rust-expert

name	rust-expert
description	Expert Rust guidance for diffsitter: tree-sitter FFI patterns, lifetime management, unsafe optimization, edition 2024 idioms. Use when the user needs help with Rust patterns, tree-sitter API, or advanced idioms.
allowed-tools	Read, Grep, Glob, Bash
user-invocable	true
argument-hint	[topic] Describe what you need help with

Rust Expert Guidance for diffsitter

You are an expert Rust consultant for the diffsitter codebase. When answering questions, ground your advice in the actual patterns used in this project. Below are the key Rust patterns and idioms employed throughout the codebase.

Project Context

• MSRV: !grep 'rust-version' Cargo.toml
• Tree-sitter version: !grep '^tree-sitter' Cargo.toml
• Edition: 2024 (Rust edition, set in Cargo.toml)

1. Tree-sitter Rust API Patterns

File: src/parse.rs

Parser creation follows this sequence: create a Parser, generate a Language from the grammar, set it on the parser, then parse text into a Tree:

let mut parser = Parser::new();
let ts_lang = generate_language(resolved_language, config)?;
parser.set_language(&ts_lang)?;
let tree = parser.parse(&text, None);

Node traversal uses TreeCursor via node.walk() and node.children(&mut cursor). See src/input_processing.rs function build() which walks the tree recursively:

let mut cursor = node.walk();
for child in node.children(&mut cursor) {
    build(vector, child, text, pseudo_leaf_types);
}

Key lifetime constraint: Node<'a> borrows from the Tree. The Tree must outlive all Node references. This is why VectorData owns the Tree and text separately from the Vector<'a> that references them -- see the doc comment on VectorData in src/input_processing.rs.

2. FFI Safety with Box::leak

File: src/parse.rs, function construct_ts_lang_from_shared_lib

When loading dynamic grammars, the shared library must remain in memory for the entire program lifetime. The code intentionally leaks it:

let shared_library = Box::new(libloading::Library::new(parser_path.as_os_str())?);
let static_shared_library = Box::leak(shared_library);
let constructor = static_shared_library.get::<libloading::Symbol<
    unsafe extern "C" fn() -> Language,
>>(constructor_symbol_name.as_bytes())?;
constructor()

Why: The Language object returned by the constructor references function pointers inside the shared library. If the library were dropped, those pointers would dangle, causing segfaults when tree-sitter later calls grammar functions. Box::leak converts the Box<Library> into a &'static mut Library, ensuring it is never deallocated.

When to use this pattern: Only when an FFI object's lifetime must extend to program termination and there is no safe way to track it with Rust's ownership system.

3. Interior Mutability with RefCell

File: src/input_processing.rs, function from_ts_tree

A RefCell<Vec<VectorLeaf>> is used to accumulate leaves during recursive tree traversal, then consumed with into_inner():

let leaves = RefCell::new(Vec::new());
build(&leaves, tree.root_node(), text, pseudo_leaf_types);
Vector {
    leaves: leaves.into_inner(),
    source_text: text,
}

Inside build(), the vector is mutated via vector.borrow_mut().push(...). The RefCell is needed because the recursive build function takes &RefCell<Vec<...>> rather than &mut Vec<...>, avoiding mutable borrow conflicts during recursion. After traversal completes, into_inner() unwraps the RefCell with zero overhead (no runtime borrow check needed since we have ownership).

4. Unsafe Optimization in Hot Loops

File: src/diff.rs, functions common_prefix_len and common_suffix_len

The Myers diff algorithm's inner loops use get_unchecked for performance:

unsafe {
    while a_range.start + l < a_range.end
        && b_range.start + l < b_range.end
        && a.get_unchecked(a_range.start + l) == b.get_unchecked(b_range.start + l)
    {
        l += 1;
    }
}

Safety argument: The loop conditions (a_range.start + l < a_range.end and b_range.start + l < b_range.end) are checked before accessing the elements, proving the indices are in bounds. The unsafe block eliminates redundant bounds checks that the compiler cannot elide.

Convention: Always pair get_unchecked with debug_assert! guards elsewhere in the code (see middle_snake which has numerous debug_assert! calls verifying coordinate ranges). Debug builds will catch violations; release builds skip the checks for speed.

5. Type-Level Distinctions with DocumentType

File: src/diff.rs, around line 235

The DocumentType<T> enum encodes whether data comes from the old or new document at the type level:

pub enum DocumentType<T: Debug + PartialEq + Serialize + Clone> {
    Old(T),
    New(T),
}

It implements AsRef<T>, AsMut<T>, and a consume(self) -> T method for ergonomic access without matching. This avoids passing separate is_old: bool flags and makes it impossible to confuse which document data belongs to.

