Borrow Checking (Static Alias and Lifetime Safety)

This document specifies Silk’s intended borrow-checking model for references.

Silk currently implements:

• call-scoped alias checks for mutable borrows (including slice range borrows),
• lexical lifetime checks for slice and reference borrows (no escaping borrows of stack locals),
• and a small explicit ownership-transfer form (move) used by the checker and lowering to prevent accidental double-drops in the safe subset.

Semantics#

• Prevent use-after-free and data races in safe code.
• Make mutation explicit and intentional.
• Reject invalid borrows at compile time (no runtime borrow errors required for safe code).
• Keep diagnostics actionable (highlight the borrow origin, conflicting use, and suggest a fix).

Notes#

Today, the language subset implemented by the compiler supports only:

• call-scoped borrow alias checks for:
• borrowed reference parameters (&T, mut p: &T), and
• slice parameters (T[], mut s: T[]) and slice range borrows (&base[start..end], mut &base[start..end], and &base[r] / mut &base[r] where r: range).
• first-class borrowed &T values created from borrowable lvalues:
• &expr (borrow operator) for borrowable lvalues, for:
• the supported &Struct subset, and
• &T where T is a single-slot scalar primitive (for example &int, &bool, &u64, &f64).
• implicit borrow coercions in contexts that expect &T are currently implemented for &Struct (for example let r: &Pair = pair;).

Additionally, the subset implements lexical lifetime checks for both slice borrows and borrowed &T values so obvious use-after-scope cases are rejected (for example returning a slice borrowed from a local fixed array, or returning &T borrowed from a local struct binding).

The currently shipped subset therefore already includes borrowed views in the positions that matter most for day-to-day code:

• local &T and T[] bindings,
• local T? bindings whose payload is a borrowed &T or T[],
• struct fields and enum payloads that carry borrowed views,
• assignments through fields and mutable reference parameters,
• and whole-value returns / assignments that are checked against lexical escape rules even when the borrowed view is carried through an aggregate.

Lexical Lifetimes#

Slices (T[]) are non-owning views. Slice range borrows create slices that point into existing storage:

• &base[start..end] creates a slice view whose lifetime is tied to base.
• &base[r] creates a slice view whose bounds are defined by the range value r (see types).
• When borrowing a range from an existing slice binding s: T[], the borrow’s underlying origin is s’s origin (sub-slicing does not extend lifetime).

Lexical lifetime rules enforced by Silk currently:

• A slice value that ultimately borrows from a local fixed array binding (T[N]) may not escape that binding’s lexical scope.
• Returning such a slice from a function is rejected.
• Assigning such a slice into outer-scope storage is rejected (including via field assignment and via mutable reference parameters).
• The same rule also applies when that borrowed slice is wrapped in T? (Some(&xs[...]) does not allow the local borrow to escape).
• The same rule also applies when that borrowed slice is carried inside a struct field or enum payload; returning or assigning the aggregate does not allow the local borrow to escape.
• Returning a slice is permitted when the returned slice ultimately borrows from a function parameter (for example returning a sub-slice of a T[] parameter).

These rules are intentionally conservative, but they are the complete lexical lifetime model for the currently supported language subset.

Lexical Reference Lifetimes#

Borrowed &T values that ultimately reference stack storage may not escape that storage’s lexical scope. This includes:

• returning a borrowed &T that points to a local stack binding (struct or single-slot scalar),
• returning such a borrow wrapped in T?,
• returning such a borrow carried inside a struct field or enum payload,
• and assigning such a borrowed reference into outer-scope storage.

Returning a reference is permitted when the returned &T ultimately refers to an input reference parameter (that is, storage owned by the caller), and not to stack locals.

When multiple input references or slices are in scope, no explicit lifetime label syntax is required in the current language. A returned borrowed view may refer to any caller-owned input borrow that reaches the return expression through the supported control-flow forms. If any path introduces a local stack or fixed-array origin, the lexical escape check still rejects the return.

Local Mutation While Borrowed#

Silk currently also rejects direct mutation of ordinary local storage while a borrow of that same storage remains live.

This applies to:

• whole-binding assignment (x = ...) when x is a local stack value or local fixed array,
• field assignment (x.f = ...) into a local aggregate that is still borrowed,
• and index assignment (xs[i] = ...) into a local fixed array that still has a live borrowed slice.

