diff --git a/compiler/commands.nim b/compiler/commands.nim
index f7de0978ed..a37cb348ae 100644
--- a/compiler/commands.nim
+++ b/compiler/commands.nim
@@ -930,7 +930,7 @@ proc processSwitch*(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
       if m.len == 0:
         localError(conf, info, "Cannot resolve filename: " & arg)
       else:
-        conf.implicitImports.add m
+        conf.implicitImports.add(if arg.startsWith(stdPrefix): arg else: m)
   of "include":
     expectArg(conf, switch, arg, pass, info)
     if pass in {passCmd2, passPP}:
diff --git a/compiler/importer.nim b/compiler/importer.nim
index a02a5e96a1..ac10720f90 100644
--- a/compiler/importer.nim
+++ b/compiler/importer.nim
@@ -13,7 +13,7 @@ import
   ast, msgs, options, idents, lookups,
   semdata, modulepaths, sigmatch, lineinfos,
   modulegraphs, wordrecg
-from std/strutils import `%`, startsWith
+from std/strutils import `%`, startsWith, replace
 from std/sequtils import addUnique
 import std/[sets, tables, intsets]
 
@@ -304,9 +304,9 @@ proc myImportModule(c: PContext, n: var PNode, importStmtResult: PNode): PSym =
       var prefix = ""
       if realModule.constraint != nil: prefix = realModule.constraint.strVal & "; "
       message(c.config, n.info, warnDeprecated, prefix & realModule.name.s & " is deprecated")
-    let moduleName = getModuleName(c.config, n)
-    if belongsToStdlib(c.graph, result) and not startsWith(moduleName, stdPrefix) and
-        not startsWith(moduleName, "system/") and not startsWith(moduleName, "packages/"):
+    let moduleNameNorm = getModuleName(c.config, n).replace("\\", "/")
+    if belongsToStdlib(c.graph, result) and not startsWith(moduleNameNorm, stdPrefix) and
+        not startsWith(moduleNameNorm, "system/") and not startsWith(moduleNameNorm, "packages/"):
       message(c.config, n.info, warnStdPrefix, realModule.name.s)
 
     proc suggestMod(n: PNode; s: PSym) =
diff --git a/compiler/lexer.nim b/compiler/lexer.nim
index 9ebec89be5..9c0b803602 100644
--- a/compiler/lexer.nim
+++ b/compiler/lexer.nim
@@ -1349,7 +1349,7 @@ proc rawGetTok*(L: var Lexer, tok: var Token) =
           lexMessage(L, errGenerated, "invalid token: no whitespace between number and identifier")
     of '-':
       if L.buf[L.bufpos+1] in {'0'..'9'} and
-          (L.bufpos-1 == 0 or L.buf[L.bufpos-1] in UnaryMinusWhitelist):
+          (L.bufpos == 0 or L.buf[L.bufpos-1] in UnaryMinusWhitelist):
         # x)-23 # binary minus
         # ,-23  # unary minus
         # \n-78 # unary minus? Yes.
diff --git a/compiler/semdata.nim b/compiler/semdata.nim
index 15d8b14fe7..32c98cdb31 100644
--- a/compiler/semdata.nim
+++ b/compiler/semdata.nim
@@ -637,6 +637,11 @@ proc renderNotLValue*(n: PNode): string =
   elif n.kind in {nkHiddenStdConv, nkHiddenSubConv} and n.len == 2:
     result = typeToString(n.typ.skipTypes(abstractVar)) & "(" & result & ")"
 
