Queries: demand-driven compilation

As described in the high-level overview of the compiler, the Rust compiler is current transitioning from a traditional "pass-based" setup to a "demand-driven" system. The Compiler Query System is the key to our new demand-driven organization. The idea is pretty simple. You have various queries that compute things about the input – for example, there is a query called type_of(def_id) that, given the def-id of some item, will compute the type of that item and return it to you.

Query execution is memoized – so the first time you invoke a query, it will go do the computation, but the next time, the result is returned from a hashtable. Moreover, query execution fits nicely into incremental computation; the idea is roughly that, when you do a query, the result may be returned to you by loading stored data from disk (but that's a separate topic we won't discuss further here).

The overall vision is that, eventually, the entire compiler control-flow will be query driven. There will effectively be one top-level query ("compile") that will run compilation on a crate; this will in turn demand information about that crate, starting from the end. For example:

This "compile" query might demand to get a list of codegen-units (i.e. modules that need to be compiled by LLVM).
But computing the list of codegen-units would invoke some subquery that returns the list of all modules defined in the Rust source.
That query in turn would invoke something asking for the HIR.
This keeps going further and further back until we wind up doing the actual parsing.

However, that vision is not fully realized. Still, big chunks of the compiler (for example, generating MIR) work exactly like this.

The Query Evaluation Model in Detail

The Query Evaluation Model in Detail 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.

Invoking queries

To invoke a query is simple. The tcx ("type context") offers a method for each defined query. So, for example, to invoke the type_of query, you would just do this:

let ty = tcx.type_of(some_def_id);

How the compiler executes a query

So you may be wondering what happens when you invoke a query method. The answer is that, for each query, the compiler maintains a cache – if your query has already been executed, then, the answer is simple: we clone the return value out of the cache and return it (therefore, you should try to ensure that the return types of queries are cheaply cloneable; insert a Rc if necessary).

Providers

If, however, the query is not in the cache, then the compiler will try to find a suitable provider. A provider is a function that has been defined and linked into the compiler somewhere that contains the code to compute the result of the query.

Providers are defined per-crate. The compiler maintains, internally, a table of providers for every crate, at least conceptually. Right now, there are really two sets: the providers for queries about the local crate (that is, the one being compiled) and providers for queries about external crates (that is, dependencies of the local crate). Note that what determines the crate that a query is targeting is not the kind of query, but the key. For example, when you invoke tcx.type_of(def_id), that could be a local query or an external query, depending on what crate the def_id is referring to (see the self::keys::Key trait for more information on how that works).

Providers always have the same signature:

fn provider<'cx, 'tcx>(tcx: TyCtxt<'cx, 'tcx, 'tcx>,
                       key: QUERY_KEY)
                       -> QUERY_RESULT
{
    ...
}

Providers take two arguments: the tcx and the query key. Note also that they take the global tcx (i.e. they use the 'tcx lifetime twice), rather than taking a tcx with some active inference context. They return the result of the query.

How providers are setup

When the tcx is created, it is given the providers by its creator using the Providers struct. This struct is generated by the macros here, but it is basically a big list of function pointers:

struct Providers {
    type_of: for<'cx, 'tcx> fn(TyCtxt<'cx, 'tcx, 'tcx>, DefId) -> Ty<'tcx>,
    ...
}

At present, we have one copy of the struct for local crates, and one for external crates, though the plan is that we may eventually have one per crate.

These Provider structs are ultimately created and populated by librustc_driver, but it does this by distributing the work throughout the other rustc_* crates. This is done by invoking various provide functions. These functions tend to look something like this:

pub fn provide(providers: &mut Providers) {
    *providers = Providers {
        type_of,
        ..*providers
    };
}

That is, they take an &mut Providers and mutate it in place. Usually we use the formulation above just because it looks nice, but you could as well do providers.type_of = type_of, which would be equivalent. (Here, type_of would be a top-level function, defined as we saw before.) So, if we want to add a provider for some other query, let's call it fubar, into the crate above, we might modify the provide() function like so:

pub fn provide(providers: &mut Providers) {
    *providers = Providers {
        type_of,
        fubar,
        ..*providers
    };
}

fn fubar<'cx, 'tcx>(tcx: TyCtxt<'cx, 'tcx>, key: DefId) -> Fubar<'tcx> { ... }

N.B. Most of the rustc_* crates only provide local providers. Almost all extern providers wind up going through the rustc_metadata crate, which loads the information from the crate metadata. But in some cases there are crates that provide queries for both local and external crates, in which case they define both a provide and a provide_extern function that rustc_driver can invoke.