In other words, an ordinary local borrow freezes the borrowed local storage against direct mutation until that borrow ends.

Writes performed through the unique mutable borrow itself remain allowed. For example, mutation through mut r: &T is permitted when it is not competing with a separate live borrow of the same local storage.

This rule is intentionally local-storage-specific. Borrowed access to caller-owned or external-handle storage is governed by the existing boundary rules instead.

Borrow-Carrying Wrappers and Conservative Control Flow#

The current checker also preserves borrow identity through a small set of wrapper and control-flow forms:

• Some(<borrow>) preserves the underlying borrow identity.
• Local T? bindings whose payload type is &T or T[] participate in the same local mutation, lexical escape, move, and await checks as direct borrowed bindings.
• Local named struct / enum bindings whose fields or payloads carry &T or T[] also participate in the same local mutation, lexical escape, move, and await checks as direct borrowed bindings.
• Refutable-pattern binders also preserve borrow identity when the scrutinee already proves a single local borrow origin. In the Supported forms, this includes:
• if let Some(x) = r { ... }
• let Some(x) = r else { ... };
• while let Some(x) = r { ... }
• statement match (r) { Some(x) => ..., None => ... } where r: T? and T is a borrowed &U or U[].
• if let Ok(x) = r { ... } / if let Err(x) = r { ... }
• let Ok(x) = r else { ... }; / let Err(x) = r else { ... };
• while let Ok(x) = r { ... } / while let Err(x) = r { ... }
• statement match (r) { Ok(x) => ..., Err(y) => ... } for supported result-shaped enums whose payload type carries a borrow, including monomorphized std::result::Result(T, E) instantiations, and the equivalent qualified enum-variant forms such as State::Ready(x).
• if expressions preserve borrow identity when:
• every borrowing branch resolves to the same local origin,
• or one branch is non-borrowing (None, for example) and the other carries the borrow,
• or all borrowing branches are caller-owned inputs.
• match expressions preserve borrow identity under the same conservative rule:
• every borrowing arm must resolve to the same local origin,
• or one or more arms are non-borrowing while the remaining borrowing arms resolve to that same origin,
• or all borrowing arms are caller-owned inputs.

When a borrowed control-flow expression could refer to multiple distinct local origins, the Supported forms rejects it with E2122 instead of guessing.

Boundary Safety#

The current compiler also enforces conservative rules at boundaries where a borrowed view could outlive the storage it refers to.

async fn boundaries#

At an async fn boundary, the result type may not contain ordinary borrowed-view types:

• non-opaque references (&T),
• and slices (T[]).

This includes such types nested inside structs, enums, optionals, and function types. The reason is suspension: an async fn call returns a Promise(T), so the eventual result may outlive the stack frame that originally produced the borrowed view.

Opaque handle references are allowed:

• &Handle is permitted when Handle is declared as an opaque struct Name;.

These are treated as external handles rather than borrow-checked views into ordinary Silk storage.

Borrowed parameters are permitted in the Supported forms, but the checker also enforces a conservative async call-site rule:

• an ordinary reference or slice that still resolves to function-local stack storage or a local fixed array may not be passed into an async call unless that call is awaited immediately in the same expression,
• opaque handle references remain allowed because they are not borrow-checked views into ordinary Silk storage.

This is the borrow model for the current async subset. Additional async surface area must define equivalent suspension and escape rules before it lands.

External ABI boundaries#

At top-level external ABI boundaries, ordinary borrowed views are also rejected:

• ext declarations may not use ordinary references or slices in parameters or results,
• unnamed C-facing root-package top-level export fn declarations are subject to the same rule because they define the compiler’s C-facing symbol surface,
• named-package Silk object exports may accept slice parameters in the compiler-owned package ABI; that does not make slices part of the external C ABI surface,
• only opaque handle references (&Handle where Handle is struct Name;) may cross that boundary.

This rule does not apply to ordinary impl/public methods inside Silk modules; those remain normal intra-Silk calls.

await suspension points#

At a concrete await / await * suspension point inside an async function, the checker also rejects live borrowed views that still resolve to ordinary function-local storage:

• a borrowed reference (&T) that still points at a local stack value,
• a slice (T[]) that still points at a local fixed array,
• and either of the above when the borrowed view is stored in a local struct field instead of a standalone local binding.

