add UFCS/MCS cast, and addrCast

changelog.md

@@ -4,7 +4,6 @@
   [nim-lang/RFCs#245](https://github.com/nim-lang/RFCs/issues/245)
 - Added module `astdsl` that contains macro `buildAst`. That is a DSL for convenient
   construction of Nim ASTs.
-- Added module `pointers` containing `toUncheckedArray`.
 - Added `filepermissions.chmod` and `filepermissions.fromFilePermissions`,
   convenience functions to change file permissions using Unix like octal file permissions.
 - Added module `scripting` providing `withDir` to switch the directory temporarily. This
@@ -19,3 +18,4 @@
 - Added `jssets` module, Set for the JavaScript target
   https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set
 - Added `jsheaders` module for [`Headers`](https://developer.mozilla.org/en-US/docs/Web/API/Headers) for the JavaScript target.
+- Added module `pointers` containing `toUncheckedArray`, UFCS/MCS `cast`, `addrCast`

src/fusion/pointers.nim

@@ -15,3 +15,32 @@ proc toUncheckedArray*[T](a: ptr T): ptr UncheckedArray[T] {.inline.} =
     pa[0] += 5
     doAssert a[1] == 105
   cast[ptr UncheckedArray[T]](a)
+
+template `cast`*[T](a:T, T2: typedesc): untyped =
+  ## Same as `cast[T2](a)` but can be used in UFCS/MCS chains.
+  ## Unsafe as it calls `cast`.
+  runnableExamples:
+    var a = [1'u8, 2, 3, 4]
+    # the MCS chain reads left to right:
+    let x1 = a.cast(array[2, uint16])[0].cast(char)
+    # instead of a zig-zag:
+    let x2 = cast[char](cast[array[2, uint16]](a)[0])
+    doAssert x1 == x2
+  cast[T2](a)
+
+template addrCast*[T](a:T, T2: typedesc): untyped =
+  ## Same as `cast[ptr T2](unsafeAddr(a))[]` but can be used in UFCS/MCS chains.
+  ## Unsafe as it calls `cast`.
+  runnableExamples:
+    proc fn(a: var auto): auto =
+      a[0].inc
+      a
+    var a = [1'u8, 2, 3, 4]
+    let b = fn(a.addrCast(array[2, uint16]))
+    doAssert a == [2'u8, 2, 3, 4]
+    when system.cpuEndian == littleEndian:
+      doAssert b == [514'u16, 1027]
+      doAssert a.addrCast(uint32) == 67305986'u32
+      a.addrCast(uint32).inc
+      doAssert a == [3'u8, 2, 3, 4]
+  cast[ptr T2](unsafeAddr(a))[]