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| <h1 id="how-salsa-works"><a class="header" href="#how-salsa-works">How Salsa works</a></h1> |
| <p>This chapter is based on the explanation given by Niko Matsakis in this |
| <a href="https://www.youtube.com/watch?v=_muY4HjSqVw">video</a> about |
| <a href="https://github.com/salsa-rs/salsa">Salsa</a>. To find out more you may |
| want to watch <a href="https://www.youtube.com/watch?v=i_IhACacPRY">Salsa In More |
| Depth</a>, also by Niko |
| Matsakis.</p> |
| <blockquote> |
| <p>As of <!-- date-check --> November 2022, although Salsa is inspired by (among |
| other things) rustc's query system, it is not used directly in rustc. It |
| <em>is</em> used in <a href="https://rust-lang.github.io/chalk/book/what_is_chalk.html">chalk</a>, an implementation of Rust's trait system, and |
| extensively in <a href="https://rust-analyzer.github.io/"><code>rust-analyzer</code></a>, the official implementation of the language |
| server protocol for Rust, but there are no medium or long-term concrete |
| plans to integrate it into the compiler.</p> |
| </blockquote> |
| <h2 id="what-is-salsa"><a class="header" href="#what-is-salsa">What is Salsa?</a></h2> |
| <p>Salsa is a library for incremental recomputation. This means it allows reusing |
| computations that were already done in the past to increase the efficiency |
| of future computations.</p> |
| <p>The objectives of Salsa are:</p> |
| <ul> |
| <li>Provide that functionality in an automatic way, so reusing old computations |
| is done automatically by the library.</li> |
| <li>Doing so in a "sound", or "correct", way, therefore leading to the same |
| results as if it had been done from scratch.</li> |
| </ul> |
| <p>Salsa's actual model is much richer, allowing many kinds of inputs and many different outputs. |
| For example, integrating Salsa with an IDE could mean that |
| the inputs could be manifests (<code>Cargo.toml</code>, <code>rust-toolchain.toml</code>), entire |
| source files (<code>foo.rs</code>), snippets and so on. The outputs of such an integration |
| could range from a binary executable, to lints, types (for example, if a user |
| selects a certain variable and wishes to see its type), completions, etc.</p> |
| <h2 id="how-does-it-work"><a class="header" href="#how-does-it-work">How does it work?</a></h2> |
| <p>The first thing that Salsa has to do is identify the "base inputs" that |
| are not something computed but given as input.</p> |
| <p>Then Salsa has to also identify intermediate, "derived" values, which are |
| something that the library produces, but, for each derived value there's a |
| "pure" function that computes the derived value.</p> |
| <p>For example, there might be a function <code>ast(x: Path) -> AST</code>. The produced |
| Abstract Syntax Tree (<code>AST</code>) isn't a final value, it's an intermediate value |
| that the library would use for the computation.</p> |
| <p>This means that when you try to compute with the library, Salsa is going to |
| compute various derived values, and eventually read the input and produce the |
| result for the asked computation.</p> |
| <p>In the course of computing, Salsa tracks which inputs were accessed and which |
| values are derived. This information is used to determine what's going to |
| happen when the inputs change: are the derived values still valid?</p> |
| <p>This doesn't necessarily mean that each computation downstream from the input |
| is going to be checked, which could be costly. Salsa only needs to check each |
| downstream computation until it finds one that isn't changed. At that point, it |
| won't check other derived computations since they wouldn't need to change.</p> |
| <p>It's helpful to think about this as a graph with nodes. Each derived value |
| has a dependency on other values, which could themselves be either base or |
| derived. Base values don't have a dependency.</p> |
| <pre><code class="language-ignore">I <- A <- C ... |
| | |
| J <- B <--+ |
| </code></pre> |
| <p>When an input <code>I</code> changes, the derived value <code>A</code> could change. The derived |
| value <code>B</code>, which does not depend on <code>I</code>, <code>A</code>, or any value derived from <code>A</code> or |
| <code>I</code>, is not subject to change. Therefore, Salsa can reuse the computation done |
| for <code>B</code> in the past, without having to compute it again.</p> |
| <p>The computation could also terminate early. Keeping the same graph as before, |
| say that input <code>I</code> has changed in some way (and input <code>J</code> hasn't), but when |
| computing <code>A</code> again, it's found that <code>A</code> hasn't changed from the previous |
| computation. This leads to an "early termination", because there's no need to |
| check if <code>C</code> needs to change, since both <code>C</code> direct inputs, <code>A</code> and <code>B</code>, |
| haven't changed.</p> |
| <h2 id="key-salsa-concepts"><a class="header" href="#key-salsa-concepts">Key Salsa concepts</a></h2> |
| <h3 id="query"><a class="header" href="#query">Query</a></h3> |
| <p>A query is some value that Salsa can access in the course of computation. Each |
| query can have a number of keys (from 0 to many), and all queries have a |
| result, akin to functions. <code>0-key</code> queries are called "input" queries.</p> |
| <h3 id="database"><a class="header" href="#database">Database</a></h3> |
