go.tools/oracle: show size and alignment for structs and named types.

Users should be familiar with the sizes of all other types.

Currently we assume amd64 (as do other parts of the oracle,
e.g. go/build tags).  Will parameterize later.

R=crawshaw
CC=golang-dev, gri
https://golang.org/cl/29710043

oracle/describe.go

@@ -639,10 +639,22 @@ func describeType(o *Oracle, qpos *QueryPos, path []ast.Node) (*describeTypeResu
 		return nil, fmt.Errorf("unexpected AST for type: %T", n)
 	}
 
+	description = description + "type " + qpos.TypeString(t)
+
+	// Show sizes for structs and named types (it's fairly obvious for others).
+	switch t.(type) {
+	case *types.Named, *types.Struct:
+		// TODO(adonovan): use o.imp.Config().TypeChecker.Sizes when
+		// we add the Config() method (needs some thought).
+		szs := types.StdSizes{8, 8}
+		description = fmt.Sprintf("%s (size %d, align %d)", description,
+			szs.Sizeof(t), szs.Alignof(t))
+	}
+
 	return &describeTypeResult{
 		qpos:        qpos,
 		node:        path[0],
-		description: description + "type " + qpos.TypeString(t),
+		description: description,
 		typ:         t,
 		methods:     accessibleMethods(t, qpos.info.Pkg),
 	}, nil

oracle/testdata/src/main/describe-json.golden

@@ -133,7 +133,7 @@
 {
 	"mode": "describe",
 	"describe": {
-		"desc": "definition of type C",
+		"desc": "definition of type C (size 8, align 8)",
 		"pos": "testdata/src/main/describe-json.go:27:6",
 		"detail": "type",
 		"type": {

oracle/testdata/src/main/describe.golden

@@ -50,13 +50,13 @@ reference to interface method func (I).f()
 defined here
 
 -------- @describe type-D --------
-reference to type D
+reference to type D (size 0, align 1)
 defined as struct{}
 Method set:
 	method (D) f()
 
 -------- @describe type-I --------
-reference to type I
+reference to type I (size 16, align 8)
 defined as interface{f()}
 Method set:
 	method (I) f()
@@ -140,11 +140,11 @@ reference to const localpi untyped float of constant value 3141/1000
 defined here
 
 -------- @describe type-def-T --------
-definition of type T
+definition of type T (size 8, align 8)
 No methods.
 
 -------- @describe type-ref-T --------
-reference to type T
+reference to type T (size 8, align 8)
 defined as int
 No methods.
 
@@ -186,7 +186,7 @@ definition of var b *int
 no points-to information: PTA did not encounter this expression (dead code?)
 
 -------- @describe def-iface-I --------
-definition of type I
+definition of type I (size 16, align 8)
 Method set:
 	method (I) f()
 

oracle/testdata/src/main/imports.golden

@@ -19,7 +19,7 @@ reference to var lib.Var int
 defined here
 
 -------- @describe ref-type --------
-reference to type lib.Type
+reference to type lib.Type (size 8, align 8)
 defined as int
 Method set:
 	method (lib.Type) Method(x *int) *int