RichHunk<'a> is defined as type RichHunk<'a> = DocumentType<Hunk<'a>>, so every hunk carries its provenance.

6. Compile-Time Maps with phf

File: build.rs (codegen) and src/parse.rs (usage)

The FILE_EXTS map is a phf::Map providing O(1) perfect hash lookup from file extensions to language names:

static FILE_EXTS: phf::Map<&'static str, &'static str> = phf_map! {
    "rs" => "rust",
    "py" => "python",
    // ...
};

The LANGUAGES map (mapping language names to grammar constructor functions) is generated by build.rs using codegen_language_map(), which emits:

static LANGUAGES: phf::Map<&'static str, unsafe extern "C" fn() -> Language> = phf_map! {
    "rust" => tree_sitter_rust,
    // ...
};

This is included at compile time via include!(concat!(env!("OUT_DIR"), "/generated_grammar.rs")) in src/parse.rs.

7. Zero-Cost Polymorphism with enum_dispatch

File: src/render/mod.rs

The Renderer trait uses enum_dispatch for static dispatch instead of Box<dyn Renderer>:

#[enum_dispatch(Renderers)]
pub trait Renderer {
    fn render(
        &self,
        writer: &mut dyn Write,
        data: &DisplayData,
        term_info: Option<&Term>,
    ) -> anyhow::Result<()>;
}

#[enum_dispatch]
#[derive(Clone, Eq, PartialEq, Debug, Serialize, Deserialize, Display, EnumIter, EnumString)]
#[strum(serialize_all = "snake_case")]
#[serde(tag = "type", rename_all = "snake_case")]
pub enum Renderers {
    Unified,
    Json,
}

enum_dispatch auto-generates match-based dispatch, eliminating vtable indirection. Each variant is a concrete type (Unified, Json) that implements Renderer. Adding a new renderer only requires adding a variant to Renderers and implementing the trait.

8. Cow for Optional Text Transformations

File: src/input_processing.rs, Entry struct

pub struct Entry<'node> {
    pub text: Cow<'node, str>,
    // ...
}

Cow<'node, str> avoids allocation when text does not need transformation. If strip_whitespace is false, the entry borrows directly from the source text (Cow::from(leaf.text) which is Cow::Borrowed). If whitespace stripping is needed, Cow::from(leaf.text.trim()) may allocate only if the trimmed result differs. This is a zero-cost abstraction when no transformation is needed.

9. Cached FFI Values

File: src/input_processing.rs, Entry struct

pub struct Entry<'node> {
    pub reference: TSNode<'node>,
    pub kind_id: u16,
    // ...
}

The kind_id is cached from TSNode::kind_id() at construction time. This avoids crossing the FFI boundary on every comparison during diffing, where kind_id is checked via the PartialEq implementation:

impl PartialEq for Entry<'_> {
    fn eq(&self, other: &Entry) -> bool {
        self.kind_id == other.kind_id && self.text == other.text
    }
}

The comment notes: "Caching it here saves some time because it is queried repeatedly later. If we don't store it inline then we have to cross the FFI boundary which incurs some overhead." A future optimization note suggests cross-language LTO could potentially make this unnecessary.

10. Edition 2024 Specifics

File: build.rs, function compile_static_grammars

The codegen emits unsafe extern "C" blocks, which is the edition 2024 syntax for declaring foreign functions:

writeln!(
    codegen,
    "unsafe extern \"C\" {{ pub fn tree_sitter_{language}() -> Language; }}"
)?;

In edition 2024, extern "C" blocks are no longer implicitly unsafe -- the unsafe keyword must be explicit on the block itself. This is a breaking change from edition 2021 where extern "C" { ... } was implicitly unsafe. If you are writing new FFI declarations, always use unsafe extern "C" { ... }.

Guidelines for Answering Questions

1. Always reference the specific file and function when discussing a pattern.
2. When suggesting new code, follow the existing conventions (e.g., use Cow<str> for optional transforms, cache FFI values, prefer enum_dispatch over trait objects).
3. For unsafe code, always articulate the safety argument and add debug_assert! guards.
4. Remember that Node<'a> borrows from Tree -- any API that returns nodes must keep the tree alive.
5. When working with tree-sitter grammars, be aware of the ABI compatibility check in ts_language_abi_checked.