sys: support type aliases (aka typedefs)

Complex types that are often repeated can be given short type aliases using the
following syntax:

```
type identifier underlying_type
```

For example:

```
type signalno int32[0:65]
type net_port proc[20000, 4, int16be]
```

Then, type alias can be used instead of the underlying type in any contexts.
Underlying type needs to be described as if it's a struct field, that is,
with the base type if it's required. However, type alias can be used as syscall
arguments as well. Underlying types are currently restricted to integer types,
`ptr`, `ptr64`, `const`, `flags` and `proc` types.

1 files changed, 14 insertions, 0 deletions

diff --git a/pkg/ast/parser.go b/pkg/ast/parser.go
index 9e1ab679e..db211ab2a 100644
--- a/pkg/ast/parser.go
+++ b/pkg/ast/parser.go
@@ -128,6 +128,9 @@ func (p *parser) parseTop() Node {
 		return p.parseResource()
 	case tokIdent:
 		name := p.parseIdent()
+		if name.Name == "type" {
+			return p.parseTypeDef()
+		}
 		switch p.tok {
 		case tokLParen:
 			return p.parseCall(name)
@@ -245,6 +248,17 @@ func (p *parser) parseResource() *Resource {
 	}
 }
 
+func (p *parser) parseTypeDef() *TypeDef {
+	pos0 := p.pos
+	name := p.parseIdent()
+	typ := p.parseType()
+	return &TypeDef{
+		Pos:  pos0,
+		Name: name,
+		Type: typ,
+	}
+}
+
 func (p *parser) parseCall(name *Ident) *Call {
 	c := &Call{
 		Pos:      name.Pos,