internal: document source_to_def and it's connection to Kotlin&Roslyn

crates/hir/src/semantics/source_to_def.rs

@@ -1,4 +1,73 @@
 //! Maps *syntax* of various definitions to their semantic ids.
+//!
+//! This is a very interesting module, and, in some sense, can be considered a
+//! heart of the IDE parts of rust-analyzer.
+//!
+//! This module solves the following problem:
+//!
+//!     Given a piece of syntax, find the corresponding semantic definition (def).
+//!
+//! This problem is a part of more-or-less every IDE feature implemented. Every
+//! IDE functionality (like goto to definition), conceptually starts with a
+//! specific cursor position in a file. Starting with this text offset, we first
+//! figure out what syntactic construct are we at: is this a pattern, an
+//! expression, an item definition.
+//!
+//! Knowing only the syntax gives us relatively little info. For example,
+//! looking at the syntax of the function we can realise that it is a part of an
+//! `impl` block, but we won't be able to tell what trait function the current
+//! function overrides, and whether it does that correctly. For that, we need to
+//! go from [`ast::Fn`] to [`crate::Function], and that's exactly what this
+//! module does.
+//!
+//! As syntax trees are values and don't know their place of origin/identity,
+//! this module also requires [`InFile`] wrappers to understand which specific
+//! real or macro-expanded file the tree comes from.
+//!
+//! The actual algorithm to resolve syntax to def is curious in two aspects:
+//!
+//!     * It is recursive
+//!     * It uses the inverse algorithm (what is the syntax for this def?)
+//!
+//! Specifically, the algorithm goes like this:
+//!
+//!     1. Find the syntactic container for the syntax. For example, field's
+//!        container is the struct, and structs container is a module.
+//!     2. Recursively get the def corresponding to container.
+//!     3. Ask the container def for all child defs. These child defs contain
+//!        the answer and answer's siblings.
+//!     4. For each child def, ask for it's source.
+//!     5. The child def whose source is the syntax node we've started with
+//!        is the answer.
+//!
+//! It's interesting that both Roslyn and Kotlin contain very similar code
+//! shape.
+//!
+//! Let's take a look at Roslyn:
+//!
+//!   <https://github.com/dotnet/roslyn/blob/36a0c338d6621cc5fe34b79d414074a95a6a489c/src/Compilers/CSharp/Portable/Compilation/SyntaxTreeSemanticModel.cs#L1403-L1429>
+//!   <https://sourceroslyn.io/#Microsoft.CodeAnalysis.CSharp/Compilation/SyntaxTreeSemanticModel.cs,1403>
+//!
+//! The `GetDeclaredType` takes `Syntax` as input, and returns `Symbol` as
+//! output. First, it retrieves a `Symbol` for parent `Syntax`:
+//!
+//! * https://sourceroslyn.io/#Microsoft.CodeAnalysis.CSharp/Compilation/SyntaxTreeSemanticModel.cs,1423
+//!
+//! Then, it iterates parent symbol's children, looking for one which has the
+//! same text span as the original node:
+//!
+//!   <https://sourceroslyn.io/#Microsoft.CodeAnalysis.CSharp/Compilation/SyntaxTreeSemanticModel.cs,1786>
+//!
+//! Now, let's look at Kotlin:
+//!
+//!   <https://github.com/JetBrains/kotlin/blob/a288b8b00e4754a1872b164999c6d3f3b8c8994a/idea/idea-frontend-fir/idea-fir-low-level-api/src/org/jetbrains/kotlin/idea/fir/low/level/api/FirModuleResolveStateImpl.kt#L93-L125>
+//!
+//! This function starts with a syntax node (`KtExpression` is syntax, like all
+//! `Kt` nodes), and returns a def. It uses
+//! `getNonLocalContainingOrThisDeclaration` to get syntactic container for a
+//! current node. Then, `findSourceNonLocalFirDeclaration` gets `Fir` for this
+//! parent. Finally, `findElementIn` function traverses `Fir` children to find
+//! one with the same source we originally started with.
 
 use base_db::FileId;
 use hir_def::{