nim-drchaos/drchaos/mutator.nim

558 lines
22 KiB
Nim

import std/[random, macros, setutils, enumutils, typetraits, options]
import common, private/[sampler, utf8fix]
when not defined(fuzzerStandalone):
proc LLVMFuzzerInitialize(): cint {.exportc.} =
{.emit: "N_CDECL(void, NimMain)(void); NimMain();".}
proc mutate(data: ptr UncheckedArray[byte], len, maxLen: int): int {.
importc: "LLVMFuzzerMutate".}
template `+!`(p: pointer, s: int): untyped =
cast[pointer](cast[ByteAddress](p) +% s)
const
RandomToDefaultRatio = 100 # The chance of returning an uninitalized type.
DefaultMutateWeight = 1_000_000 # The default weight of items sampled by the reservoir sampler.
MaxInitializeDepth = 200 # The post-processor prunes nested non-copyMem types.
type
ByteSized* = int8|uint8|byte|bool|char # Run LibFuzzer's mutate for sequences of these types.
PostProcessTypes* = (object|tuple|ref|seq|string|array|set|distinct) ## The post-processor runs only on these types.
proc runMutator*[T: SomeNumber](x: var T; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand)
proc runMutator*[T](x: var seq[T]; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand)
proc runMutator*(x: var bool; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand)
proc runMutator*(x: var char; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand)
proc runMutator*[T: enum](x: var T; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand)
proc runMutator*[T](x: var set[T]; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand)
proc runMutator*(x: var string; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand)
proc runMutator*[T: tuple|object](x: var T; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand)
proc runMutator*[T](x: var ref T; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand)
proc runMutator*[S, T](x: var array[S, T]; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand)
proc flipBit*(bytes: ptr UncheckedArray[byte]; len: int; r: var Rand) =
## Flips random bit in the buffer.
let bit = rand(r, len * 8 - 1)
bytes[bit div 8] = bytes[bit div 8] xor (1'u8 shl (bit mod 8))
proc flipBit*[T](value: T; r: var Rand): T =
## Flips random bit in the value.
result = value
flipBit(cast[ptr UncheckedArray[byte]](addr result), sizeof(T), r)
when defined(fuzzerStandalone):
proc mutateValue*[T](value: T; r: var Rand): T =
flipBit(value, r)
else:
proc mutateValue*[T](value: T; r: var Rand): T =
result = value
let size = mutate(cast[ptr UncheckedArray[byte]](addr result), sizeof(T), sizeof(T))
zeroMem(result.addr +! size, sizeof(T) - size)
proc mutateEnum*(index, itemCount: int; r: var Rand): int =
if itemCount <= 1: 0
else: (index + 1 + r.rand(itemCount - 1)) mod itemCount
proc newInput*[T](sizeIncreaseHint: int; r: var Rand): T =
## Creates new input with a chance of returning default(T).
runMutator(result, sizeIncreaseHint, false, r)
proc mutateSeq*[T](value: var seq[T]; previous: seq[T]; userMax, sizeIncreaseHint: int;
r: var Rand): bool =
let previousSize = previous.byteSize
while value.len > 0 and r.rand(bool):
value.delete(rand(r, value.high))
var currentSize = value.byteSize
template remainingSize: untyped = sizeIncreaseHint-currentSize+previousSize
while value.len < userMax and remainingSize > 0 and r.rand(bool):
let index = rand(r, value.len)
value.insert(newInput[T](remainingSize, r), index)
currentSize = value.byteSize
if value != previous:
result = true
elif value.len == 0:
value.add(newInput[T](remainingSize, r))
result = true
else:
let index = rand(r, value.high)
runMutator(value[index], remainingSize, true, r)
result = value != previous # runMutator item may still fail to generate a new mutation.
