conversions to unsigned numbers are not checked anymore (#12688) [backport]

* conversions to unsigned numbers are not checked anymore; implements / fixes https://github.com/nim-lang/RFCs/issues/175

* change the spec yet again to be less consistent but to make more sense; updated the changelog

(cherry picked from commit c98e0e22ad)

changelog.md

@@ -27,6 +27,9 @@
 
 ## Language changes
 
+- Conversions to unsigned integers are unchecked at runtime, imitating earlier Nim
+  versions. The documentation was improved to acknowledge this special case.
+  See https://github.com/nim-lang/RFCs/issues/175 for more details.
 
 
 ### Tool changes

compiler/ccgexprs.nim

@@ -1931,7 +1931,8 @@ proc genCast(p: BProc, e: PNode, d: var TLoc) =
 proc genRangeChck(p: BProc, n: PNode, d: var TLoc, magic: string) =
   var a: TLoc
   var dest = skipTypes(n.typ, abstractVar)
-  if optRangeCheck notin p.options:
+  if optRangeCheck notin p.options or (dest.kind in {tyUInt..tyUInt64} and
+      checkUnsignedConversions notin p.config.legacyFeatures):
     initLocExpr(p, n.sons[0], a)
     putIntoDest(p, d, n, "(($1) ($2))" %
         [getTypeDesc(p.module, dest), rdCharLoc(a)], a.storage)

compiler/jsgen.nim

@@ -2155,7 +2155,10 @@ proc upConv(p: PProc, n: PNode, r: var TCompRes) =
 proc genRangeChck(p: PProc, n: PNode, r: var TCompRes, magic: string) =
   var a, b: TCompRes
   gen(p, n.sons[0], r)
-  if optRangeCheck in p.options:
+  if optRangeCheck notin p.options or (skipTypes(n.typ, abstractVar).kind in {tyUInt..tyUInt64} and
+      checkUnsignedConversions notin p.config.legacyFeatures):
+    discard "XXX maybe emit masking instructions here"
+  else:
     gen(p, n.sons[1], a)
     gen(p, n.sons[2], b)
     useMagic(p, "chckRange")

compiler/options.nim

@@ -144,6 +144,10 @@ type
       ## Allows to modify a NimNode where the type has already been
       ## flagged with nfSem. If you actually do this, it will cause
       ## bugs.
+    checkUnsignedConversions
+      ## Historically and especially in version 1.0.0 of the language
+      ## conversions to unsigned numbers were checked. In 1.0.4 they
+      ## are not anymore.
 
   SymbolFilesOption* = enum
     disabledSf, writeOnlySf, readOnlySf, v2Sf

compiler/semfold.nim

@@ -433,15 +433,12 @@ proc foldConv(n, a: PNode; g: ModuleGraph; check = false): PNode =
   let dstTyp = skipTypes(n.typ, abstractRange - {tyTypeDesc})
   let srcTyp = skipTypes(a.typ, abstractRange - {tyTypeDesc})
 
-
   # if srcTyp.kind == tyUInt64 and "FFFFFF" in $n:
   #   echo "n: ", n, " a: ", a
   #   echo "from: ", srcTyp, " to: ", dstTyp, " check: ", check
   #   echo getInt(a)
   #   echo high(int64)
   #   writeStackTrace()
-
-  # XXX range checks?
   case dstTyp.kind
   of tyInt..tyInt64, tyUInt..tyUInt64:
     case srcTyp.kind
@@ -449,9 +446,10 @@ proc foldConv(n, a: PNode; g: ModuleGraph; check = false): PNode =
       result = newIntNodeT(BiggestInt(getFloat(a)), n, g)
     of tyChar, tyUInt..tyUInt64, tyInt..tyInt64:
       var val = a.getOrdValue
+
       if check: rangeCheck(n, val, g)
       result = newIntNodeT(val, n, g)
-      if dstTyp.kind in {tyUInt .. tyUInt64}:
+      if dstTyp.kind in {tyUInt..tyUInt64}:
         result.kind = nkUIntLit
     else:
       result = a

doc/manual.rst

@@ -3185,6 +3185,7 @@ has lots of advantages:
 
 Type conversions
 ----------------
+
 Syntactically a `type conversion` is like a procedure call, but a
 type name replaces the procedure name. A type conversion is always
 safe in the sense that a failure to convert a type to another
@@ -3204,6 +3205,19 @@ A type conversion can also be used to disambiguate overloaded routines:
   let procVar = (proc(x: string))(p)
   procVar("a")
 
+Since operations on unsigned numbers wrap around and are unchecked so are
+type conversion to unsigned integers and between unsigned integers. The
+rationale for this is mostly better interoperability with the C Programming
+language when algorithms are ported from C to Nim.
+
+Exception: Values that are converted to an unsigned type at compile time
+are checked so that code like ``byte(-1)`` does not compile.
+
+**Note**: Historically the operations
+were unchecked and the conversions were sometimes checked but starting with
+the revision 1.0.4 of this document and the language implementation the
+conversions too are now *always unchecked*.
+
 
 Type casts
 ----------