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// Copyright 2012 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! # The Borrow Checker //! //! This pass has the job of enforcing memory safety. This is a subtle //! topic. This docs aim to explain both the practice and the theory //! behind the borrow checker. They start with a high-level overview of //! how it works, and then proceed to dive into the theoretical //! background. Finally, they go into detail on some of the more subtle //! aspects. //! //! # Table of contents //! //! These docs are long. Search for the section you are interested in. //! //! - Overview //! - Formal model //! - Borrowing and loans //! - Moves and initialization //! - Drop flags and structural fragments //! - Future work //! //! # Overview //! //! The borrow checker checks one function at a time. It operates in two //! passes. The first pass, called `gather_loans`, walks over the function //! and identifies all of the places where borrows (e.g., `&` expressions //! and `ref` bindings) and moves (copies or captures of a linear value) //! occur. It also tracks initialization sites. For each borrow and move, //! it checks various basic safety conditions at this time (for example, //! that the lifetime of the borrow doesn't exceed the lifetime of the //! value being borrowed, or that there is no move out of an `&T` //! referent). //! //! It then uses the dataflow module to propagate which of those borrows //! may be in scope at each point in the procedure. A loan is considered //! to come into scope at the expression that caused it and to go out of //! scope when the lifetime of the resulting reference expires. //! //! Once the in-scope loans are known for each point in the program, the //! borrow checker walks the IR again in a second pass called //! `check_loans`. This pass examines each statement and makes sure that //! it is safe with respect to the in-scope loans. //! //! # Formal model //! //! Throughout the docs we'll consider a simple subset of Rust in which //! you can only borrow from lvalues, defined like so: //! //! ```text //! LV = x | LV.f | *LV //! ``` //! //! Here `x` represents some variable, `LV.f` is a field reference, //! and `*LV` is a pointer dereference. There is no auto-deref or other //! niceties. This means that if you have a type like: //! //! ```text //! struct S { f: uint } //! ``` //! //! and a variable `a: Box<S>`, then the rust expression `a.f` would correspond //! to an `LV` of `(*a).f`. //! //! Here is the formal grammar for the types we'll consider: //! //! ```text //! TY = () | S<'LT...> | Box<TY> | & 'LT MQ TY //! MQ = mut | imm | const //! ``` //! //! Most of these types should be pretty self explanatory. Here `S` is a //! struct name and we assume structs are declared like so: //! //! ```text //! SD = struct S<'LT...> { (f: TY)... } //! ``` //! //! # Borrowing and loans //! //! ## An intuitive explanation //! //! ### Issuing loans //! //! Now, imagine we had a program like this: //! //! ```text //! struct Foo { f: uint, g: uint } //! ... //! 'a: { //! let mut x: Box<Foo> = ...; //! let y = &mut (*x).f; //! x = ...; //! } //! ``` //! //! This is of course dangerous because mutating `x` will free the old //! value and hence invalidate `y`. The borrow checker aims to prevent //! this sort of thing. //! //! #### Loans and restrictions //! //! The way the borrow checker works is that it analyzes each borrow //! expression (in our simple model, that's stuff like `&LV`, though in //! real life there are a few other cases to consider). For each borrow //! expression, it computes a `Loan`, which is a data structure that //! records (1) the value being borrowed, (2) the mutability and scope of //! the borrow, and (3) a set of restrictions. In the code, `Loan` is a //! struct defined in `middle::borrowck`. Formally, we define `LOAN` as //! follows: //! //! ```text //! LOAN = (LV, LT, MQ, RESTRICTION*) //! RESTRICTION = (LV, ACTION*) //! ACTION = MUTATE | CLAIM | FREEZE //! ``` //! //! Here the `LOAN` tuple defines the lvalue `LV` being borrowed; the //! lifetime `LT` of that borrow; the mutability `MQ` of the borrow; and a //! list of restrictions. The restrictions indicate actions which, if //! taken, could invalidate the loan and lead to type safety violations. //! //! Each `RESTRICTION` is a pair of a restrictive lvalue `LV` (which will //! either be the path that was borrowed or some prefix of the path that //! was borrowed) and a set of restricted actions. There are three kinds //! of actions that may be restricted for the path `LV`: //! //! - `MUTATE` means that `LV` cannot be assigned to; //! - `CLAIM` means that the `LV` cannot be borrowed mutably; //! - `FREEZE` means that the `LV` cannot be borrowed immutably; //! //! Finally, it is never possible to move from an lvalue that appears in a //! restriction. This implies that the "empty restriction" `(LV, [])`, //! which contains an empty set of actions, still has a purpose---it //! prevents moves from `LV`. I chose not to make `MOVE` a fourth kind of //! action because that would imply that sometimes moves are permitted //! from restrictived values, which is not the case. //! //! #### Example //! //! To give you a better feeling for what kind of restrictions derived //! from a loan, let's look at the loan `L` that would be issued as a //! result of the borrow `&mut (*x).f` in the example above: //! //! ```text //! L = ((*x).f, 'a, mut, RS) where //! RS = [((*x).f, [MUTATE, CLAIM, FREEZE]), //! (*x, [MUTATE, CLAIM, FREEZE]), //! (x, [MUTATE, CLAIM, FREEZE])] //! ``` //! //! The loan states that the expression `(*x).f` has been loaned as //! mutable for the lifetime `'a`. Because the loan is mutable, that means //! that the value `(*x).f` may be mutated via the newly created reference //! (and *only* via that pointer). This is reflected in the //! restrictions `RS` that accompany the loan. //! //! The first restriction `((*x).f, [MUTATE, CLAIM, FREEZE])` states that //! the lender may not mutate, freeze, nor alias `(*x).f`. Mutation is //! illegal because `(*x).f` is only supposed to be mutated via the new //! reference, not by mutating the original path `(*x).f`. Freezing is //! illegal because the path now has an `&mut` alias; so even if we the //! lender were to consider `(*x).f` to be immutable, it might be mutated //! via this alias. They will be enforced for the lifetime `'a` of the //! loan. After the loan expires, the restrictions no longer apply. //! //! The second restriction on `*x` is interesting because it does not //! apply to the path that was lent (`(*x).f`) but rather to a prefix of //! the borrowed path. This is due to the rules of inherited mutability: //! if the user were to assign to (or freeze) `*x`, they would indirectly //! overwrite (or freeze) `(*x).f`, and thus invalidate the reference //! that was created. In general it holds that when a path is //! lent, restrictions are issued for all the owning prefixes of that //! path. In this case, the path `*x` owns the path `(*x).f` and, //! because `x` is an owned pointer, the path `x` owns the path `*x`. //! Therefore, borrowing `(*x).f` yields restrictions on both //! `*x` and `x`. //! //! ### Checking for illegal assignments, moves, and reborrows //! //! Once we have computed the loans introduced by each borrow, the borrow //! checker uses a data flow propagation to compute the full set of loans //! in scope at each expression and then uses that set to decide whether //! that expression is legal. Remember that the scope of loan is defined //! by its lifetime LT. We sometimes say that a loan which is in-scope at //! a particular point is an "outstanding loan", and the set of //! restrictions included in those loans as the "outstanding //! restrictions". //! //! The kinds of expressions which in-scope loans can render illegal are: //! - *assignments* (`lv = v`): illegal if there is an in-scope restriction //! against mutating `lv`; //! - *moves*: illegal if there is any in-scope restriction on `lv` at all; //! - *mutable borrows* (`&mut lv`): illegal there is an in-scope restriction //! against claiming `lv`; //! - *immutable borrows* (`&lv`): illegal there is an in-scope restriction //! against freezing `lv`. //! //! ## Formal rules //! //! Now that we hopefully have some kind of intuitive feeling for how the //! borrow checker works, let's look a bit more closely now at the precise //! conditions that it uses. For simplicity I will ignore const loans. //! //! I will present the rules in a modified form of standard inference //! rules, which looks as follows: //! //! ```text //! PREDICATE(X, Y, Z) // Rule-Name //! Condition 1 //! Condition 2 //! Condition 3 //! ``` //! //! The initial line states the predicate that is to be satisfied. The //! indented lines indicate the conditions that must be met for the //! predicate to be satisfied. The right-justified comment states the name //! of this rule: there are comments in the borrowck source referencing //! these names, so that you can cross reference to find the actual code //! that corresponds to the formal rule. //! //! ### Invariants //! //! I want to collect, at a high-level, the invariants the borrow checker //! maintains. I will give them names and refer to them throughout the //! text. Together these invariants are crucial for the overall soundness //! of the system. //! //! **Mutability requires uniqueness.** To mutate a path //! //! **Unique mutability.** There is only one *usable* mutable path to any //! given memory at any given time. This implies that when claiming memory //! with an expression like `p = &mut x`, the compiler must guarantee that //! the borrowed value `x` can no longer be mutated so long as `p` is //! live. (This is done via restrictions, read on.) //! //! **.** //! //! //! ### The `gather_loans` pass //! //! We start with the `gather_loans` pass, which walks the AST looking for //! borrows. For each borrow, there are three bits of information: the //! lvalue `LV` being borrowed and the mutability `MQ` and lifetime `LT` //! of the resulting pointer. Given those, `gather_loans` applies four //! validity tests: //! //! 1. `MUTABILITY(LV, MQ)`: The mutability of the reference is //! compatible with the mutability of `LV` (i.e., not borrowing immutable //! data as mutable). //! //! 2. `ALIASABLE(LV, MQ)`: The aliasability of the reference is //! compatible with the aliasability of `LV`. The goal is to prevent //! `&mut` borrows of aliasability data. //! //! 3. `LIFETIME(LV, LT, MQ)`: The lifetime of the borrow does not exceed //! the lifetime of the value being borrowed. //! //! 4. `RESTRICTIONS(LV, LT, ACTIONS) = RS`: This pass checks and computes the //! restrictions to maintain memory safety. These are the restrictions //! that will go into the final loan. We'll discuss in more detail below. //! //! ## Checking mutability //! //! Checking mutability is fairly straightforward. We just want to prevent //! immutable data from being borrowed as mutable. Note that it is ok to //! borrow mutable data as immutable, since that is simply a //! freeze. Formally we define a predicate `MUTABLE(LV, MQ)` which, if //! defined, means that "borrowing `LV` with mutability `MQ` is ok. The //! Rust code corresponding to this predicate is the function //! `check_mutability` in `middle::borrowck::gather_loans`. //! //! ### Checking mutability of variables //! //! *Code pointer:* Function `check_mutability()` in `gather_loans/mod.rs`, //! but also the code in `mem_categorization`. //! //! Let's begin with the rules for variables, which state that if a //! variable is declared as mutable, it may be borrowed any which way, but //! otherwise the variable must be borrowed as immutable or const: //! //! ```text //! MUTABILITY(X, MQ) // M-Var-Mut //! DECL(X) = mut //! //! MUTABILITY(X, MQ) // M-Var-Imm //! DECL(X) = imm //! MQ = imm | const //! ``` //! //! ### Checking mutability of owned content //! //! Fields and owned pointers inherit their mutability from //! their base expressions, so both of their rules basically //! delegate the check to the base expression `LV`: //! //! ```text //! MUTABILITY(LV.f, MQ) // M-Field //! MUTABILITY(LV, MQ) //! //! MUTABILITY(*LV, MQ) // M-Deref-Unique //! TYPE(LV) = Box<Ty> //! MUTABILITY(LV, MQ) //! ``` //! //! ### Checking mutability of immutable pointer types //! //! Immutable pointer types like `&T` can only //! be borrowed if MQ is immutable or const: //! //! ```text //! MUTABILITY(*LV, MQ) // M-Deref-Borrowed-Imm //! TYPE(LV) = &Ty //! MQ == imm | const //! ``` //! //! ### Checking mutability of mutable