proc mutateByteSizedSeq*[T: ByteSized and not range](value: sink seq[T]; userMax, sizeIncreaseHint: int;
r: var Rand): seq[T] =
if r.rand(0..20) == 0:
result = @[]
else:
let oldSize = value.len
result = value
result.setLen(max(1, oldSize + r.rand(sizeIncreaseHint)))
result.setLen(mutate(cast[ptr UncheckedArray[byte]](addr result[0]), oldSize, result.len))
when T is bool:
# Fix bool values so UBSan stops complaining.
for i in 0..<result.len: result[i] = cast[seq[byte]](result)[i] != 0.byte
elif T is range:
for i in 0..<result.len: result[i] = clamp(result[i], low(T), high(T))
proc mutateString*(value: sink string; userMax, sizeIncreaseHint: int; r: var Rand): string =
if r.rand(0..20) == 0:
result = ""
else:
let oldSize = value.len
result = value
result.setLen(max(1, oldSize + r.rand(sizeIncreaseHint)))
result.setLen(mutate(cast[ptr UncheckedArray[byte]](addr result[0]), oldSize, result.len))
proc mutateUtf8String*(value: sink string; userMax, sizeIncreaseHint: int; r: var Rand): string {.inline.} =
result = mutateString(value, userMax, sizeIncreaseHint, r)
fixUtf8(result, r)
proc mutateArray*[S, T](value: array[S, T]; r: var Rand): array[S, T] {.inline.} =
result = mutateValue(value, r)
when T is bool:
for i in low(result)..high(result): result[i] = cast[array[S, byte]](result)[i] != 0.byte
elif T is range:
for i in low(result)..high(result): result[i] = clamp(result[i], low(T), high(T))
template repeatMutate*(call: untyped) =
if not enforceChanges and rand(r, RandomToDefaultRatio - 1) == 0:
discard
else:
var tmp = value
for i in 1..10:
value = call
if not enforceChanges or value != tmp: return
template repeatMutateInplace*(call: untyped) =
if not enforceChanges and rand(r, RandomToDefaultRatio - 1) == 0:
discard
else:
var tmp {.inject.} = value
for i in 1..10:
let notEqual = call
if not enforceChanges or notEqual: return
proc mutate*(value: var bool; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
value = not value
proc mutate*(value: var char; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
repeatMutate(mutateValue(value, r))
proc mutate*[T: range](value: var T; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
repeatMutate(clamp(mutateValue(value, r), low(T), high(T)))
proc mutate*[T](value: var set[T]; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
repeatMutate(mutateValue(value, r) * fullSet(T))
macro enumFullRange(a: typed): untyped =
nnkBracket.newTree(a.getType[1][1..^1])
proc mutate*[T: HoleyEnum](value: var T; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
repeatMutate(enumFullRange(T)[mutateEnum(value.symbolRank, enumLen(T), r)])
proc mutate*[T: OrdinalEnum](value: var T; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
repeatMutate(T(mutateEnum(value.symbolRank, enumLen(T), r)+low(T).ord))
proc mutate*[T: SomeNumber](value: var T; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
repeatMutate(mutateValue(value, r))
proc mutate*[T: not ByteSized](value: var seq[T]; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
repeatMutateInplace(mutateSeq(value, tmp, high(int), sizeIncreaseHint, r))
proc mutate*[T: ByteSized](value: var seq[T]; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
repeatMutate(mutateByteSizedSeq(move value, high(int), sizeIncreaseHint, r))