+proc isSsoStringIndex*(conf: ConfigRef; n: PNode): bool =
+  result = conf.usesSso() and n.kind == nkBracketExpr and n.len >= 1 and
+      n[0].typ != nil and
+      n[0].typ.skipTypes(abstractVar + abstractInst - {tyTypeDesc}).kind == tyString
+
 proc isAssignable(c: PContext, n: PNode): TAssignableResult =
   result = parampatterns.isAssignable(c.p.owner, n)
 
diff --git a/compiler/sempass2.nim b/compiler/sempass2.nim
index 35901ed960..9c84b721ad 100644
--- a/compiler/sempass2.nim
+++ b/compiler/sempass2.nim
@@ -809,6 +809,10 @@ proc trackOperandForIndirectCall(tracked: PEffects, n: PNode, formals: PType; ar
         markSideEffect(tracked, a, n.info)
   let paramType = if formals != nil and argIndex < formals.signatureLen: formals[argIndex] else: nil
   if paramType != nil and paramType.kind in {tyVar}:
+    let arg = n.skipAddr()
+    if isSsoStringIndex(tracked.config, arg):
+      localError(tracked.config, arg.info,
+        "expression '$1' is immutable, not 'var'" % renderNotLValue(arg))
     invalidateFacts(tracked.guards, n)
     if n.kind == nkSym and isLocalSym(tracked, n.sym):
       makeVolatile(tracked, n.sym)
diff --git a/doc/manual.md b/doc/manual.md
index 40897ac108..0970d0f5b8 100644
--- a/doc/manual.md
+++ b/doc/manual.md
@@ -34,10 +34,10 @@ To learn how to compile Nim programs and generate documentation see
 the [Compiler User Guide](nimc.html) and the [DocGen Tools Guide](docgen.html).
 
 The language constructs are explained using an extended BNF, in which `(a)*`
-means 0 or more `a`'s, `a+` means 1 or more `a`'s, and `(a)?` means an
+means 0 or more *a*'s, `a+` means 1 or more *a*'s, and `(a)?` means an
 optional *a*. Parentheses may be used to group elements.
 
-`&` is the lookahead operator; `&a` means that an `a` is expected but
+`&` is the lookahead operator; `&a` means that an *a* is expected but
 not consumed. It will be consumed in the following rule.
 
 The `|`, `/` symbols are used to mark alternatives and have the lowest
@@ -8874,7 +8874,7 @@ Byref pragma
 The `byref` pragma can be applied to an object or tuple type or a proc param.
 When applied to a type it instructs the compiler to pass the type by reference
 (hidden pointer) to procs. When applied to a param it will take precedence, even
-if the the type was marked as `bycopy`. When an `importc` type has a `byref` pragma or
+if the type was marked as `bycopy`. When an `importc` type has a `byref` pragma or
 parameters are marked as `byref` in an `importc` proc, these params translate to pointers.
 When an `importcpp` type has a `byref` pragma, these params translate to
 C++ references `&`.
diff --git a/doc/tut1.md b/doc/tut1.md
index 072d8f3ba6..be13f8fbb5 100644
--- a/doc/tut1.md
+++ b/doc/tut1.md
@@ -1144,7 +1144,7 @@ there is a difference between the `$` and `repr` outputs:
   echo myCharacter, ":", repr(myCharacter)
   # --> n:'n'
   echo myString, ":", repr(myString)
-  # --> nim:0x10fa8c050"nim"
+  # --> nim:"nim"
   echo myInteger, ":", repr(myInteger)
   # --> 42:42
   echo myFloat, ":", repr(myFloat)
diff --git a/lib/system.nim b/lib/system.nim
index c76d096426..63989b1502 100644
--- a/lib/system.nim
+++ b/lib/system.nim
@@ -2694,7 +2694,9 @@ when hasAlloc or defined(nimscript):
     setLen(x, xl+item.len)
     var j = xl-1
     while j >= i:
-      when defined(gcArc) or defined(gcOrc) or defined(gcYrc) or defined(gcAtomicArc):
+      when defined(nimsso):
+        x[j+item.len] = x[j]
+      elif defined(gcArc) or defined(gcOrc) or defined(gcYrc) or defined(gcAtomicArc):
         x[j+item.len] = move x[j]
       else:
         shallowCopy(x[j+item.len], x[j])
diff --git a/lib/system/alloc.nim b/lib/system/alloc.nim
index 925f20d906..256c8afd80 100644
--- a/lib/system/alloc.nim
+++ b/lib/system/alloc.nim
@@ -691,7 +691,7 @@ proc getBigChunk(a: var MemRegion, size: int): PBigChunk =
     removeChunkFromMatrix2(a, result, fl, sl)
     if result.size >= size + PageSize:
       splitChunk(a, result, size)
-  # set 'used' to to true:
+  # set 'used' to true:
   result.prevSize = 1
   track("setUsedToFalse", addr result.size, sizeof(int))
   sysAssert result.owner == addr a, "getBigChunk: No owner set!"
@@ -708,7 +708,7 @@ proc getHugeChunk(a: var MemRegion; size: int): PBigChunk =
   result.next = nil
   result.prev = nil
   result.size = size
-  # set 'used' to to true:
+  # set 'used' to true:
   result.prevSize = 1
   result.owner = addr a
   incl(a, a.chunkStarts, pageIndex(result))
diff --git a/lib/system/gc_common.nim b/lib/system/gc_common.nim
index 08e8798b08..1569d12e14 100644
--- a/lib/system/gc_common.nim
+++ b/lib/system/gc_common.nim
@@ -143,7 +143,7 @@ when nimCoroutines:
 
