Add documentation on top of rustc_middle/src/query/mod.rs

compiler/rustc_middle/src/query/mod.rs

@@ -1,8 +1,63 @@
-//! Defines the various compiler queries.
 //!
-//! For more information on the query system, see
-//! ["Queries: demand-driven compilation"](https://rustc-dev-guide.rust-lang.org/query.html).
-//! This chapter includes instructions for adding new queries.
+//! # The rustc Query System: Query Definitions and Modifiers
+//!
+//! The core processes in rustc are shipped as queries. Each query is a demand-driven function from some key to a value.
+//! The execution result of the function is cached and directly read during the next request, thereby improving compilation efficiency.
+//! Some results are saved locally and directly read during the next compilation, which are core of incremental compilation.
+//!
+//! ## How to Read This Module
+//!
+//! Each `query` block in this file defines a single query, specifying its key and value types, along with various modifiers.
+//! These query definitions are processed by the macro system, which expands them into the necessary boilerplate code
+//! for the query system—including the [`Providers`] struct (a function table for all query implementations, where each field is
+//! a function pointer to the actual provider), caching, and dependency graph integration.
+//! **Note:** The `Providers` struct is not a Rust trait, but a struct generated by the macro system to hold all provider functions.
+//! The macro system also supports a set of **query modifiers** (see below) that control the behavior of each query.
+//!
+//! The actual provider functions are implemented in various modules and registered into the `Providers` struct
+//! during compiler initialization (see [`rustc_interface::passes::DEFAULT_QUERY_PROVIDERS`]).
+//!
+//! [`rustc_interface::passes::DEFAULT_QUERY_PROVIDERS`]: ../../rustc_interface/passes/static.DEFAULT_QUERY_PROVIDERS.html
+//!
+//! ## Query Modifiers
+//!
+//! Query modifiers are special flags that alter the behavior of a query. They are parsed and processed by the macro system
+//! The main modifiers are:
+//!
+//! - `desc { ... }`: Sets the human-readable description for diagnostics and profiling. Required for every query.
+//! - `arena_cache`: Use an arena for in-memory caching of the query result.
+//! - `cache_on_disk_if { ... }`: Cache the query result to disk if the provided block evaluates to true.
+//! - `fatal_cycle`: If a dependency cycle is detected, abort compilation with a fatal error.
+//! - `cycle_delay_bug`: If a dependency cycle is detected, emit a delayed bug instead of aborting immediately.
+//! - `cycle_stash`: If a dependency cycle is detected, stash the error for later handling.
+//! - `no_hash`: Do not hash the query result for incremental compilation; just mark as dirty if recomputed.
+//! - `anon`: Make the query anonymous in the dependency graph (no dep node is created).
+//! - `eval_always`: Always evaluate the query, ignoring its dependencies and cached results.
+//! - `depth_limit`: Impose a recursion depth limit on the query to prevent stack overflows.
+//! - `separate_provide_extern`: Use separate provider functions for local and external crates.
+//! - `feedable`: Allow the query result to be set from another query ("fed" externally).
+//! - `return_result_from_ensure_ok`: When called via `tcx.ensure_ok()`, return `Result<(), ErrorGuaranteed>` instead of `()`.
+//!   If the query needs to be executed and returns an error, the error is returned to the caller.
+//!   Only valid for queries returning `Result<_, ErrorGuaranteed>`.
+//!
+//! For the up-to-date list, see the `QueryModifiers` struct in
+//! [`rustc_macros/src/query.rs`](https://github.com/rust-lang/rust/blob/master/compiler/rustc_macros/src/query.rs)
+//! and for more details in incremental compilation, see the
+//! [Query modifiers in incremental compilation](https://rustc-dev-guide.rust-lang.org/queries/incremental-compilation-in-detail.html#query-modifiers) section of the rustc-dev-guide.
+//!
+//! ## Query Expansion and Code Generation
+//!
+//! The [`rustc_macros::rustc_queries`] macro expands each query definition into:
+//! - A method on [`TyCtxt`] (and [`TyCtxtAt`]) for invoking the query.
+//! - Provider traits and structs for supplying the query's value.
+//! - Caching and dependency graph integration.
+//! - Support for incremental compilation, disk caching, and arena allocation as controlled by the modifiers.
+//!
+//! [`rustc_macros::rustc_queries`]: ../../rustc_macros/macro.rustc_queries.html
+//!
+//! The macro-based approach allows the query system to be highly flexible and maintainable, while minimizing boilerplate.
+//!
+//! For more details, see the [rustc-dev-guide](https://rustc-dev-guide.rust-lang.org/query.html).
 
 #![allow(unused_parens)]