proc mutate*(value: var string; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
when defined(fuzzerUtf8Strings):
repeatMutate(mutateUtf8String(move value, high(int), sizeIncreaseHint, r))
else:
repeatMutate(mutateString(move value, high(int), sizeIncreaseHint, r))
proc mutate*[S; T: SomeNumber|bool|char](value: var array[S, T]; sizeIncreaseHint: int;
enforceChanges: bool; r: var Rand) =
repeatMutate(mutateArray(value, r))
proc mutate*[T](value: var Option[T]; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
if not enforceChanges and rand(r, RandomToDefaultRatio - 1) == 0:
discard
else:
if not isSome(value):
value = some(default(T))
runMutator(value.get, sizeIncreaseHint, enforceChanges, r)
template sampleAttempt(call: untyped) =
inc res
call
proc sample[T: distinct](x: T; s: var Sampler; r: var Rand; res: var int) =
when compiles(mutate(x, 0, false, r)):
sampleAttempt(attempt(s, r, DefaultMutateWeight, res))
else:
sample(x.distinctBase, s, r, res)
proc sample(x: bool; s: var Sampler; r: var Rand; res: var int) =
sampleAttempt(attempt(s, r, DefaultMutateWeight, res))
proc sample(x: char; s: var Sampler; r: var Rand; res: var int) =
sampleAttempt(attempt(s, r, DefaultMutateWeight, res))
proc sample[T: enum](x: T; s: var Sampler; r: var Rand; res: var int) =
sampleAttempt(attempt(s, r, DefaultMutateWeight, res))
proc sample[T](x: set[T]; s: var Sampler; r: var Rand; res: var int) =
sampleAttempt(attempt(s, r, DefaultMutateWeight, res))
proc sample[T: SomeNumber](x: T; s: var Sampler; r: var Rand; res: var int) =
sampleAttempt(attempt(s, r, DefaultMutateWeight, res))
proc sample[T](x: seq[T]; s: var Sampler; r: var Rand; res: var int) =
sampleAttempt(attempt(s, r, DefaultMutateWeight, res))
proc sample(x: string; s: var Sampler; r: var Rand; res: var int) =
sampleAttempt(attempt(s, r, DefaultMutateWeight, res))
proc sample[T: tuple|object](x: T; s: var Sampler; r: var Rand; res: var int) =
when compiles(mutate(x, 0, false, r)):
sampleAttempt(attempt(s, r, DefaultMutateWeight, res))
else:
for v in fields(x):
sample(v, s, r, res)
proc sample[T](x: ref T; s: var Sampler; r: var Rand; res: var int) =
when compiles(mutate(x, 0, false, r)):
sampleAttempt(attempt(s, r, DefaultMutateWeight, res))
else:
if x != nil: sample(x[], s, r, res)
proc sample[S, T](x: array[S, T]; s: var Sampler; r: var Rand; res: var int) =
when compiles(mutate(x, 0, false, r)):
sampleAttempt(attempt(s, r, DefaultMutateWeight, res))
else:
for i in low(x)..high(x):
sample(x[i], s, r, res)
template pickMutate(call: untyped) =
if res > 0:
dec res
if res == 0:
call
proc pick[T: distinct](x: var T; sizeIncreaseHint: int; enforceChanges: bool;
r: var Rand; res: var int) =
when compiles(mutate(x, sizeIncreaseHint, enforceChanges, r)):
pickMutate(mutate(x, sizeIncreaseHint, enforceChanges, r))
else:
pick(x.distinctBase, sizeIncreaseHint, enforceChanges, r, res)
proc pick(x: var bool; sizeIncreaseHint: int; enforceChanges: bool;
r: var Rand; res: var int) =
pickMutate(mutate(x, sizeIncreaseHint, enforceChanges, r))
proc pick(x: var char; sizeIncreaseHint: int; enforceChanges: bool;
r: var Rand; res: var int) =
pickMutate(mutate(x, sizeIncreaseHint, enforceChanges, r))
