nil in string concats does not produce crashes anymore

compiler/ccgexprs.nim

@@ -1034,7 +1034,7 @@ proc genStrConcat(p: BProc, e: PNode, d: var TLoc) =
       if e.sons[i + 1].kind in {nkStrLit..nkTripleStrLit}:
         inc(L, len(e.sons[i + 1].strVal))
       else:
-        addf(lens, "$1->$2 + ", [rdLoc(a), lenField(p)])
+        addf(lens, "($1 ? $1->$2 : 0) + ", [rdLoc(a), lenField(p)])
       add(appends, rfmt(p.module, "#appendString($1, $2);$n", tmp.r, rdLoc(a)))
   linefmt(p, cpsStmts, "$1 = #rawNewString($2$3);$n", tmp.r, lens, rope(L))
   add(p.s(cpsStmts), appends)

lib/system/sysstr.nim

@@ -161,14 +161,16 @@ proc hashString(s: string): int {.compilerproc.} =
 
 proc addChar(s: NimString, c: char): NimString =
   # is compilerproc!
-  result = s
-  if result.len >= result.space:
-    let r = resize(result.space)
-    result = cast[NimString](growObj(result,
-      sizeof(TGenericSeq) + r + 1))
-    result.reserved = r
-  elif wasMoved(s):
-    result = newOwnedString(s, s.len)
+  if s == nil:
+    result = rawNewStringNoInit(1)
+    result.len = 0
+  else:
+    result = s
+    if result.len >= result.space:
+      let r = resize(result.space)
+      result = cast[NimString](growObj(result,
+        sizeof(TGenericSeq) + r + 1))
+      result.reserved = r
   result.data[result.len] = c
   result.data[result.len+1] = '\0'
   inc(result.len)
@@ -205,7 +207,9 @@ proc addChar(s: NimString, c: char): NimString =
 #   s = rawNewString(0);
 
 proc resizeString(dest: NimString, addlen: int): NimString {.compilerRtl.} =
-  if dest.len + addlen <= dest.space and not wasMoved(dest):
+  if dest == nil:
+    result = rawNewStringNoInit(addlen)
+  elif dest.len + addlen <= dest.space and not wasMoved(dest):
     result = dest
   else: # slow path:
     var sp = max(resize(dest.space), dest.len + addlen)
@@ -216,8 +220,9 @@ proc resizeString(dest: NimString, addlen: int): NimString {.compilerRtl.} =
     # DO NOT UPDATE LEN YET: dest.len = newLen
 
 proc appendString(dest, src: NimString) {.compilerproc, inline.} =
-  copyMem(addr(dest.data[dest.len]), addr(src.data), src.len + 1)
-  inc(dest.len, src.len)
+  if src != nil:
+    copyMem(addr(dest.data[dest.len]), addr(src.data), src.len + 1)
+    inc(dest.len, src.len)
 
 proc appendChar(dest: NimString, c: char) {.compilerproc, inline.} =
   dest.data[dest.len] = c

tests/system/tnilconcats.nim

@@ -0,0 +1,25 @@
+discard """
+  output: '''@[nil, nil, nil, nil, nil, nil, nil, "meh"]'''
+  exitcode: "0"
+"""
+
+when true:
+  var ab: string
+  ab &= "more"
+
+  doAssert ab == "more"
+
+  var x: seq[string]
+
+  setLen(x, 7)
+
+  x.add "meh"
+
+  var s: string
+  var z = "abc"
+  var zz: string
+  s &= "foo" & z & zz
+
+  doAssert s == "fooabc"
+
+  echo x