   proc find(first: var GcStack, bottom: pointer): ptr GcStack =
     ## Find stack struct based on bottom pointer. If `bottom` is nil then main
-    ## thread stack is is returned.
+    ## thread stack is returned.
     if bottom == nil:
       return addr(gch.stack)
 
diff --git a/lib/system/indices.nim b/lib/system/indices.nim
index 6230b36788..8f20af5ec5 100644
--- a/lib/system/indices.nim
+++ b/lib/system/indices.nim
@@ -59,16 +59,35 @@ template `[]=`*(s: string; i: int; val: char) = arrPut(s, i, val)
 template `^^`(s, i: untyped): untyped =
   (when i is BackwardsIndex: s.len - int(i) else: int(i))
 
-template spliceImpl(s, a, L, b: typed): untyped =
+template spliceStringImpl(s, a, L, b: typed): untyped =
   # make room for additional elements or cut:
   var shift = b.len - max(0,L)  # ignore negative slice size
   var newLen = s.len + shift
   if shift > 0:
     # enlarge:
     setLen(s, newLen)
-    for i in countdown(newLen-1, a+b.len): movingCopy(s[i], s[i-shift])
+    for i in countdown(newLen-1, a+b.len):
+      s[i] = s[i-shift]
   else:
-    for i in countup(a+b.len, newLen-1): movingCopy(s[i], s[i-shift])
+    for i in countup(a+b.len, newLen-1):
+      s[i] = s[i-shift]
+    # cut down:
+    setLen(s, newLen)
+  # fill the hole:
+  for i in 0 ..< b.len: s[a+i] = b[i]
+
+template spliceSeqImpl(s, a, L, b: typed): untyped =
+  # make room for additional elements or cut:
+  var shift = b.len - max(0,L)  # ignore negative slice size
+  var newLen = s.len + shift
+  if shift > 0:
+    # enlarge:
+    setLen(s, newLen)
+    for i in countdown(newLen-1, a+b.len):
+      movingCopy(s[i], s[i-shift])
+  else:
+    for i in countup(a+b.len, newLen-1):
+      movingCopy(s[i], s[i-shift])
     # cut down:
     setLen(s, newLen)
   # fill the hole:
@@ -102,7 +121,7 @@ proc `[]=`*[T, U: Ordinal](s: var string, x: HSlice[T, U], b: string) {.systemRa
   if L == b.len:
     for i in 0..<L: s[i+a] = b[i]
   else:
-    spliceImpl(s, a, L, b)
+    spliceStringImpl(s, a, L, b)
 
