init checks and 'out' parameters (#14521)

* I don't care about observable stores
* enforce explicit initializations
* cleaner code for the stdlib
* stdlib: use explicit initializations
* make tests green
* algorithm.nim: set result explicitly
* remove out parameters and bring the PR into a mergable state
* updated the changelog

8 files changed, 20 insertions, 14 deletions

diff --git a/lib/system/alloc.nim b/lib/system/alloc.nim
index 7ace0d536..95658c49a 100644
--- a/lib/system/alloc.nim
+++ b/lib/system/alloc.nim
@@ -586,7 +586,8 @@ proc freeBigChunk(a: var MemRegion, c: PBigChunk) =
 proc getBigChunk(a: var MemRegion, size: int): PBigChunk =
   sysAssert(size > 0, "getBigChunk 2")
   var size = size # roundup(size, PageSize)
-  var fl, sl: int
+  var fl = 0
+  var sl = 0
   mappingSearch(size, fl, sl)
   sysAssert((size and PageMask) == 0, "getBigChunk: unaligned chunk")
   result = findSuitableBlock(a, fl, sl)
diff --git a/lib/system/assertions.nim b/lib/system/assertions.nim
index ca3bd7bc1..c6283c89c 100644
--- a/lib/system/assertions.nim
+++ b/lib/system/assertions.nim
@@ -11,6 +11,7 @@ proc `$`(x: int): string {.magic: "IntToStr", noSideEffect.}
 proc `$`(info: InstantiationInfo): string =
   # The +1 is needed here
   # instead of overriding `$` (and changing its meaning), consider explicit name.
+  result = ""
   result.toLocation(info.filename, info.line, info.column+1)
 
 # ---------------------------------------------------------------------------
diff --git a/lib/system/gc_common.nim b/lib/system/gc_common.nim
index d75ada791..7f6c41e1b 100644
--- a/lib/system/gc_common.nim
+++ b/lib/system/gc_common.nim
@@ -217,7 +217,7 @@ proc stackSize(stack: ptr GcStack): int {.noinline.} =
   when nimCoroutines:
     var pos = stack.pos
   else:
-    var pos {.volatile.}: pointer
+    var pos {.volatile, noinit.}: pointer
     pos = addr(pos)
 
   if pos != nil:
@@ -229,6 +229,7 @@ proc stackSize(stack: ptr GcStack): int {.noinline.} =
     result = 0
 
 proc stackSize(): int {.noinline.} =
+  result = 0
   for stack in gch.stack.items():
     result = result + stack.stackSize()
 
@@ -303,7 +304,7 @@ when not defined(useNimRtl):
 {.pop.}
 
 proc isOnStack(p: pointer): bool =
-  var stackTop {.volatile.}: pointer
+  var stackTop {.volatile, noinit.}: pointer
   stackTop = addr(stackTop)
   var a = cast[ByteAddress](gch.getActiveStack().bottom)
   var b = cast[ByteAddress](stackTop)
diff --git a/lib/system/io.nim b/lib/system/io.nim
index d1a7f1fc7..482057214 100644
--- a/lib/system/io.nim
+++ b/lib/system/io.nim
@@ -424,12 +424,12 @@ proc write*(f: File, b: bool) {.tags: [WriteIOEffect], benign.} =
   else: write(f, "false")
 
 proc write*(f: File, r: float32) {.tags: [WriteIOEffect], benign.} =
-  var buffer: array[65, char]
+  var buffer {.noinit.}: array[65, char]
   discard writeFloatToBuffer(buffer, r)
   if c_fprintf(f, "%s", buffer[0].addr) < 0: checkErr(f)
 
 proc write*(f: File, r: BiggestFloat) {.tags: [WriteIOEffect], benign.} =
-  var buffer: array[65, char]
+  var buffer {.noinit.}: array[65, char]
   discard writeFloatToBuffer(buffer, r)
   if c_fprintf(f, "%s", buffer[0].addr) < 0: checkErr(f)
 
