cmd/viewer: add support for map-like container types

This PR modifies viewTypeForContainerType to use the last type parameter of a container type
as the value type, enabling the implementation of map-like container types where the second-to-last
(usually first) type parameter serves as the key type.

It also adds a MapContainer type to test the code generation.

Updates #12736

Signed-off-by: Nick Khyl <nickk@tailscale.com>

cmd/viewer/tests/tests.go

@@ -152,6 +152,44 @@ func ContainerViewOf[T views.ViewCloner[T, V], V views.StructView[T]](c *Contain
 	return ContainerView[T, V]{c}
 }
 
+// MapContainer is a predefined map-like container type.
+// Unlike [Container], it has two type parameters, where the value
+// is the second parameter.
+type MapContainer[K comparable, V views.Cloner[V]] struct {
+	Items map[K]V
+}
+
+func (c *MapContainer[K, V]) Clone() *MapContainer[K, V] {
+	if c == nil {
+		return nil
+	}
+	var m map[K]V
+	if c.Items != nil {
+		m = make(map[K]V, len(c.Items))
+		for i := range m {
+			m[i] = c.Items[i].Clone()
+		}
+	}
+	return &MapContainer[K, V]{m}
+}
+
+// MapContainerView is a pre-defined readonly view of a [MapContainer][K, T].
+type MapContainerView[K comparable, T views.ViewCloner[T, V], V views.StructView[T]] struct {
+	// ж is the underlying mutable value, named with a hard-to-type
+	// character that looks pointy like a pointer.
+	// It is named distinctively to make you think of how dangerous it is to escape
+	// to callers. You must not let callers be able to mutate it.
+	ж *MapContainer[K, T]
+}
+
+func (cv MapContainerView[K, T, V]) Items() views.MapFn[K, T, V] {
+	return views.MapFnOf(cv.ж.Items, func(t T) V { return t.View() })
+}
+
+func MapContainerViewOf[K comparable, T views.ViewCloner[T, V], V views.StructView[T]](c *MapContainer[K, T]) MapContainerView[K, T, V] {
+	return MapContainerView[K, T, V]{c}
+}
+
 type GenericBasicStruct[T BasicType] struct {
 	Value T
 }
@@ -160,5 +198,7 @@ type StructWithContainers struct {
 	IntContainer              Container[int]
 	CloneableContainer        Container[*StructWithPtrs]
 	BasicGenericContainer     Container[GenericBasicStruct[int]]
-	ClonableGenericContainer Container[*GenericNoPtrsStruct[int]]
+	CloneableGenericContainer Container[*GenericNoPtrsStruct[int]]
+	CloneableMap              MapContainer[int, *StructWithPtrs]
+	CloneableGenericMap       MapContainer[int, *GenericNoPtrsStruct[int]]
 }

cmd/viewer/tests/tests_clone.go

@@ -426,7 +426,9 @@ func (src *StructWithContainers) Clone() *StructWithContainers {
 	dst := new(StructWithContainers)
 	*dst = *src
 	dst.CloneableContainer = *src.CloneableContainer.Clone()
-	dst.ClonableGenericContainer = *src.ClonableGenericContainer.Clone()
+	dst.CloneableGenericContainer = *src.CloneableGenericContainer.Clone()
+	dst.CloneableMap = *src.CloneableMap.Clone()
+	dst.CloneableGenericMap = *src.CloneableGenericMap.Clone()
 	return dst
 }
 
@@ -435,5 +437,7 @@ func (src *StructWithContainers) Clone() *StructWithContainers {
 	IntContainer              Container[int]
 	CloneableContainer        Container[*StructWithPtrs]
 	BasicGenericContainer     Container[GenericBasicStruct[int]]
-	ClonableGenericContainer Container[*GenericNoPtrsStruct[int]]
+	CloneableGenericContainer Container[*GenericNoPtrsStruct[int]]
+	CloneableMap              MapContainer[int, *StructWithPtrs]
+	CloneableGenericMap       MapContainer[int, *GenericNoPtrsStruct[int]]
 }{})

cmd/viewer/tests/tests_view.go

@@ -657,8 +657,14 @@ func (v StructWithContainersView) CloneableContainer() ContainerView[*StructWith
 func (v StructWithContainersView) BasicGenericContainer() Container[GenericBasicStruct[int]] {
 	return v.ж.BasicGenericContainer
 }
-func (v StructWithContainersView) ClonableGenericContainer() ContainerView[*GenericNoPtrsStruct[int], GenericNoPtrsStructView[int]] {
-	return ContainerViewOf(&v.ж.ClonableGenericContainer)
+func (v StructWithContainersView) CloneableGenericContainer() ContainerView[*GenericNoPtrsStruct[int], GenericNoPtrsStructView[int]] {
+	return ContainerViewOf(&v.ж.CloneableGenericContainer)
+}
+func (v StructWithContainersView) CloneableMap() MapContainerView[int, *StructWithPtrs, StructWithPtrsView] {
+	return MapContainerViewOf(&v.ж.CloneableMap)
+}
+func (v StructWithContainersView) CloneableGenericMap() MapContainerView[int, *GenericNoPtrsStruct[int], GenericNoPtrsStructView[int]] {
+	return MapContainerViewOf(&v.ж.CloneableGenericMap)
 }
 
 // A compilation failure here means this code must be regenerated, with the command at the top of this file.
@@ -666,5 +672,7 @@ func (v StructWithContainersView) ClonableGenericContainer() ContainerView[*Gene
 	IntContainer              Container[int]
 	CloneableContainer        Container[*StructWithPtrs]
 	BasicGenericContainer     Container[GenericBasicStruct[int]]
-	ClonableGenericContainer Container[*GenericNoPtrsStruct[int]]
+	CloneableGenericContainer Container[*GenericNoPtrsStruct[int]]
+	CloneableMap              MapContainer[int, *StructWithPtrs]
+	CloneableGenericMap       MapContainer[int, *GenericNoPtrsStruct[int]]
 }{})

cmd/viewer/viewer.go

@@ -448,7 +448,7 @@ func viewTypeForContainerType(typ types.Type) (*types.Named, *types.Func) {
 	}
 	// ...and add the element view type.
 	// For that, we need to first determine the named elem type...
-	elemType, ok := baseType(containerType.TypeArgs().At(0)).(*types.Named)
+	elemType, ok := baseType(containerType.TypeArgs().At(containerType.TypeArgs().Len() - 1)).(*types.Named)
 	if !ok {
 		return nil, nil
 	}