| <p>The database is basically the context for the entire computation, it's meant to |
| store Salsa's internal state, all intermediate values for each query, and |
| anything else that the computation might need. The database must know all the |
| queries the library is going to do before it can be built, but they don't need |
| to be specified in the same place.</p> |
| <p>After the database is formed, it can be accessed with queries that are very |
| similar to functions. Since each query's result is stored in the database, when |
| a query is invoked <code>N</code>-times, it will return <code>N</code>-<strong>cloned</strong> results, without having |
| to recompute the query (unless the input has changed in such a way that it |
| warrants recomputation).</p> |
| <p>For each input query (<code>0-key</code>), a "set" method is generated, allowing the user to |
| change the output of such query, and trigger previous memoized values to be |
| potentially invalidated.</p> |
| <h3 id="query-groups"><a class="header" href="#query-groups">Query Groups</a></h3> |
| <p>A query group is a set of queries which have been defined together as a unit. |
| The database is formed by combining query groups. Query groups are akin to |
| "Salsa modules".</p> |
| <p>A set of queries in a query group are just a set of methods in a trait.</p> |
| <p>To create a query group a trait annotated with a specific attribute |
| (<code>#[salsa::query_group(...)]</code>) has to be created.</p> |
| <p>An argument must also be provided to said attribute as it will be used by Salsa |
| to create a <code>struct</code> to be used later when the database is created.</p> |
| <p>Example input query group:</p> |
| <pre><code class="language-rust ignore">/// This attribute will process this tree, produce this tree as output, and produce |
| /// a bunch of intermediate stuff that Salsa also uses. One of these things is a |
| /// "StorageStruct", whose name we have specified in the attribute. |
| /// |
| /// This query group is a bunch of **input** queries, that do not rely on any |
| /// derived input. |
| #[salsa::query_group(InputsStorage)] |
| pub trait Inputs { |
| /// This attribute (`#[salsa::input]`) indicates that this query is a base |
| /// input, therefore `set_manifest` is going to be auto-generated |
| #[salsa::input] |
| fn manifest(&self) -> Manifest; |
| |
| #[salsa::input] |
| fn source_text(&self, name: String) -> String; |
| }</code></pre> |
| <p>To create a <strong>derived</strong> query group, one must specify which other query groups |
| this one depends on by specifying them as supertraits, as seen in the following |
| example:</p> |
| <pre><code class="language-rust ignore">/// This query group is going to contain queries that depend on derived values. |
| /// A query group can access another query group's queries by specifying the |
| /// dependency as a supertrait. Query groups can be stacked as much as needed using |
| /// that pattern. |
| #[salsa::query_group(ParserStorage)] |
| pub trait Parser: Inputs { |
| /// This query `ast` is not an input query, it's a derived query this means |
| /// that a definition is necessary. |
| fn ast(&self, name: String) -> String; |
| }</code></pre> |
| <p>When creating a derived query the implementation of said query must be defined |
| outside the trait. The definition must take a database parameter as an <code>impl Trait</code> (or <code>dyn Trait</code>), where trait is the query group that the definition |
| belongs to, in addition to the other keys.</p> |
| <pre><code class="language-rust ignore">/// This is going to be the definition of the `ast` query in the `Parser` trait. |
| /// So, when the query `ast` is invoked, and it needs to be recomputed, Salsa is |
| /// going to call this function and it's going to give it the database as `impl |
| /// Parser`. The function doesn't need to be aware of all the queries of all the |
| /// query groups |
| fn ast(db: &impl Parser, name: String) -> String { |
| //! Note, `impl Parser` is used here but `dyn Parser` works just as well |
| /* code */ |
| ///By passing an `impl Parser`, this is allowed |
| let source_text = db.input_file(name); |
| /* do the actual parsing */ |
| return ast; |
| }</code></pre> |
| <p>Eventually, after all the query groups have been defined, the database can be |
| created by declaring a <code>struct</code>.</p> |
| <p>To specify which query groups are going to be part of the database an <code>attribute</code> |
| (<code>#[salsa::database(...)]</code>) must be added. The argument of said <code>attribute</code> is a |
| list of <code>identifiers</code>, specifying the query groups <strong>storages</strong>.</p> |
| <pre><code class="language-rust ignore">///This attribute specifies which query groups are going to be in the database |
| #[salsa::database(InputsStorage, ParserStorage)] |
| #[derive(Default)] //optional! |
| struct MyDatabase { |
| ///You also need this one field |
| runtime : salsa::Runtime<MyDatabase>, |
| } |
| ///And this trait has to be implemented |
| impl salsa::Database for MyDatabase { |
| fn salsa_runtime(&self) -> &salsa::Runtime<MyDatabase> { |
| &self.runtime |
| } |
| }</code></pre> |
| <p>Example usage:</p> |
| <pre><code class="language-rust ignore">fn main() { |
| let db = MyDatabase::default(); |
| db.set_manifest(...); |
| db.set_source_text(...); |
| loop { |
| db.ast(...); //will reuse results |
| db.set_source_text(...); |
| } |
| }</code></pre> |
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