code class="language-rust ignore">pub fn provide(providers: &mut Providers) {
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*providers = Providers {
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fubar,
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..*providers
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||
};
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}</code></pre>
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</li>
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||
</ol>
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<hr />
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<h2 id="adding-a-new-query"><a class="header" href="#adding-a-new-query">Adding a new query</a></h2>
|
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<p>How do you add a new query?
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Defining a query takes place in two steps:</p>
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<ol>
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<li>Declare the query name, its arguments and description.</li>
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<li>Supply query providers where needed.</li>
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||
</ol>
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<p>To declare the query name and arguments, you simply add an entry to
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the big macro invocation in <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/index.html"><code>compiler/rustc_middle/src/query/mod.rs</code></a>.
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Then you need to add a documentation comment to it with some <em>internal</em> description.
|
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Then, provide the <code>desc</code> attribute which contains a <em>user-facing</em> description of the query.
|
||
The <code>desc</code> attribute is shown to the user in query cycles.</p>
|
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<p>This looks something like:</p>
|
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<pre><code class="language-rust ignore">rustc_queries! {
|
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/// Records the type of every item.
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query type_of(key: DefId) -> Ty<'tcx> {
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cache_on_disk_if { key.is_local() }
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desc { |tcx| "computing the type of `{}`", tcx.def_path_str(key) }
|
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}
|
||
...
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||
}</code></pre>
|
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<p>A query definition has the following form:</p>
|
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<pre><code class="language-rust ignore">query type_of(key: DefId) -> Ty<'tcx> { ... }
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^^^^^ ^^^^^^^ ^^^^^ ^^^^^^^^ ^^^
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| | | | |
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| | | | query modifiers
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| | | result type
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| | query key type
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| name of query
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query keyword</code></pre>
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<p>Let's go over these elements one by one:</p>
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<ul>
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<li><strong>Query keyword:</strong> indicates a start of a query definition.</li>
|
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<li><strong>Name of query:</strong> the name of the query method
|
||
(<code>tcx.type_of(..)</code>). Also used as the name of a struct
|
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(<code>ty::queries::type_of</code>) that will be generated to represent
|
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this query.</li>
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<li><strong>Query key type:</strong> the type of the argument to this query.
|
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This type must implement the <a href="https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/query/keys/trait.Key.html"><code>ty::query::keys::Key</code></a> trait, which
|
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defines (for example) how to map it to a crate, and so forth.</li>
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<li><strong>Result type of query:</strong> the type produced by this query. This type
|
||
should (a) not use <code>RefCell</code> or other interior mutability and (b) be
|
||
cheaply cloneable. Interning or using <code>Rc</code> or <code>Arc</code> is recommended for
|
||
non-trivial data types.<sup class="footnote-reference" id="fr-steal-1"><a href="#footnote-steal">2</a></sup></li>
|
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<li><strong>Query modifiers:</strong> various flags and options that customize how the
|
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query is processed (mostly with respect to <a href="queries/incremental-compilation-in-detail.html#query-modifiers">incremental compilation</a>).</li>
|
||
</ul>
|
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<p>So, to add a query:</p>
|
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<ul>
|
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<li>Add an entry to <code>rustc_queries!</code> using the format above.</li>
|
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<li>Link the provider by modifying the appropriate <code>provide</code> method;
|
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or add a new one if needed and ensure that <code>rustc_driver</code> is invoking it.</li>
|
||
</ul>
|
||
<h2 id="external-links"><a class="header" href="#external-links">External links</a></h2>
|
||
<p>Related design ideas, and tracking issues:</p>
|
||
<ul>
|
||
<li>Design document: <a href="https://github.com/nikomatsakis/rustc-on-demand-incremental-design-doc/blob/master/0000-rustc-on-demand-and-incremental.md">On-demand Rustc incremental design doc</a></li>
|
||
<li>Tracking Issue: <a href="https://github.com/rust-lang/rust/issues/42293">"Red/Green" dependency tracking in compiler</a></li>
|
||
</ul>
|
||
<p>More discussion and issues:</p>
|
||
<ul>
|
||
<li><a href="https://github.com/rust-lang/rust/issues/42633">GitHub issue #42633</a></li>
|
||
<li><a href="https://internals.rust-lang.org/t/incremental-compilation-beta/4721">Incremental Compilation Beta</a></li>
|
||
<li><a href="https://blog.rust-lang.org/2016/09/08/incremental.html">Incremental Compilation Announcement</a></li>
|
||
</ul>
|
||
<hr>
|
||
<ol class="footnote-definition"><li id="footnote-incr-comp-detail">
|
||
<p>The <a href="queries/incremental-compilation-in-detail.html">Incremental compilation in detail</a> chapter gives a more
|
||
in-depth description of what queries are and how they work.
|
||
If you intend to write a query of your own, this is a good read. <a href="#fr-incr-comp-detail-1">↩</a></p>
|
||
</li>
|
||
<li id="footnote-steal">
|
||
<p>The one exception to those rules is the <code>ty::steal::Steal</code> type,
|
||
which is used to cheaply modify MIR in place. See the definition
|
||
of <code>Steal</code> for more details. New uses of <code>Steal</code> should <strong>not</strong> be
|
||
added without alerting <code>@rust-lang/compiler</code>. <a href="#fr-steal-1">↩</a></p>
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