Strict not nil checking ========================= .. default-role:: code .. include:: rstcommon.rst **Note:** This feature is experimental, you need to enable it with .. code-block:: nim {.experimental: "strictNotNil".} or .. code-block:: cmd nim c --experimental:strictNotNil In the second case it would check builtin and imported modules as well. It checks the nilability of ref-like types and makes dereferencing safer based on flow typing and `not nil` annotations. Its implementation is different than the `notnil` one: defined under `strictNotNil`. Keep in mind the difference in option names, be careful with distinguishing them. We check several kinds of types for nilability: - ref types - pointer types - proc types - cstrings nil ------- The default kind of nilability types is the nilable kind: they can have the value `nil`. If you have a non-nilable type `T`, you can use `T nil` to get a nilable type for it. not nil -------- You can annotate a type where nil isn't a valid value with `not nil`. .. code-block:: nim type NilableObject = ref object a: int Object = NilableObject not nil Proc = (proc (x, y: int)) proc p(x: Object) = echo x.a # ensured to dereference without an error # compiler catches this: p(nil) # and also this: var x: NilableObject if x.isNil: p(x) else: p(x) # ok If a type can include `nil` as a valid value, dereferencing values of the type is checked by the compiler: if a value which might be nil is derefenced, this produces a warning by default, you can turn this into an error using the compiler options `--warningAsError:strictNotNil`:option:. If a type is nilable, you should dereference its values only after a `isNil` or equivalent check. local turn on/off --------------------- You can still turn off nil checking on function/module level by using a `{.strictNotNil: off.}` pragma. Note: test that/TODO for code/manual. nilability state ----------------- Currently a nilable value can be `Safe`, `MaybeNil` or `Nil` : we use internally `Parent` and `Unreachable` but this is an implementation detail(a parent layer has the actual nilability). - `Safe` means it shouldn't be nil at that point: e.g. after assignment to a non-nil value or `not a.isNil` check - `MaybeNil` means it might be nil, but it might not be nil: e.g. an argument, a call argument or a value after an `if` and `else`. - `Nil` means it should be nil at that point; e.g. after an assignment to `nil` or a `.isNil` check. - `Unreachable` means it shouldn't be possible to access this in this branch: so we do generate a warning as well. We show an error for each dereference (`[]`, `.field`, `[index]` `()` etc) which is of a tracked expression which is in `MaybeNil` or `Nil` state. type nilability ---------------- Types are either nilable or non-nilable. When you pass a param or a default value, we use the type : for nilable types we return `MaybeNil` and for non-nilable `Safe`. TODO: fix the manual here. (This is not great, as default values for non-nilables and nilables are usually actually `nil` , so we should think a bit more about this section.) params rules ------------ Param's nilability is detected based on type nilability. We use the type of the argument to detect the nilability. assignment rules ----------------- Let's say we have `left = right`. When we assign, we pass the right's nilability to the left's expression. There should be special handling of aliasing and compound expressions which we specify in their sections. (Assignment is a possible alias `move` or `move out`). call args rules ----------------- When we call with arguments, we have two cases when we might change the nilability. .. code-block:: nim callByVar(a) Here `callByVar` can re-assign `a`, so this might change `a`'s nilability, so we change it to `MaybeNil`. This is also a possible aliasing `move out` (moving out of a current alias set). .. code-block:: nim call(a) Here `call` can change a field or element of `a`, so if we have a dependant expression of `a` : e.g. `a.field`. Dependats become `MaybeNil`. branches rules --------------- Branches are the reason we do nil checking like this: with flow checking. Sources of brancing are `if`, `while`, `for`, `and`, `or`, `case`, `try` and combinations with `return`, `break`, `continue` and `raise` We create a new layer/"scope" for each branch where we map expressions to nilability. This happens when we "fork": usually on the beginning of a construct. When branches "join" we usually unify their expression maps or/and nilabilities. Merging usually merges maps and alias sets: nilabilities are merged like this: .. code-block:: nim template union(l: Nilability, r: Nilability): Nilability = ## unify two states if l == r: l else: MaybeNil Special handling is for `.isNil` and `== nil`, also for `not`, `and` and `or`. `not` reverses the nilability, `and` is similar to "forking" : the right expression is checked in the layer resulting from the left one and `or` is similar to "merging": the right and left expression should be both checked in the original layer. `isNil`, `== nil` make expressions `Nil`. If there is a `not` or `!= nil`, they make them `Safe`. We also reverse the nilability in the opposite branch: e.g. `else`. compound expressions: field, index expressions ----------------------------------------------- We want to track also field(dot) and index(bracket) expressions. We track some of those compound expressions which might be nilable as dependants of their bases: `a.field` is changed if `a` is moved (re-assigned), similarly `a[index]` is dependent on `a` and `a.field.field` on `a.field`. When we move the base, we update dependants to `MaybeNil`. Otherwise we usually start with type nilability. When we call args, we update the nilability of their dependants to `MaybeNil` as the calls usually can change them. We might need to check for `strictFuncs` pure funcs and not do that then. For field expressions `a.field`, we calculate an integer value based on a hash of the tree and just accept equivalent trees as equivalent expressions. For item expression `a[index]`, we also calculate an integer value based on a hash of the tree and accept equivalent trees as equivalent expressions: for static values only. For now we support only constant indices: we dont track expression with no-const indices. For those we just report a warning even if they are safe for now: one can use a local variable to workaround. For loops this might be annoying: so one should be able to turn off locally the warning using the `{.warning[StrictCheckNotNil]:off}.`. For bracket expressions, in the future we might count `a[]` as the same general expression. This means we should should the index but otherwise handle it the same for assign (maybe "aliasing" all the non-static elements) and differentiate only for static: e.g. `a[0]` and `a[1]`. element tracking ----------------- When we assign an object construction, we should track the fields as well: .. code-block:: nim var a = Nilable(field: Nilable()) # a : Safe, a.field: Safe Usually we just track the result of an expression: probably this should apply for elements in other cases as well. Also related to tracking initialization of expressions/fields. unstructured control flow rules ------------------------------- Unstructured control flow keywords as `return`, `break`, `continue`, `raise` mean that we jump from a branch out. This means that if there is code after the finishing of the branch, it would be ran if one hasn't hit the direct parent branch of those: so it is similar to an `else`. In those cases we should use the reverse nilabilities for the local to the condition expressions. E.g. .. code-block:: nim for a in c: if not a.isNil: b() break code # here a: Nil , because if not, we would have breaked aliasing ------------ We support alias detection for local expressions. We track sets of aliased expressions. We start with all nilable local expressions in separate sets. Assignments and other changes to nilability can move / move out expressions of sets. `move`: Moving `left` to `right` means we remove `left` from its current set and unify it with the `right`'s set. This means it stops being aliased with its previous aliases. .. code-block:: nim var left = b left = right # moving left to right `move out`: Moving out `left` might remove it from the current set and ensure that it's in its own set as a single element. e.g. .. code-block:: nim var left = b left = nil # moving out initialization of non nilable and nilable values ------------------------------------------------- TODO warnings and errors --------------------- We show an error for each dereference (`[]`, `.field`, `[index]` `()` etc) which is of a tracked expression which is in `MaybeNil` or `Nil` state. We might also show a history of the transitions and the reasons for them that might change the nilability of the expression.