pointer types //! //! `&mut T` can be frozen, so it is acceptable to borrow it as either imm or mut: //! //! ```text //! MUTABILITY(*LV, MQ) // M-Deref-Borrowed-Mut //! TYPE(LV) = &mut Ty //! ``` //! //! ## Checking aliasability //! //! The goal of the aliasability check is to ensure that we never permit //! `&mut` borrows of aliasable data. Formally we define a predicate //! `ALIASABLE(LV, MQ)` which if defined means that //! "borrowing `LV` with mutability `MQ` is ok". The //! Rust code corresponding to this predicate is the function //! `check_aliasability()` in `middle::borrowck::gather_loans`. //! //! ### Checking aliasability of variables //! //! Local variables are never aliasable as they are accessible only within //! the stack frame. //! //! ```text //! ALIASABLE(X, MQ) // M-Var-Mut //! ``` //! //! ### Checking aliasable of owned content //! //! Owned content is aliasable if it is found in an aliasable location: //! //! ```text //! ALIASABLE(LV.f, MQ) // M-Field //! ALIASABLE(LV, MQ) //! //! ALIASABLE(*LV, MQ) // M-Deref-Unique //! ALIASABLE(LV, MQ) //! ``` //! //! ### Checking mutability of immutable pointer types //! //! Immutable pointer types like `&T` are aliasable, and hence can only be //! borrowed immutably: //! //! ```text //! ALIASABLE(*LV, imm) // M-Deref-Borrowed-Imm //! TYPE(LV) = &Ty //! ``` //! //! ### Checking mutability of mutable pointer types //! //! `&mut T` can be frozen, so it is acceptable to borrow it as either imm or mut: //! //! ```text //! ALIASABLE(*LV, MQ) // M-Deref-Borrowed-Mut //! TYPE(LV) = &mut Ty //! ``` //! //! ## Checking lifetime //! //! These rules aim to ensure that no data is borrowed for a scope that exceeds //! its lifetime. These two computations wind up being intimately related. //! Formally, we define a predicate `LIFETIME(LV, LT, MQ)`, which states that //! "the lvalue `LV` can be safely borrowed for the lifetime `LT` with mutability //! `MQ`". The Rust code corresponding to this predicate is the module //! `middle::borrowck::gather_loans::lifetime`. //! //! ### The Scope function //! //! Several of the rules refer to a helper function `SCOPE(LV)=LT`. The //! `SCOPE(LV)` yields the lifetime `LT` for which the lvalue `LV` is //! guaranteed to exist, presuming that no mutations occur. //! //! The scope of a local variable is the block where it is declared: //! //! ```text //! SCOPE(X) = block where X is declared //! ``` //! //! The scope of a field is the scope of the struct: //! //! ```text //! SCOPE(LV.f) = SCOPE(LV) //! ``` //! //! The scope of a unique referent is the scope of the pointer, since //! (barring mutation or moves) the pointer will not be freed until //! the pointer itself `LV` goes out of scope: //! //! ```text //! SCOPE(*LV) = SCOPE(LV) if LV has type Box<T> //! ``` //! //! The scope of a borrowed referent is the scope associated with the //! pointer. This is a conservative approximation, since the data that //! the pointer points at may actually live longer: //! //! ```text //! SCOPE(*LV) = LT if LV has type &'LT T or &'LT mut T //! ``` //! //! ### Checking lifetime of variables //! //! The rule for variables states that a variable can only be borrowed a //! lifetime `LT` that is a subregion of the variable's scope: //! //! ```text //! LIFETIME(X, LT, MQ) // L-Local //! LT <= SCOPE(X) //! ``` //! //! ### Checking lifetime for owned content //! //! The lifetime of a field or owned pointer is the same as the lifetime //! of its owner: //! //! ```text //! LIFETIME(LV.f, LT, MQ) // L-Field //! LIFETIME(LV, LT, MQ) //! //! LIFETIME(*LV, LT, MQ) // L-Deref-Send //! TYPE(LV) = Box<Ty> //! LIFETIME(LV, LT, MQ) //! ``` //! //! ### Checking lifetime for derefs of references //! //! References have a lifetime `LT'` associated with them. The //! data they point at has been guaranteed to be valid for at least this //! lifetime. Therefore, the borrow is valid so long as the lifetime `LT` //! of the borrow is shorter than the lifetime `LT'` of the pointer //! itself: //! //! ```text //! LIFETIME(*LV, LT, MQ) // L-Deref-Borrowed //! TYPE(LV) = <' Ty OR <' mut Ty //! LT <= LT' //! ``` //! //! ## Computing the restrictions //! //! The final rules govern the computation of *restrictions*, meaning that //! we compute the set of actions that will be illegal for the life of the //! loan. The predicate is written `RESTRICTIONS(LV, LT, ACTIONS) = //! RESTRICTION*`, which can be read "in order to prevent `ACTIONS` from //! occurring on `LV`, the restrictions `RESTRICTION*` must be respected //! for the lifetime of the loan". //! //! Note that there is an initial set of restrictions: these restrictions //! are computed based on the kind of borrow: //! //! ```text //! &mut LV => RESTRICTIONS(LV, LT, MUTATE|CLAIM|FREEZE) //! &LV => RESTRICTIONS(LV, LT, MUTATE|CLAIM) //! &const LV => RESTRICTIONS(LV, LT, []) //! ``` //! //! The reasoning here is that a mutable borrow must be the only writer, //! therefore it prevents other writes (`MUTATE`), mutable borrows //! (`CLAIM`), and immutable borrows (`FREEZE`). An immutable borrow //! permits other immutable borrows but forbids writes and mutable borrows. //! Finally, a const borrow just wants to be sure that the value is not //! moved out from under it, so no actions are forbidden. //! //! ### Restrictions for loans of a local variable //! //! The simplest case is a borrow of a local variable `X`: //! //! ```text //! RESTRICTIONS(X, LT, ACTIONS) = (X, ACTIONS) // R-Variable //! ``` //! //! In such cases we just record the actions that are not permitted. //! //! ### Restrictions for loans of fields //! //! Restricting a field is the same as restricting the owner of that //! field: //! //! ```text //! RESTRICTIONS(LV.f, LT, ACTIONS) = RS, (LV.f, ACTIONS) // R-Field //! RESTRICTIONS(LV, LT, ACTIONS) = RS //! ``` //! //! The reasoning here is as follows. If the field must not be mutated, //! then you must not mutate the owner of the field either, since that //! would indirectly modify the field. Similarly, if the field cannot be //! frozen or aliased, we cannot allow the owner to be frozen or aliased, //! since doing so indirectly freezes/aliases the field. This is the //! origin of inherited mutability. //! //! ### Restrictions for loans of owned referents //! //! Because the mutability of owned referents is