 proc `[]`*[Idx, T; U, V: Ordinal](a: array[Idx, T], x: HSlice[U, V]): seq[T] {.systemRaisesDefect.} =
   ## Slice operation for arrays.
@@ -162,4 +181,4 @@ proc `[]=`*[T; U, V: Ordinal](s: var seq[T], x: HSlice[U, V], b: openArray[T]) {
   if L == b.len:
     for i in 0 ..< L: s[i+a] = b[i]
   else:
-    spliceImpl(s, a, L, b)
+    spliceSeqImpl(s, a, L, b)
diff --git a/lib/system/strs_v3.nim b/lib/system/strs_v3.nim
index 173737edea..58462ba522 100644
--- a/lib/system/strs_v3.nim
+++ b/lib/system/strs_v3.nim
@@ -224,13 +224,14 @@ proc cmpStringPtrs(a, b: ptr SmallString): int {.inline.} =
         minLen - AlwaysAvail)
     if result == 0: result = aslen - bslen
     return
-  # At least one is long. Hot prefix: inlinePtr[0..AlwaysAvail-1] mirrors heap data.
-  let pfxLen = min(min(aslen, bslen), AlwaysAvail)
-  result = cmpInlineBytes(inlinePtrOf(a), inlinePtrOf(b), pfxLen)
-  if result != 0: return
+  # At least one is long. Hot prefix mirrors heap data, but only up to fullLen:
+  # shrinking can leave stale bytes in the inline cache past the logical length.
   let la = if aslen > PayloadSize: a.more.fullLen else: aslen
   let lb = if bslen > PayloadSize: b.more.fullLen else: bslen
   let minLen = min(la, lb)
+  let pfxLen = min(minLen, AlwaysAvail)
+  result = cmpInlineBytes(inlinePtrOf(a), inlinePtrOf(b), pfxLen)
+  if result != 0: return
   if minLen <= AlwaysAvail:
     result = la - lb
     return
diff --git a/tests/compiler/tcmdline_import_std_prefix.nim b/tests/compiler/tcmdline_import_std_prefix.nim
new file mode 100644
index 0000000000..9b9eca776f
--- /dev/null
+++ b/tests/compiler/tcmdline_import_std_prefix.nim
@@ -0,0 +1,10 @@
+discard """
+  matrix: "-d:nimPreviewSlimSystem --warning:StdPrefix:on --warningAsError:StdPrefix:on --import:std/objectdollar"
+  output: "(a: 23, b: 45)"
+"""
+
+type Foo = object
+  a, b: int
+
+let x = Foo(a: 23, b: 45)
+echo x
diff --git a/tests/errmsgs/tsso_string_index_var.nim b/tests/errmsgs/tsso_string_index_var.nim
new file mode 100644
index 0000000000..401491e0a5
--- /dev/null
+++ b/tests/errmsgs/tsso_string_index_var.nim
@@ -0,0 +1,13 @@
+discard """
+  cmd: "nim check --strings:sso --mm:orc --hints:off $file"
+  action: "reject"
+  nimout: '''
+tsso_string_index_var.nim(13, 12) Error: expression 's[0]' is immutable, not 'var'
+'''
+"""
+
+proc passByVar(c: var char) =
+  c = 'x'
+
+var s = "abc"
+passByVar(s[0])
diff --git a/tests/stdlib/tstrformat.nim b/tests/stdlib/tstrformat.nim
index 74f23b953b..258c190a1d 100644
--- a/tests/stdlib/tstrformat.nim
+++ b/tests/stdlib/tstrformat.nim
@@ -544,6 +544,12 @@ proc main() =
     var x = 5
     doAssert fmt"{(x=7;123.456)=:13e}" == "(x=7;123.456)= 1.234560e+02"
     doAssert x==7
+
+  block: # binary operators in interpolated expressions
+    let n = 1
+    doAssert &"{n-1}" == "0"
+    doAssert fmt"{n-1}" == "0"
+
   block: #curly bracket expressions and tuples
     proc formatValue(result: var string; value:Table|bool|JsonNode; specifier:string) = result.add $value