@@ -591,7 +591,7 @@ when defined(posix) and not defined(nimscript):
 
 proc open*(f: var File, filename: string,
           mode: FileMode = fmRead,
-          bufSize: int = -1): bool  {.tags: [], raises: [], benign.} =
+          bufSize: int = -1): bool {.tags: [], raises: [], benign.} =
   ## Opens a file named `filename` with given `mode`.
   ##
   ## Default mode is readonly. Returns true if the file could be opened.
@@ -605,7 +605,7 @@ proc open*(f: var File, filename: string,
       # How `fopen` handles opening a directory is not specified in ISO C and
       # POSIX. We do not want to handle directories as regular files that can
       # be opened.
-      var res: Stat
+      var res {.noinit.}: Stat
       if c_fstat(getFileHandle(f2), res) >= 0'i32 and modeIsDir(res.st_mode):
         close(f2)
         return false
@@ -759,7 +759,7 @@ proc readFile*(filename: string): TaintedString {.tags: [ReadIOEffect], benign.}
   ## Raises an IO exception in case of an error. If you need to call
   ## this inside a compile time macro you can use `staticRead
   ## <system.html#staticRead,string>`_.
-  var f: File
+  var f: File = nil
   if open(f, filename):
     try:
       result = readAll(f)
@@ -772,7 +772,7 @@ proc writeFile*(filename, content: string) {.tags: [WriteIOEffect], benign.} =
   ## Opens a file named `filename` for writing. Then writes the
   ## `content` completely to the file and closes the file afterwards.
   ## Raises an IO exception in case of an error.
-  var f: File
+  var f: File = nil
   if open(f, filename, fmWrite):
     try:
       f.write(content)
@@ -785,7 +785,7 @@ proc writeFile*(filename: string, content: openArray[byte]) {.since: (1, 1).} =
   ## Opens a file named `filename` for writing. Then writes the
   ## `content` completely to the file and closes the file afterwards.
   ## Raises an IO exception in case of an error.
-  var f: File
+  var f: File = nil
   if open(f, filename, fmWrite):
     try:
       f.writeBuffer(unsafeAddr content[0], content.len)
@@ -799,7 +799,7 @@ proc readLines*(filename: string, n: Natural): seq[TaintedString] =
   ## in case of an error. Raises EOF if file does not contain at least `n` lines.
   ## Available at compile time. A line of text may be delimited by ``LF`` or ``CRLF``.
   ## The newline character(s) are not part of the returned strings.
-  var f: File
+  var f: File = nil
   if open(f, filename):
     try:
       result = newSeq[TaintedString](n)
diff --git a/lib/system/repr.nim b/lib/system/repr.nim
index 526839aa2..318e95ebb 100644
--- a/lib/system/repr.nim
+++ b/lib/system/repr.nim
@@ -102,6 +102,7 @@ proc reprSetAux(result: var string, p: pointer, typ: PNimType) =
   of 4: u = cast[ptr uint32](p)[]
   of 8: u = cast[ptr uint64](p)[]
   else:
+    u = uint64(0)
     var a = cast[PByteArray](p)
     for i in 0 .. typ.size*8-1:
       if (uint(a[i shr 3]) and (1'u shl (i and 7))) != 0:
diff --git a/lib/system/sets.nim b/lib/system/sets.nim
index 0df50957d..42c448848 100644
--- a/lib/system/sets.nim
+++ b/lib/system/sets.nim
@@ -31,6 +31,7 @@ proc countBits64(n: uint64): int {.compilerproc, inline.} =
 
 proc cardSet(s: NimSet, len: int): int {.compilerproc, inline.} =
   var i = 0
+  result = 0
   when defined(x86) or defined(amd64):
     while i < len - 8:
       inc(result, countBits64((cast[ptr uint64](s[i].unsafeAddr))[]))
diff --git a/lib/system/strmantle.nim b/lib/system/strmantle.nim
index c3ac5fc1b..cb26833ac 100644
--- a/lib/system/strmantle.nim
+++ b/lib/system/strmantle.nim
@@ -94,7 +94,7 @@ proc addFloat*(result: var string; x: float) =
   when nimvm:
     result.add $x
   else:
-    var buffer: array[65, char]
+    var buffer {.noinit.}: array[65, char]
     let n = writeFloatToBuffer(buffer, x)
     result.addCstringN(cstring(buffer[0].addr), n)
 
@@ -131,8 +131,8 @@ proc nimParseBiggestFloat(s: string, number: var BiggestFloat,
     i = start
     sign = 1.0
     kdigits, fdigits = 0
-    exponent: int
-    integer: uint64
+    exponent = 0
+    integer = uint64(0)
     fracExponent = 0
     expSign = 1
     firstDigit = -1
diff --git a/lib/system/widestrs.nim b/lib/system/widestrs.nim
index 3048de82a..a7a514446 100644
--- a/lib/system/widestrs.nim
+++ b/lib/system/widestrs.nim
@@ -63,6 +63,7 @@ proc ord(arg: Utf16Char): int = int(cast[uint16](arg))
 proc len*(w: WideCString): int =
   ## returns the length of a widestring. This traverses the whole string to
   ## find the binary zero end marker!
+  result = 0
   while int16(w[result]) != 0'i16: inc result
 
 const