inherited, restricting an //! owned referent is similar to restricting a field, in that it implies //! restrictions on the pointer. However, owned pointers have an important //! twist: if the owner `LV` is mutated, that causes the owned referent //! `*LV` to be freed! So whenever an owned referent `*LV` is borrowed, we //! must prevent the owned pointer `LV` from being mutated, which means //! that we always add `MUTATE` and `CLAIM` to the restriction set imposed //! on `LV`: //! //! ```text //! RESTRICTIONS(*LV, LT, ACTIONS) = RS, (*LV, ACTIONS) // R-Deref-Send-Pointer //! TYPE(LV) = Box<Ty> //! RESTRICTIONS(LV, LT, ACTIONS|MUTATE|CLAIM) = RS //! ``` //! //! ### Restrictions for loans of immutable borrowed referents //! //! Immutable borrowed referents are freely aliasable, meaning that //! the compiler does not prevent you from copying the pointer. This //! implies that issuing restrictions is useless. We might prevent the //! user from acting on `*LV` itself, but there could be another path //! `*LV1` that refers to the exact same memory, and we would not be //! restricting that path. Therefore, the rule for `&Ty` pointers //! always returns an empty set of restrictions, and it only permits //! restricting `MUTATE` and `CLAIM` actions: //! //! ```text //! RESTRICTIONS(*LV, LT, ACTIONS) = [] // R-Deref-Imm-Borrowed //! TYPE(LV) = <' Ty //! LT <= LT' // (1) //! ACTIONS subset of [MUTATE, CLAIM] //! ``` //! //! The reason that we can restrict `MUTATE` and `CLAIM` actions even //! without a restrictions list is that it is never legal to mutate nor to //! borrow mutably the contents of a `&Ty` pointer. In other words, //! those restrictions are already inherent in the type. //! //! Clause (1) in the rule for `&Ty` deserves mention. Here I //! specify that the lifetime of the loan must be less than the lifetime //! of the `&Ty` pointer. In simple cases, this clause is redundant, since //! the `LIFETIME()` function will already enforce the required rule: //! //! ``` //! fn foo(point: &'a Point) -> &'static f32 { //! &point.x // Error //! } //! ``` //! //! The above example fails to compile both because of clause (1) above //! but also by the basic `LIFETIME()` check. However, in more advanced //! examples involving multiple nested pointers, clause (1) is needed: //! //! ``` //! fn foo(point: &'a &'b mut Point) -> &'b f32 { //! &point.x // Error //! } //! ``` //! //! The `LIFETIME` rule here would accept `'b` because, in fact, the //! *memory is* guaranteed to remain valid (i.e., not be freed) for the //! lifetime `'b`, since the `&mut` pointer is valid for `'b`. However, we //! are returning an immutable reference, so we need the memory to be both //! valid and immutable. Even though `point.x` is referenced by an `&mut` //! pointer, it can still be considered immutable so long as that `&mut` //! pointer is found in an aliased location. That means the memory is //! guaranteed to be *immutable* for the lifetime of the `&` pointer, //! which is only `'a`, not `'b`. Hence this example yields an error. //! //! As a final twist, consider the case of two nested *immutable* //! pointers, rather than a mutable pointer within an immutable one: //! //! ``` //! fn foo(point: &'a &'b Point) -> &'b f32 { //! &point.x // OK //! } //! ``` //! //! This function is legal. The reason for this is that the inner pointer //! (`*point : &'b Point`) is enough to guarantee the memory is immutable //! and valid for the lifetime `'b`. This is reflected in //! `RESTRICTIONS()` by the fact that we do not recurse (i.e., we impose //! no restrictions on `LV`, which in this particular case is the pointer //! `point : &'a &'b Point`). //! //! #### Why both `LIFETIME()` and `RESTRICTIONS()`? //! //! Given the previous text, it might seem that `LIFETIME` and //! `RESTRICTIONS` should be folded together into one check, but there is //! a reason that they are separated. They answer separate concerns. //! The rules pertaining to `LIFETIME` exist to ensure that we don't //! create a borrowed pointer that outlives the memory it points at. So //! `LIFETIME` prevents a function like this: //! //! ``` //! fn get_1<'a>() -> &'a int { //! let x = 1; //! &x //! } //! ``` //! //! Here we would be returning a pointer into the stack. Clearly bad. //! //! However, the `RESTRICTIONS` rules are more concerned with how memory //! is used. The example above doesn't generate an error according to //! `RESTRICTIONS` because, for local variables, we don't require that the //! loan lifetime be a subset of the local variable lifetime. The idea //! here is that we *can* guarantee that `x` is not (e.g.) mutated for the //! lifetime `'a`, even though `'a` exceeds the function body and thus //! involves unknown code in the caller -- after all, `x` ceases to exist //! after we return and hence the remaining code in `'a` cannot possibly //! mutate it. This distinction is important for type checking functions //! like this one: //! //! ``` //! fn inc_and_get<'a>(p: &'a mut Point) -> &'a int { //! p.x += 1; //! &p.x //! } //! ``` //! //! In this case, we take in a `&mut` and return a frozen borrowed pointer //! with the same lifetime. So long as the lifetime of the returned value //! doesn't exceed the lifetime of the `&mut` we receive as input, this is //! fine, though it may seem surprising at first (it surprised me when I //! first worked it through). After all, we're guaranteeing that `*p` //! won't be mutated for the lifetime `'a`, even though we can't "see" the //! entirety of the code during that lifetime, since some of it occurs in //! our caller. But we *do* know that nobody can mutate `*p` except //! through `p`. So if we don't mutate `*p` and we don't return `p`, then //! we know that the right to mutate `*p` has been lost to our caller -- //! in terms of capability, the caller passed in the ability to mutate //! `*p`, and we never gave it back. (Note that we can't return `p` while //! `*p` is borrowed since that would be a move of `p`, as `&mut` pointers //! are affine.) //! //! ### Restrictions for loans of const aliasable referents //! //! Freeze pointers are read-only. There may be `&mut` or `&` aliases, and //! we can not prevent *anything* but moves in that case. So the //! `RESTRICTIONS` function is only defined if `ACTIONS` is the empty set. //! Because