This rule is intentionally conservative. It applies only to borrows rooted in ordinary local Silk storage. The following remain allowed:

• borrowed views rooted in caller-owned storage that have already passed the boundary rules,
• and opaque handle references (&Handle where Handle is an opaque struct Name;).

The practical rule is: if an await may suspend, end any live borrow of local stack / fixed-array storage before the suspension point.

External ABI boundaries#

At ext boundaries, the same borrowed-view restriction applies:

• ordinary references and slices may not cross the boundary,
• and only opaque handle references (&Handle where Handle is opaque) are permitted by reference.

This keeps Silk’s borrow rules out of the C ABI and avoids exposing non-stable borrowed layouts to foreign code.

Ownership Transfer (move)#

Silk’s safe subset includes a small explicit ownership-transfer form:

• move <name>

This expression:

• consumes the binding <name> when its type requires ownership tracking (for example values that are dropped on scope exit),
• and makes <name> unavailable for further use until it is reinitialized (for var) or permanently (for let).

This enables moving values into other values (for example as call arguments or as the payload of Some(...)) without accidentally copying a resource-owning value and dropping it twice.

In the Supported forms, ownership transfer is intentionally conservative:

• A binding may not be moved while it has any live borrows (reference or slice views) in scope, including borrows stored in struct fields.
• A by-value call argument that requires ownership tracking is treated as a move, and is rejected when the same binding is also borrowed in that call.
• When a value type requires ownership tracking, binding initialization and assignment from a name are also treated as moves:
• let y = x; consumes x,
• y = x; consumes x.
• Passing an ownership-tracked value as an enum payload consumes that value. This applies equally to qualified constructors such as Result::Ok(value) and type-directed constructors such as Ok(value), including constructors of monomorphized generic enums. Returning the constructed enum transfers the payload to the caller; function-scope cleanup must not destroy the consumed local after its slots have been copied into the return value.
• Assignment of a value into a struct field, including through a mutable reference parameter (owner.field = value), follows the same rule when the field type requires ownership tracking. The source binding is consumed, the old field value is released exactly once, and the installed value becomes the aggregate's owned field.
• Reading a direct ownership-tracked field in an ownership-consuming position moves that field. For example, both let item = owner.item; and item = owner.item; transfer the field out of owner; owner.item cannot be read or moved again until that exact field is reinitialized. This applies equally when owner is a mutable local or a mutable reference parameter and includes transfers nested inside a newly constructed struct, enum payload, optional, or other aggregate. This supports explicit take operations that immediately install an empty or invalid value in the source aggregate. Ownership classification is transparent through named aliases, applied generic aliases such as Vector(Item), and optional wrappers around an owning type.
• Reinitializing a moved-out field does not run Drop for the field's stale storage, because ownership has already transferred. Once the replacement is installed, the field is initialized again and later overwrites release the then-current value normally.
• A partially moved aggregate must be reinitialized before it is otherwise used or leaves the current block/function. The Supported forms deliberately rejects other aggregate access while a direct field is moved, which keeps a custom aggregate Drop implementation from observing stale field storage.
• Consequently, return owner.item; is rejected when item requires ownership tracking. A function that transfers the field must first take it into a local, reinitialize owner.item, and then return that local.
• let move / var move are the equivalent binding-level spellings for an explicit initialization-time move:
• let move y = x; and var move y = x; consume x when x requires ownership tracking,
• let mut move y = x;, let move mut y = x;, var mut move y = x;, and var move mut y = x; are accepted combined modifier forms,
• copyable existing sources still copy, so both bindings remain independent,
• let move Some(value) = maybe;, let move Some(value) = maybe else { ... };, if let move Some(value) = maybe { ... }, else if let move Some(value) = maybe { ... }, and while let move Some(value) = next() { ... } consume the pattern scrutinee under the same move and borrow checks.

Completeness#

The borrow checker is complete for the currently documented and regression-tested Silk language subset, including the wrapper and control-flow forms described above. New language features may still require new borrow rules, but those are not treated as pre-declared borrow-checker roadmap items. Any such extension must be specified in grammar and in this document before implementation lands, and must be reflected in diagnostics (diagnostics) and tests.