proc pick[T: enum](x: var T; sizeIncreaseHint: int; enforceChanges: bool;
r: var Rand; res: var int) =
pickMutate(mutate(x, sizeIncreaseHint, enforceChanges, r))
proc pick[T](x: var set[T]; sizeIncreaseHint: int; enforceChanges: bool;
r: var Rand; res: var int) =
pickMutate(mutate(x, sizeIncreaseHint, enforceChanges, r))
proc pick[T: SomeNumber](x: var T; sizeIncreaseHint: int; enforceChanges: bool;
r: var Rand; res: var int) =
pickMutate(mutate(x, sizeIncreaseHint, enforceChanges, r))
proc pick[T](x: var seq[T]; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand;
res: var int) =
pickMutate(mutate(x, sizeIncreaseHint, enforceChanges, r))
proc pick(x: var string; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand;
res: var int) =
pickMutate(mutate(x, sizeIncreaseHint, enforceChanges, r))
proc pick[T: tuple](x: var T; sizeIncreaseHint: int; enforceChanges: bool;
r: var Rand; res: var int) =
when compiles(mutate(x, sizeIncreaseHint, enforceChanges, r)):
pickMutate(mutate(x, sizeIncreaseHint, enforceChanges, r))
else:
for v in fields(x):
pick(v, sizeIncreaseHint, enforceChanges, r, res)
proc pick[T: object](x: var T; sizeIncreaseHint: int; enforceChanges: bool;
r: var Rand; res: var int) =
when compiles(mutate(x, sizeIncreaseHint, enforceChanges, r)):
pickMutate(mutate(x, sizeIncreaseHint, enforceChanges, r))
else:
template pickFunc(x: untyped) =
pick(x, sizeIncreaseHint, enforceChanges, r, res)
assignObjectImpl(x, pickFunc)
proc pick[T](x: var ref T; sizeIncreaseHint: int; enforceChanges: bool;
r: var Rand; res: var int) =
when compiles(mutate(x, sizeIncreaseHint, enforceChanges, r)):
pickMutate(mutate(x, sizeIncreaseHint, enforceChanges, r))
else:
if x != nil: pick(x[], sizeIncreaseHint, enforceChanges, r, res)
proc pick[S, T](x: var array[S, T]; sizeIncreaseHint: int; enforceChanges: bool;
r: var Rand; res: var int) =
when compiles(mutate(x, sizeIncreaseHint, enforceChanges, r)):
pickMutate(mutate(x, sizeIncreaseHint, enforceChanges, r))
else:
for i in low(x)..high(x):
pick(x[i], sizeIncreaseHint, enforceChanges, r, res)
proc runMutator*[T: distinct](x: var T; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
when compiles(mutate(x, sizeIncreaseHint, enforceChanges, r)):
mutate(x, sizeIncreaseHint, enforceChanges, r)
else:
runMutator(x.distinctBase, sizeIncreaseHint, enforceChanges, r)
proc runMutator*[T: SomeNumber](x: var T; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
mutate(x, sizeIncreaseHint, enforceChanges, r)
proc runMutator*[T](x: var seq[T]; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
mutate(x, sizeIncreaseHint, enforceChanges, r)
proc runMutator*(x: var string; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
mutate(x, sizeIncreaseHint, enforceChanges, r)
proc runMutator*(x: var bool; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
mutate(x, sizeIncreaseHint, enforceChanges, r)
proc runMutator*(x: var char; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
mutate(x, sizeIncreaseHint, enforceChanges, r)
proc runMutator*[T: enum](x: var T; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
mutate(x, sizeIncreaseHint, enforceChanges, r)
proc runMutator*[T](x: var set[T]; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