moves from a `&const` lvalue are never legal, it is not //! necessary to add any restrictions at all to the final result. //! //! ```text //! RESTRICTIONS(*LV, LT, []) = [] // R-Deref-Freeze-Borrowed //! TYPE(LV) = &const Ty //! ``` //! //! ### Restrictions for loans of mutable borrowed referents //! //! Mutable borrowed pointers are guaranteed to be the only way to mutate //! their referent. This permits us to take greater license with them; for //! example, the referent can be frozen simply be ensuring that we do not //! use the original pointer to perform mutate. Similarly, we can allow //! the referent to be claimed, so long as the original pointer is unused //! while the new claimant is live. //! //! The rule for mutable borrowed pointers is as follows: //! //! ```text //! RESTRICTIONS(*LV, LT, ACTIONS) = RS, (*LV, ACTIONS) // R-Deref-Mut-Borrowed //! TYPE(LV) = <' mut Ty //! LT <= LT' // (1) //! RESTRICTIONS(LV, LT, ACTIONS) = RS // (2) //! ``` //! //! Let's examine the two numbered clauses: //! //! Clause (1) specifies that the lifetime of the loan (`LT`) cannot //! exceed the lifetime of the `&mut` pointer (`LT'`). The reason for this //! is that the `&mut` pointer is guaranteed to be the only legal way to //! mutate its referent -- but only for the lifetime `LT'`. After that //! lifetime, the loan on the referent expires and hence the data may be //! modified by its owner again. This implies that we are only able to //! guarantee that the referent will not be modified or aliased for a //! maximum of `LT'`. //! //! Here is a concrete example of a bug this rule prevents: //! //! ``` //! // Test region-reborrow-from-shorter-mut-ref.rs: //! fn copy_pointer<'a,'b,T>(x: &'a mut &'b mut T) -> &'b mut T { //! &mut **p // ERROR due to clause (1) //! } //! fn main() { //! let mut x = 1; //! let mut y = &mut x; // <-'b-----------------------------+ //! // +-'a--------------------+ | //! // v v | //! let z = copy_borrowed_ptr(&mut y); // y is lent | //! *y += 1; // Here y==z, so both should not be usable... | //! *z += 1; // ...and yet they would be, but for clause 1. | //! } // <------------------------------------------------------+ //! ``` //! //! Clause (2) propagates the restrictions on the referent to the pointer //! itself. This is the same as with an owned pointer, though the //! reasoning is mildly different. The basic goal in all cases is to //! prevent the user from establishing another route to the same data. To //! see what I mean, let's examine various cases of what can go wrong and //! show how it is prevented. //! //! **Example danger 1: Moving the base pointer.** One of the simplest //! ways to violate the rules is to move the base pointer to a new name //! and access it via that new name, thus bypassing the restrictions on //! the old name. Here is an example: //! //! ``` //! // src/test/compile-fail/borrowck-move-mut-base-ptr.rs //! fn foo(t0: &mut int) { //! let p: &int = &*t0; // Freezes `*t0` //! let t1 = t0; //~ ERROR cannot move out of `t0` //! *t1 = 22; // OK, not a write through `*t0` //! } //! ``` //! //! Remember that `&mut` pointers are linear, and hence `let t1 = t0` is a //! move of `t0` -- or would be, if it were legal. Instead, we get an //! error, because clause (2) imposes restrictions on `LV` (`t0`, here), //! and any restrictions on a path make it impossible to move from that //! path. //! //! **Example danger 2: Claiming the base pointer.** Another possible //! danger is to mutably borrow the base path. This can lead to two bad //! scenarios. The most obvious is that the mutable borrow itself becomes //! another path to access the same data, as shown here: //! //! ``` //! // src/test/compile-fail/borrowck-mut-borrow-of-mut-base-ptr.rs //! fn foo<'a>(mut t0: &'a mut int, //! mut t1: &'a mut int) { //! let p: &int = &*t0; // Freezes `*t0` //! let mut t2 = &mut t0; //~ ERROR cannot borrow `t0` //! **t2 += 1; // Mutates `*t0` //! } //! ``` //! //! In this example, `**t2` is the same memory as `*t0`. Because `t2` is //! an `&mut` pointer, `**t2` is a unique path and hence it would be //! possible to mutate `**t2` even though that memory was supposed to be //! frozen by the creation of `p`. However, an error is reported -- the //! reason is that the freeze `&*t0` will restrict claims and mutation //! against `*t0` which, by clause 2, in turn prevents claims and mutation //! of `t0`. Hence the claim `&mut t0` is illegal. //! //! Another danger with an `&mut` pointer is that we could swap the `t0` //! value away to create a new path: //! //! ``` //! // src/test/compile-fail/borrowck-swap-mut-base-ptr.rs //! fn foo<'a>(mut t0: &'a mut int, //! mut t1: &'a mut int) { //! let p: &int = &*t0; // Freezes `*t0` //! swap(&mut t0, &mut t1); //~ ERROR cannot borrow `t0` //! *t1 = 22; //! } //! ``` //! //! This is illegal for the same reason as above. Note that if we added //! back a swap operator -- as we used to have -- we would want to be very //! careful to ensure this example is still illegal. //! //! **Example danger 3: Freeze the base pointer.** In the case where the //! referent is claimed, even freezing the base pointer can be dangerous, //! as shown in the following example: //! //! ``` //! // src/test/compile-fail/borrowck-borrow-of-mut-base-ptr.rs //! fn foo<'a>(mut t0: &'a mut int, //! mut t1: &'a mut int) { //! let p: &mut int = &mut *t0; // Claims `*t0` //! let mut t2 = &t0; //~ ERROR cannot borrow `t0` //! let q: &int = &*t2; // Freezes `*t0` but not through `*p` //! *p += 1; // violates type of `*q` //! } //! ``` //! //! Here the problem is that `*t0` is claimed by `p`, and hence `p` wants //! to be the controlling pointer through which mutation or freezes occur. //! But `t2` would -- if it were legal -- have the type `& &mut int`, and //! hence would be a mutable pointer in an aliasable location, which is //! considered frozen (since no one can write to `**t2` as it is not a //! unique path). Therefore, we could reasonably create a frozen `&int` //! pointer pointing at `*t0` that coexists with the mutable pointer `p`, //! which is clearly unsound. //! //! However, it is not always unsafe to freeze the base pointer. In //! particular, if the referent is frozen, there is no harm in it: //! //! ``` //! // src/test/run-pass/borrowck-borrow-of-mut-base-ptr-safe.rs //! fn foo<'a>(mut t0: &'a mut int, //! mut t1: &'a mut int) { //! let p: &int = &*t0; // Freezes `*t0` //! let mut t2 = &t0; //! let q: &int = &*t2; // Freezes `*t0`, but that's ok... //! let r: &int = &*t0; // ...after all, could do same thing directly. //! } //! ``` //! //! In this case, creating the alias `t2` of `t0` is safe because the only //! thing `t2` can be used for is to further freeze `*t0`, which is //! already frozen. In particular, we cannot assign to `*t0` through the //! new alias `t2`, as demonstrated in this test case: //! //! ``` //! // src/test/run-pass/borrowck-borrow-mut-base-ptr-in-aliasable-loc.rs //! fn foo(t0: & &mut int) { //! let t1 = t0; //! let p: &int = &**t0; //! **t1 = 22; //~ ERROR cannot assign //! } //! ``` //! //! This distinction is reflected in the rules. When doing an `&mut` //! borrow -- as in the first example -- the set `ACTIONS` will be //! `CLAIM|MUTATE|FREEZE`, because claiming the referent implies that it //! cannot be claimed, mutated, or frozen by anyone else. These //! restrictions are propagated back to the base path and hence the base //! path is considered unfreezable. //! //! In contrast, when the referent is merely frozen -- as in the second //! example -- the set `ACTIONS` will be `CLAIM|MUTATE`, because freezing //! the referent implies that it cannot be claimed or mutated but permits //! others to freeze. Hence when these restrictions are propagated back to //! the base path, it will still be considered freezable. //! //! //! //! **FIXME #10520: Restrictions against mutating the base pointer.** When //! an `&mut` pointer is frozen or claimed, we currently pass along the //! restriction against MUTATE to the base pointer. I do not believe this //! restriction is needed. It dates from the days when we had a way to //! mutate that preserved the value being mutated (i.e., swap). Nowadays //! the only form of mutation is assignment, which destroys the pointer //! being mutated -- therefore, a mutation cannot create a new path to the //! same data. Rather, it removes an existing path. This implies that not //! only can we permit mutation, we can have mutation kill restrictions in //! the dataflow sense. //! //! **WARNING:** We do not currently have `const` borrows in the //! language. If they are added back in, we must ensure that they are //! consistent with all of these examples. The crucial question will be //! what sorts of actions are permitted with a `&const &mut` pointer. I //! would suggest that an `&mut` referent found in an `&const` location be //! prohibited from both freezes and claims. This would avoid the need to //! prevent `const` borrows of the base pointer when the referent is //! borrowed. //! //! # Moves and initialization //! //! The borrow checker is also in charge of ensuring that: //! //! - all memory which is accessed is initialized //! - immutable local variables are assigned at most once. //! //! These are two separate dataflow analyses built on the same //! framework. Let's look at checking that memory is initialized first; //! the checking of immutable local variable assignments works in a very //! similar way. //! //! To track the initialization of memory, we actually track all the //! points in the program that *create uninitialized memory*, meaning //! moves and the declaration of uninitialized variables. For each of //! these points, we create a bit in the dataflow set. Assignments to a //! variable `x` or path `a.b.c` kill the move/uninitialization bits for //! those paths and any subpaths (e.g., `x`, `x.y`, `a.b.c`, `*a.b.c`). //! Bits are unioned when two control-flow paths join. Thus, the //! presence of a bit indicates that the move may have occurred without an //! intervening assignment to the same memory. At each use of a variable, //! we examine the bits in scope, and check that none of them are //! moves/uninitializations of the variable that is being used. //! //! Let's look at a simple example: //! //! ``` //! fn foo(a: Box<int>) { //! let b: Box<int>; // Gen bit 0. //! //! if cond { // Bits: 0 //! use(&*a); //! b = a; // Gen bit 1, kill bit 0. //! use(&*b); //! } else { //! // Bits: 0 //! } //! // Bits: 0,1 //! use(&*a); // Error. //! use(&*b); // Error. //! } //! //! fn use(a: &int) { } //! ``` //! //! In this example, the variable `b` is created uninitialized. In one //! branch of an `if`, we then move the variable `a` into `b`. Once we //! exit the `if`, therefore, it is an error to use `a` or `b` since both //! are only conditionally initialized. I have annotated the dataflow //! state using comments. There are two dataflow bits, with bit 0 //! corresponding to the creation of `b` without an initializer, and bit 1 //! corresponding to the move of `a`. The assignment `b = a` both //! generates bit 1, because it is a move of `a`, and kills bit 0, because //! `b` is now initialized. On the else branch, though, `b` is never //! initialized, and so bit 0 remains untouched. When the two flows of //! control join, we union the bits from both sides, resulting in both //! bits 0 and 1 being set. Thus any attempt to use `a` uncovers the bit 1 //! from the "then" branch, showing that `a` may be moved, and any attempt //! to use `b` uncovers bit 0, from the "else" branch, showing that `b` //! may not be initialized. //! //! ## Initialization of immutable variables //! //! Initialization of immutable variables works in a very similar way, //! except that: //! //! 1. we generate bits for each assignment to a variable; //! 