Salsa: Incremental Computation for Rust

Salsa is a framework for incremental recomputation. You define inputs and pure functions over them. Salsa memoizes every function call. When inputs change, it re-executes only the functions whose dependencies actually changed — skipping everything else.

Salsa powers rust-analyzer (Rust IDE), ty (Python type checker), and Cairo, all of which need sub-second response times on large codebases after small edits.

The Mental Model

                    Salsa Database
                    ┌─────────────────────────────────────┐
 External world     │                                     │
 (editor, CLI,      │  Inputs ──→ Tracked Fns ──→ Output  │
  filesystem)       │    │            │                    │
        │           │    └── memoized ┘                    │
        │           │    dependencies tracked automatically│
        ▼           └─────────────────────────────────────┘
  Mutate inputs             │
  (new revision)            ▼
                    Only re-run what changed

The core loop:

let db = MyDatabase::default();

// 1. Create inputs
let file = SourceFile::new(&db, "fn main() {}".into(), path);

// 2. Compute (Salsa memoizes everything)
let result = analyze(&db, file);

// 3. Mutate an input (starts a new "revision")
file.set_text(&mut db).to("fn main() { 42 }".into());

// 4. Re-compute — Salsa reuses what it can
let result = analyze(&db, file);  // Only re-runs what depends on text

Core Concepts

The Database

A struct with #[salsa::db] that stores all cached data. It's the single source of truth — every Salsa operation takes a &db or &mut db.

#[salsa::db]
#[derive(Default, Clone)]
pub struct Database {
    storage: salsa::Storage<Self>,
}

#[salsa::db]
impl salsa::Database for Database {}

→ Deep dive: database-architecture.md for layered traits, test vs production, and side tables.

Inputs — The Roots

External data entering the system. The only mutable Salsa structs. Just newtypes around integer IDs (Copy, no lifetime).

#[salsa::input]
pub struct SourceFile {
    #[returns(ref)]
    pub text: String,
    pub path: PathBuf,
}

Tracked Functions — The Computation

Pure functions whose results are memoized. Salsa records which inputs/fields each call reads. On re-execution, it checks if those dependencies changed.

#[salsa::tracked]
fn parse(db: &dyn Db, file: SourceFile) -> Ast<'_> {
    let text = file.text(db);  // dependency recorded
    // ... parse ...
    Ast::new(db, statements)
}

→ Deep dive: query-pipeline.md for return modes, LRU, no_eq, specify, and granularity.

Tracked Structs — Intermediate Entities

Created inside tracked functions. Have per-field change tracking. Carry a 'db lifetime.

#[salsa::tracked]
pub struct Function<'db> {
    #[id]
    pub name: Word<'db>,         // used for cross-revision matching
    #[tracked]
    #[returns(ref)]
    pub body: Expression<'db>,   // per-field tracking
}

Interned Structs — Cheap Equality

Same data → same integer ID. Used for identifiers, type representations, module paths. Carry a 'db lifetime.

#[salsa::interned]
pub struct Word<'db> {
    #[returns(ref)]
    pub text: String,
}

→ Deep dive: struct-selection.md for the decision framework, including ty's "no tracked structs" and rust-analyzer's "intern every definition" styles.

Accumulators — Side-Channel Output

Report diagnostics from tracked functions without affecting the return value.

#[salsa::accumulator]
pub struct Diagnostics(Diagnostic);

// Inside a tracked function:
Diagnostics::push(db, Diagnostic { message: "type error".into(), .. });

→ Deep dive: accumulators.md.

Revisions and the Red-Green Algorithm

Every input mutation increments a revision counter. When you call a tracked function, Salsa checks its dependencies since the last cached result. If they changed, it re-executes; if the new result equals the old one (backdating), it stops propagation.

Durability — Optimization Hint

Inputs can be tagged with LOW, MEDIUM, or HIGH durability. When only LOW-durability inputs change, Salsa skips validating stable subgraphs.

→ Deep dive: durability.md.

Where to Go Deeper

• For a complete walkthrough of a calculator project: see references/overview/minimal-example.md.

Key Vocabulary

• Revision: A version of the database between input mutations.
• Ingredient: Any Salsa-managed item (input, tracked struct, interned struct, tracked fn, accumulator).
• Backdating: If a re-executed function produces the same result, it's marked unchanged to prevent downstream re-execution.
• Red-green algorithm: Salsa's strategy for deciding what to re-execute.
• Durability: A promise about how often an input changes.
• LRU: Least Recently Used eviction for bounding cache size.
• no_eq: Skip equality comparison on result — always propagate changes.
• Cycle: When tracked functions form a dependency loop.

Real-World Scale

• ty: 2 inputs, 0 tracked structs for types (all interned), 60+ cycle sites.
• rust-analyzer: 5+ inputs, tracked structs for collection caches, 6-layer DB hierarchy.
• Cairo: 4 singleton inputs, 127+ tracked functions, 29 cycle sites, parallel compilation via CloneableDatabase.
• django-language-server: 1 input, 2 accumulators, 5-layer DB (~78 Rust files — very approachable).
• BAML: 15 tracked structs, 6-layer DB, documented early cutoff strategies.
• Fe: marker traits for compilation phase enforcement, hybrid accumulator/return-value diagnostics.
• stc [Legacy API]: 7 tracked functions wrapping the SWC TypeScript ecosystem — the "thin Salsa shell" approach to incrementalizing an existing tool.
• wgsl-analyzer [Legacy API]: ~30 queries across 4 layers (~110 Rust files) — validates the rust-analyzer LSP architecture in a GPU shader language.
• Mun [Legacy API]: ~40 queries across 6 query groups (~150 Rust files) — the only project extending Salsa through LLVM codegen to shared library output, with a hot-reloading compiler daemon.

The hardest part of Salsa is the design decisions about granularity, identity, and what to track. django-language-server and BAML are the best starting points for learning. stc [Legacy API] is the best example of wrapping an existing non-Salsa library with minimal Salsa code. Mun [Legacy API] is the best example of extending Salsa queries all the way to machine code generation and hot reloading.

Further Reading

• Salsa book: https://salsa-rs.github.io/salsa/
• Salsa repo: https://github.com/salsa-rs/salsa (see examples/calc/, examples/lazy-input/)
• ty: crates/ty_* and crates/ruff_db in https://github.com/astral-sh/ruff
• rust-analyzer: https://github.com/rust-lang/rust-analyzer
• Cairo: https://github.com/starkware-libs/cairo
• BAML: https://github.com/BoundaryML/baml
• Fe: https://github.com/argotorg/fe
• django-language-server: https://github.com/joshuadavidthomas/django-language-server
• stc [Legacy API]: https://github.com/dudykr/stc
• Mun [Legacy API]: https://github.com/mun-lang/mun