Adding a new kind of query

So suppose you want to add a new kind of query, how do you do so? Well, defining a query takes place in two steps:

first, you have to specify the query name and arguments; and then,
you have to supply query providers where needed.

To specify the query name and arguments, you simply add an entry to the big macro invocation in src/librustc/query/mod.rs, which looks something like:

rustc_queries! {
    Other {
        /// Records the type of every item.
        query type_of(key: DefId) -> Ty<'tcx> {
            cache { key.is_local() }
        }
    }

    ...
}

Queries are grouped into categories (Other, Codegen, TypeChecking, etc.). Each group contains one or more queries. Each query definition is broken up like this:

query type_of(key: DefId) -> Ty<'tcx> { ... }
^^    ^^^^^^^      ^^^^^     ^^^^^^^^   ^^^
|     |            |         |          |
|     |            |         |          query modifiers
|     |            |         result type of query
|     |            query key type
|     name of query
query keyword

Let's go over them one by one:

Query keyword: indicates a start of a query definition.
Name of query: the name of the query method (tcx.type_of(..)). Also used as the name of a struct (ty::queries::type_of) that will be generated to represent this query.
Query key type: the type of the argument to this query. This type must implement the ty::query::keys::Key trait, which defines (for example) how to map it to a crate, and so forth.
Result type of query: the type produced by this query. This type should (a) not use RefCell or other interior mutability and (b) be cheaply cloneable. Interning or using Rc or Arc is recommended for non-trivial data types.
- The one exception to those rules is the ty::steal::Steal type, which is used to cheaply modify MIR in place. See the definition of Steal for more details. New uses of Steal should not be added without alerting @rust-lang/compiler.
Query modifiers: various flags and options that customize how the query is processed.

So, to add a query:

Add an entry to rustc_queries! using the format above.
Link the provider by modifying the appropriate provide method; or add a new one if needed and ensure that rustc_driver is invoking it.

Query structs and descriptions

For each kind, the rustc_queries macro will generate a "query struct" named after the query. This struct is a kind of a place-holder describing the query. Each such struct implements the self::config::QueryConfig trait, which has associated types for the key/value of that particular query. Basically the code generated looks something like this:

// Dummy struct representing a particular kind of query:
pub struct type_of<'tcx> { data: PhantomData<&'tcx ()> }

impl<'tcx> QueryConfig for type_of<'tcx> {
  type Key = DefId;
  type Value = Ty<'tcx>;

  const NAME: QueryName = QueryName::type_of;
  const CATEGORY: ProfileCategory = ProfileCategory::Other;
}

There is an additional trait that you may wish to implement called self::config::QueryDescription. This trait is used during cycle errors to give a "human readable" name for the query, so that we can summarize what was happening when the cycle occurred. Implementing this trait is optional if the query key is DefId, but if you don't implement it, you get a pretty generic error ("processing foo..."). You can put new impls into the config module. They look something like this:

impl<'tcx> QueryDescription for queries::type_of<'tcx> {
    fn describe(tcx: TyCtxt, key: DefId) -> String {
        format!("computing the type of `{}`", tcx.def_path_str(key))
    }
}

Another option is to add desc modifier:

rustc_queries! {
    Other {
        /// Records the type of every item.
        query type_of(key: DefId) -> Ty<'tcx> {
            desc { |tcx| "computing the type of `{}`", tcx.def_path_str(key) }
        }
    }
}

rustc_queries macro will generate an appropriate impl automatically.

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