2. the bits are never killed except when the variable goes out of scope. //! //! Thus the presence of an assignment bit indicates that the assignment //! may have occurred. Note that assignments are only killed when the //! variable goes out of scope, as it is not relevant whether or not there //! has been a move in the meantime. Using these bits, we can declare that //! an assignment to an immutable variable is legal iff there is no other //! assignment bit to that same variable in scope. //! //! ## Why is the design made this way? //! //! It may seem surprising that we assign dataflow bits to *each move* //! rather than *each path being moved*. This is somewhat less efficient, //! since on each use, we must iterate through all moves and check whether //! any of them correspond to the path in question. Similar concerns apply //! to the analysis for double assignments to immutable variables. The //! main reason to do it this way is that it allows us to print better //! error messages, because when a use occurs, we can print out the //! precise move that may be in scope, rather than simply having to say //! "the variable may not be initialized". //! //! ## Data structures used in the move analysis //! //! The move analysis maintains several data structures that enable it to //! cross-reference moves and assignments to determine when they may be //! moving/assigning the same memory. These are all collected into the //! `MoveData` and `FlowedMoveData` structs. The former represents the set //! of move paths, moves, and assignments, and the latter adds in the //! results of a dataflow computation. //! //! ### Move paths //! //! The `MovePath` tree tracks every path that is moved or assigned to. //! These paths have the same form as the `LoanPath` data structure, which //! in turn is the "real world version of the lvalues `LV` that we //! introduced earlier. The difference between a `MovePath` and a `LoanPath` //! is that move paths are: //! //! 1. Canonicalized, so that we have exactly one copy of each, and //! we can refer to move paths by index; //! 2. Cross-referenced with other paths into a tree, so that given a move //! path we can efficiently find all parent move paths and all //! extensions (e.g., given the `a.b` move path, we can easily find the //! move path `a` and also the move paths `a.b.c`) //! 3. Cross-referenced with moves and assignments, so that we can //! easily find all moves and assignments to a given path. //! //! The mechanism that we use is to create a `MovePath` record for each //! move path. These are arranged in an array and are referenced using //! `MovePathIndex` values, which are newtype'd indices. The `MovePath` //! structs are arranged into a tree, representing using the standard //! Knuth representation where each node has a child 'pointer' and a "next //! sibling" 'pointer'. In addition, each `MovePath` has a parent //! 'pointer'. In this case, the 'pointers' are just `MovePathIndex` //! values. //! //! In this way, if we want to find all base paths of a given move path, //! we can just iterate up the parent pointers (see `each_base_path()` in //! the `move_data` module). If we want to find all extensions, we can //! iterate through the subtree (see `each_extending_path()`). //! //! ### Moves and assignments //! //! There are structs to represent moves (`Move`) and assignments //! (`Assignment`), and these are also placed into arrays and referenced //! by index. All moves of a particular path are arranged into a linked //! lists, beginning with `MovePath.first_move` and continuing through //! `Move.next_move`. //! //! We distinguish between "var" assignments, which are assignments to a //! variable like `x = foo`, and "path" assignments (`x.f = foo`). This //! is because we need to assign dataflows to the former, but not the //! latter, so as to check for double initialization of immutable //! variables. //! //! ### Gathering and checking moves //! //! Like loans, we distinguish two phases. The first, gathering, is where //! we uncover all the moves and assignments. As with loans, we do some //! basic sanity checking in this phase, so we'll report errors if you //! attempt to move out of a borrowed pointer etc. Then we do the dataflow //! (see `FlowedMoveData::new`). Finally, in the `check_loans.rs` code, we //! walk back over, identify all uses, assignments, and captures, and //! check that they are legal given the set of dataflow bits we have //! computed for that program point. //! //! # Drop flags and structural fragments //! //! In addition to the job of enforcing memory safety, the borrow checker //! code is also responsible for identifying the *structural fragments* of //! data in the function, to support out-of-band dynamic drop flags //! allocated on the stack. (For background, see [RFC PR #320].) //! //! [RFC PR #320]: https://github.com/rust-lang/rfcs/pull/320 //! //! Semantically, each piece of data that has a destructor may need a //! boolean flag to indicate whether or not its destructor has been run //! yet. However, in many cases there is no need to actually maintain such //! a flag: It can be apparent from the code itself that a given path is //! always initialized (or always deinitialized) when control reaches the //! end of its owner's scope, and thus we can unconditionally emit (or //! not) the destructor invocation for that path. //! //! A simple example of this is the following: //! //! ```rust //! struct D { p: int } //! impl D { fn new(x: int) -> D { ... } //! impl Drop for D { ... } //! //! fn foo(a: D, b: D, t: || -> bool) { //! let c: D; //! let d: D; //! if t() { c = b; } //! } //! ``` //! //! At the end of the body of `foo`, the compiler knows that `a` is //! initialized, introducing a drop obligation (deallocating the boxed //! integer) for the end of `a`'s scope that is run unconditionally. //! Likewise the compiler knows that `d` is not initialized, and thus it //! leave out the drop code for `d`. //! //! The compiler cannot statically know the drop-state of `b` nor `c` at //! the end of their scope, since that depends on the value of //! `t`. Therefore, we need to insert boolean flags to track whether we //! need to drop `b` and `c`. //! //! However, the matter is not as simple as just mapping local variables //! to their corresponding drop flags when necessary. In particular, in //! addition to being able to move data out of local variables, Rust //! allows one to move values in and out of structured data. //! //! Consider the following: //! //! ```rust //! struct S { x: D, y: D, z: D } //! //! fn foo(a: S, mut b: S, t: || -> bool) { //! let mut c: S; //! let d: S; //! let e: S = a.clone(); //! if t() { //! c = b; //! b.x = e.y; //! } //! if t() { c.y = D::new(4); } //! } //! ``` //! //! As before, the drop obligations of `a` and `d` can be statically //! determined, and again the state of `b` and `c` depend on dynamic //! state. But additionally, the dynamic drop obligations introduced by //! `b` and `c` are not just per-local boolean flags. For example, if the //! first call to `t` returns `false` and the second