fixes #24720; std lib iterators unnecessarily require value copies

compiler/semmagic.nim

@@ -65,7 +65,7 @@ proc semTypeOf(c: PContext; n: PNode): PNode =
       t.incl tfNonConstExpr
     else:
       t = base
-  result.typ = makeTypeDesc(c, t)
+  result.typ = makeTypeDesc(c, decayTypeOfView(c, t))
 
 type
   SemAsgnMode = enum asgnNormal, noOverloadedSubscript, noOverloadedAsgn

compiler/semtypes.nim

@@ -1879,6 +1879,38 @@ proc fixupTypeOf(c: PContext, prev: PType, typ: PType) =
     if prev.kind != tyGenericBody:
       assignType(prev, result)
 
+proc decayTypeOfView(c: PContext, typ: PType): PType =
+  if typ == nil: return nil
+  let t = typ.skipTypes({tyGenericInst, tyAlias, tySink})
+  case t.kind
+  of tyVar, tyLent:
+    result = decayTypeOfView(c, t.elementType)
+  of tyTuple:
+    var changed = false
+    var kids = newSeq[PType](t.len)
+    for i in 0..<t.len:
+      kids[i] = decayTypeOfView(c, t[i])
+      if kids[i] != t[i]: changed = true
+    if changed:
+      result = copyType(t, c.idgen, t.owner)
+      for i in 0..<kids.len:
+        result[i] = kids[i]
+      if t.n != nil:
+        result.n = copyNode(t.n)
+        for it in t.n:
+          if it.kind == nkSym and it.sym.kind == skField:
+            let field = copySym(it.sym, c.idgen)
+            field.ast = it.sym.ast
+            if field.position >= 0 and field.position < kids.len:
+              field.typ = kids[field.position]
+            result.n.add newSymNode(field, it.info)
+          else:
+            result.n.add copyTree(it)
+    else:
+      result = typ
+  else:
+    result = typ
+
 proc semTypeExpr(c: PContext, n: PNode; prev: PType): PType =
   var n = semExprWithType(c, n, {efDetermineType})
   if n.typ.kind == tyTypeDesc:
@@ -2078,6 +2110,7 @@ proc semTypeOf(c: PContext; n: PNode; prev: PType): PType =
       result.incl tfNonConstExpr
     else:
       result = base
+  result = decayTypeOfView(c, result)
   fixupTypeOf(c, prev, result)
 
 proc semTypeOf2(c: PContext; n: PNode; prev: PType): PType =
@@ -2103,6 +2136,7 @@ proc semTypeOf2(c: PContext; n: PNode; prev: PType): PType =
       result.incl tfNonConstExpr
     else:
       result = base
+  result = decayTypeOfView(c, result)
   fixupTypeOf(c, prev, result)
 
 proc semTypeIdent(c: PContext, n: PNode): PSym =

lib/pure/collections/tables.nim

@@ -739,7 +739,7 @@ template withValue*[A, B](t: Table[A, B], key: A,
     discard
 
 
-iterator pairs*[A, B](t: Table[A, B]): (A, B) =
+iterator pairs*[A, B](t: Table[A, B]): (lent A, lent B) =
   ## Iterates over any `(key, value)` pair in the table `t`.
   ##
   ## See also:
@@ -1201,7 +1201,7 @@ proc `==`*[A, B](s, t: TableRef[A, B]): bool =
 
 
 
-iterator pairs*[A, B](t: TableRef[A, B]): (A, B) =
+iterator pairs*[A, B](t: TableRef[A, B]): (lent A, lent B) =
   ## Iterates over any `(key, value)` pair in the table `t`.
   ##
   ## See also:
@@ -1789,7 +1789,7 @@ proc `==`*[A, B](s, t: OrderedTable[A, B]): bool =
 
 
 
-iterator pairs*[A, B](t: OrderedTable[A, B]): (A, B) =
+iterator pairs*[A, B](t: OrderedTable[A, B]): (lent A, lent B) =
   ## Iterates over any `(key, value)` pair in the table `t` in insertion
   ## order.
   ##
@@ -2212,7 +2212,7 @@ proc `==`*[A, B](s, t: OrderedTableRef[A, B]): bool =
 
 
 
-iterator pairs*[A, B](t: OrderedTableRef[A, B]): (A, B) =
+iterator pairs*[A, B](t: OrderedTableRef[A, B]): (lent A, lent B) =
   ## Iterates over any `(key, value)` pair in the table `t` in insertion
   ## order.
   ##
@@ -2622,7 +2622,7 @@ proc `==`*[A](s, t: CountTable[A]): bool =
   equalsImpl(s, t)
 
 
-iterator pairs*[A](t: CountTable[A]): (A, int) =
+iterator pairs*[A](t: CountTable[A]): (lent A, int) =
   ## Iterates over any `(key, value)` pair in the table `t`.
   ##
   ## See also:
@@ -2899,7 +2899,7 @@ proc `==`*[A](s, t: CountTableRef[A]): bool =
   else: result = s[] == t[]
 
 
-iterator pairs*[A](t: CountTableRef[A]): (A, int) =
+iterator pairs*[A](t: CountTableRef[A]): (lent A, int) =
   ## Iterates over any `(key, value)` pair in the table `t`.
   ##
   ## See also:

tests/arc/t24720.nim

@@ -0,0 +1,20 @@
+discard """
+  matrix: "--mm:orc"
+  output: '''
+found entry
+'''
+"""
+
+import std/tables
+type NoCopies = object
+
+proc `=copy`(a: var NoCopies, b: NoCopies) {.error.}
+
+# bug #24720
+proc foo() =
+  var t: Table[int, NoCopies]
+  t[3] = NoCopies() # only moves
+  for k, v in t.pairs(): # lent values, no need to copy!
+    echo "found entry"
+
+foo()