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

tests/assert/tassert_c.nim

@@ -6,8 +6,8 @@ discard """
 const expected = """
 tassert_c.nim(35)        tassert_c
 tassert_c.nim(34)        foo
-assertions.nim(29)       failedAssertImpl
-assertions.nim(22)       raiseAssert
+assertions.nim(30)       failedAssertImpl
+assertions.nim(23)       raiseAssert
 fatal.nim(49)            sysFatal"""
 
 proc tmatch(x, p: string): bool =
@@ -36,4 +36,5 @@ try:
 except AssertionDefect:
   let e = getCurrentException()
   let trace = e.getStackTrace
-  if tmatch(trace, expected): echo true else: echo "wrong trace:\n" & trace
+  if tmatch(trace, expected): echo true
+  else: echo "wrong trace:\n" & trace

tests/effects/toutparam.nim

@@ -0,0 +1,28 @@
+discard """
+  cmd: '''nim c --warningAsError[Uninit]:on --skipCfg --skipParentCfg $file'''
+  errormsg: "use explicit initialization of 'x' for clarity [Uninit]"
+  line: 24
+  disabled: "true"
+"""
+
+proc gah[T](x: out T) =
+  x = 3
+
+proc main =
+  var a: array[2, int]
+  var x: int
+  gah(x)
+  a[0] = 3
+  a[x] = 3
+  echo x
+
+main()
+
+proc mainB =
+  var a: array[2, int]
+  var x: int
+  a[0] = 3
+  a[x] = 3
+  echo x
+
+mainB()

tests/init/tuninit1.nim

@@ -1,5 +1,5 @@
 discard """
-  nimout: "Warning: 'y' might not have been initialized [Uninit]"
+  nimout: "Warning: use explicit initialization of 'y' for clarity [Uninit]"
   line:34
   action: compile
 """