call `true`, then at //! the end of their scope, `b` will be completely initialized, but only //! `c.y` in `c` will be initialized. If both calls to `t` return `true`, //! then at the end of their scope, `c` will be completely initialized, //! but only `b.x` will be initialized in `b`, and only `e.x` and `e.z` //! will be initialized in `e`. //! //! Note that we need to cover the `z` field in each case in some way, //! since it may (or may not) need to be dropped, even though `z` is never //! directly mentioned in the body of the `foo` function. We call a path //! like `b.z` a *fragment sibling* of `b.x`, since the field `z` comes //! from the same structure `S` that declared the field `x` in `b.x`. //! //! In general we need to maintain boolean flags that match the //! `S`-structure of both `b` and `c`. In addition, we need to consult //! such a flag when doing an assignment (such as `c.y = D::new(4);` //! above), in order to know whether or not there is a previous value that //! needs to be dropped before we do the assignment. //! //! So for any given function, we need to determine what flags are needed //! to track its drop obligations. Our strategy for determining the set of //! flags is to represent the fragmentation of the structure explicitly: //! by starting initially from the paths that are explicitly mentioned in //! moves and assignments (such as `b.x` and `c.y` above), and then //! traversing the structure of the path's type to identify leftover //! *unmoved fragments*: assigning into `c.y` means that `c.x` and `c.z` //! are leftover unmoved fragments. Each fragment represents a drop //! obligation that may need to be tracked. Paths that are only moved or //! assigned in their entirety (like `a` and `d`) are treated as a single //! drop obligation. //! //! The fragment construction process works by piggy-backing on the //! existing `move_data` module. We already have callbacks that visit each //! direct move and assignment; these form the basis for the sets of //! moved_leaf_paths and assigned_leaf_paths. From these leaves, we can //! walk up their parent chain to identify all of their parent paths. //! We need to identify the parents because of cases like the following: //! //! ```rust //! struct Pair<X,Y>{ x: X, y: Y } //! fn foo(dd_d_d: Pair<Pair<Pair<D, D>, D>, D>) { //! other_function(dd_d_d.x.y); //! } //! ``` //! //! In this code, the move of the path `dd_d.x.y` leaves behind not only //! the fragment drop-obligation `dd_d.x.x` but also `dd_d.y` as well. //! //! Once we have identified the directly-referenced leaves and their //! parents, we compute the left-over fragments, in the function //! `fragments::add_fragment_siblings`. As of this writing this works by //! looking at each directly-moved or assigned path P, and blindly //! gathering all sibling fields of P (as well as siblings for the parents //! of P, etc). After accumulating all such siblings, we filter out the //! entries added as siblings of P that turned out to be //! directly-referenced paths (or parents of directly referenced paths) //! themselves, thus leaving the never-referenced "left-overs" as the only //! thing left from the gathering step. //! //! ## Array structural fragments //! //! A special case of the structural fragments discussed above are //! the elements of an array that has been passed by value, such as //! the following: //! //! ```rust //! fn foo(a: [D, ..10], i: uint) -> D { //! a[i] //! } //! ``` //! //! The above code moves a single element out of the input array `a`. //! The remainder of the array still needs to be dropped; i.e., it //! is a structural fragment. Note that after performing such a move, //! it is not legal to read from the array `a`. There are a number of //! ways to deal with this, but the important thing to note is that //! the semantics needs to distinguish in some manner between a //! fragment that is the *entire* array versus a fragment that represents //! all-but-one element of the array. A place where that distinction //! would arise is the following: //! //! ```rust //! fn foo(a: [D, ..10], b: [D, ..10], i: uint, t: bool) -> D { //! if t { //! a[i] //! } else { //! b[i] //! } //! //! // When control exits, we will need either to drop all of `a` //! // and all-but-one of `b`, or to drop all of `b` and all-but-one //! // of `a`. //! } //! ``` //! //! There are a number of ways that the trans backend could choose to //! compile this (e.g. a `[bool, ..10]` array for each such moved array; //! or an `Option<uint>` for each moved array). From the viewpoint of the //! borrow-checker, the important thing is to record what kind of fragment //! is implied by the relevant moves. //! //! # Future work //! //! While writing up these docs, I encountered some rules I believe to be //! stricter than necessary: //! //! - I think restricting the `&mut` LV against moves and `ALIAS` is sufficient, //! `MUTATE` and `CLAIM` are overkill. `MUTATE` was necessary when swap was //! a built-in operator, but as it is not, it is implied by `CLAIM`, //! and `CLAIM` is implied by `ALIAS`. The only net effect of this is an //! extra error message in some cases, though. //! - I have not described how closures interact. Current code is unsound. //! I am working on describing and implementing the fix. //! - If we wish, we can easily extend the move checking to allow finer-grained //! tracking of what is initialized and what is not, enabling code like //! this: //! //! a = x.f.g; // x.f.g is now uninitialized //! // here, x and x.f are not usable, but x.f.h *is* //! x.f.g = b; // x.f.g is not initialized //! // now x, x.f, x.f.g, x.f.h are all usable //! //! What needs to change here, most likely, is that the `moves` module //! should record not only what paths are moved, but what expressions //! are actual *uses*. For example, the reference to `x` in `x.f.g = b` //! is not a true *use* in the sense that it requires `x` to be fully //! initialized. This is in fact why the above code produces an error //! today: the reference to `x` in `x.f.g = b` is considered illegal //! because `x` is not fully initialized. //! //! There are also some possible refactorings: //! //! - It might be nice to replace all loan paths with the MovePath mechanism, //! since they allow lightweight comparison using an integer.