mutate(x, sizeIncreaseHint, enforceChanges, r)
proc runMutator*[T: tuple|object](x: var T; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
when compiles(mutate(x, sizeIncreaseHint, enforceChanges, r)):
mutate(x, sizeIncreaseHint, enforceChanges, r)
else:
if not enforceChanges and rand(r, RandomToDefaultRatio - 1) == 0:
discard
else:
var res = 0
var s: Sampler[int]
sample(x, s, r, res)
res = s.selected
pick(x, sizeIncreaseHint, enforceChanges, r, res)
proc runMutator*[T](x: var ref T; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
when compiles(mutate(x, sizeIncreaseHint, enforceChanges, r)):
mutate(x, sizeIncreaseHint, enforceChanges, r)
else:
if not enforceChanges and rand(r, RandomToDefaultRatio - 1) == 0:
discard
else:
if x == nil: new(x)
runMutator(x[], sizeIncreaseHint, enforceChanges, r)
proc runMutator*[S, T](x: var array[S, T]; sizeIncreaseHint: int; enforceChanges: bool; r: var Rand) =
when compiles(mutate(x, sizeIncreaseHint, enforceChanges, r)):
mutate(x, sizeIncreaseHint, enforceChanges, r)
else:
if not enforceChanges and rand(r, RandomToDefaultRatio - 1) == 0:
discard
else:
var res = 0
var s: Sampler[int]
sample(x, s, r, res)
res = s.selected
pick(x, sizeIncreaseHint, enforceChanges, r, res)
proc runPostProcessor*(x: var string, depth: int; r: var Rand)
proc runPostProcessor*[T](x: var seq[T], depth: int; r: var Rand)
proc runPostProcessor*[T](x: var set[T], depth: int; r: var Rand)
proc runPostProcessor*[T: tuple](x: var T, depth: int; r: var Rand)
proc runPostProcessor*[T: object](x: var T, depth: int; r: var Rand)
proc runPostProcessor*[T](x: var ref T, depth: int; r: var Rand)
proc runPostProcessor*[S, T](x: var array[S, T], depth: int; r: var Rand)
proc runPostProcessor*[T: distinct](x: var T, depth: int; r: var Rand) =
# Allow post-processor functions for all distinct types.
when compiles(postProcess(x, r)):
if depth < 0:
when not supportsCopyMem(T): reset(x)
else:
postProcess(x, r)
else:
when x.distinctBase is PostProcessTypes:
runPostProcessor(x.distinctBase, depth-1, r)
proc runPostProcessor*(x: var string, depth: int; r: var Rand) =
if depth < 0:
reset(x)
else:
when compiles(postProcess(x, r)):
postProcess(x, r)
proc runPostProcessor*[T](x: var seq[T], depth: int; r: var Rand) =
if depth < 0:
reset(x)
else:
when compiles(postProcess(x, r)):
postProcess(x, r)
else:
when T is PostProcessTypes:
for i in 0..<x.len:
runPostProcessor(x[i], depth-1, r)
proc runPostProcessor*[T](x: var set[T], depth: int; r: var Rand) =
when compiles(postProcess(x, r)):
if depth >= 0:
postProcess(x, r)
proc runPostProcessor*[T: tuple](x: var T, depth: int; r: var Rand) =
if depth < 0:
when not supportsCopyMem(T): reset(x)
else:
when compiles(postProcess(x, r)):
postProcess(x, r)
else:
for v in fields(x):
when typeof(v) is PostProcessTypes:
runPostProcessor(v, depth-1, r)
proc runPostProcessor*[T: object](x: var T, depth: int; r: var Rand) =
if depth < 0:
when not supportsCopyMem(T): reset(x)
else:
when compiles(postProcess(x, r)):
postProcess(x, r)
# When there is a user-provided mutator, don't touch private fields.
elif compiles(mutate(x, 0, false, r)):
# Guess how to traverse a data structure, if it's even one.
when compiles(for v in mitems(x): discard):
# Run the post-processor only for compatible types as there is an overhead.
when typeof(for v in mitems(x): v) is PostProcessTypes:
for v in mitems(x):
runPostProcessor(v, depth-1, r)
elif compiles(for k, v in mpairs(x): discard):
when typeof(for k, v in mpairs(x): v) is PostProcessTypes:
for k, v in mpairs(x):
runPostProcessor(v, depth-1, r)
else:
template runPostFunc(x: untyped) =
when typeof(x) is PostProcessTypes:
runPostProcessor(x, depth-1, r)
assignObjectImpl(x, runPostFunc)
proc runPostProcessor*[T](x: var ref T, depth: int; r: var Rand) =
if depth < 0:
reset(x)
else:
when compiles(postProcess(x, r)):
postProcess(x, r)
else:
when T is PostProcessTypes:
if x != nil: runPostProcessor(x[], depth-1, r)
proc runPostProcessor*[S, T](x: var array[S, T], depth: int; r: var Rand) =
if depth < 0:
when not supportsCopyMem(T): reset(x)
else:
when compiles(postProcess(x, r)):
postProcess(x, r)
else:
when T is PostProcessTypes:
for i in low(x)..high(x):
runPostProcessor(x[i], depth-1, r)
proc myMutator[T](x: var T; sizeIncreaseHint: Natural; r: var Rand) {.nimcall.} =
runMutator(x, sizeIncreaseHint, true, r)
when T is PostProcessTypes:
runPostProcessor(x, MaxInitializeDepth, r)
template mutatorImpl(target, mutator, typ: untyped) =
{.pragma: nocov, codegenDecl: "__attribute__((no_sanitize(\"coverage\"))) $# $#$#".}
{.pragma: nosan, codegenDecl: "__attribute__((disable_sanitizer_instrumentation)) $# $#$#".}
type
FuzzTarget = proc (x: typ) {.nimcall, noSideEffect.}
FuzzMutator = proc (x: var typ; sizeIncreaseHint: Natural, r: var Rand) {.nimcall.}
var
buffer: seq[byte] = @[0xf1'u8]
cached: typ
proc getInput(x: var typ; data: openArray[byte]): var typ {.nocov, nosan.} =
if equals(data, buffer):
result = cached
else:
var pos = 1
fromData(data, pos, x)
result = x
proc setInput(x: var typ; data: openArray[byte]; len: int) {.inline.} =
setLen(buffer, len)
var pos = 1
toData(buffer, pos, x)
assert pos == len
copyMem(addr data, addr buffer[0], len)
cached = move x
proc clearBuffer() {.inline.} =
setLen(buffer, 1)
proc testOneInputImpl[T](x: var T; data: openArray[byte]) =
if data.len > 1: # Ignore '\n' passed by LibFuzzer.
try:
FuzzTarget(target)(getInput(x, data))
finally:
# Call Nim's compiler api to report unhandled exceptions.
{.emit: "nimTestErrorFlag();".}
proc customMutatorImpl(x: var typ; data: openArray[byte]; maxLen: int;
r: var Rand): int {.nosan.} =
if data.len > 1:
#var pos = 1
#fromData(data, pos, x)
x = getInput(x, data)
FuzzMutator(mutator)(x, maxLen-x.byteSize, r)
result = x.byteSize+1 # +1 for the skipped byte
if result <= maxLen:
setInput(x, data, result)
else:
clearBuffer()
result = data.len
proc LLVMFuzzerTestOneInput(data: ptr UncheckedArray[byte], len: int): cint {.exportc.} =
result = 0
var x: typ
testOneInputImpl(x, toOpenArray(data, 0, len-1))
proc LLVMFuzzerCustomMutator(data: ptr UncheckedArray[byte], len, maxLen: int,
seed: int64): int {.
exportc.} =
var r = initRand(seed)
var x: typ
customMutatorImpl(x, toOpenArray(data, 0, len-1), maxLen, r)
proc commonImpl(target, mutator: NimNode): NimNode =
let typ = getTypeImpl(target).params[^1][1]
result = getAst(mutatorImpl(target, mutator, typ))
macro defaultMutator*(target: proc) =
## Implements the interface for running LibFuzzer's fuzzing loop, where func `target`'s
## single immutatable parameter, is the structured input type.
## It uses the default mutator that also includes the post-processor.
## It's recommended that the experimental "strict funcs" feature is enabled.
commonImpl(target, bindSym"myMutator")
macro customMutator*(target, mutator: proc) =
## Implements the interface for running LibFuzzer's fuzzing loop, where func `target`'s
## single immutatable parameter, is the structured input type.
## It uses `mutator: proc (x: var T; sizeIncreaseHint: Natural, r: var Rand)`
## to generate new mutations. This has the flexibility of transforming the input and/or
## mutating some part of it via the `runMutator` proc. Then applying the reverse transform to
## convert it back to the original representation.
## It's recommended that the experimental "strict funcs" feature is enabled.
commonImpl(target, mutator)