cgo: refactor constant expressions

Put them in a separate file for separation of concerns (making them
testable) and add some tests.

cgo/const.go

@@ -0,0 +1,59 @@
+package cgo
+
+// This file implements a parser of a subset of the C language, just enough to
+// parse common #define statements to Go constant expressions.
+
+import (
+	"go/ast"
+	"go/token"
+	"strings"
+)
+
+// parseConst parses the given string as a C constant.
+func parseConst(pos token.Pos, value string) *ast.BasicLit {
+	for len(value) != 0 && value[0] == '(' && value[len(value)-1] == ')' {
+		value = strings.TrimSpace(value[1 : len(value)-1])
+	}
+	if len(value) == 0 {
+		// Pretend it doesn't exist at all.
+		return nil
+	}
+	// For information about integer literals:
+	// https://en.cppreference.com/w/cpp/language/integer_literal
+	if value[0] == '"' {
+		// string constant
+		return &ast.BasicLit{ValuePos: pos, Kind: token.STRING, Value: value}
+	}
+	if value[0] == '\'' {
+		// char constant
+		return &ast.BasicLit{ValuePos: pos, Kind: token.CHAR, Value: value}
+	}
+	// assume it's a number (int or float)
+	value = strings.Replace(value, "'", "", -1) // remove ' chars
+	value = strings.TrimRight(value, "lu")      // remove llu suffixes etc.
+	// find the first non-number
+	nonnum := byte(0)
+	for i := 0; i < len(value); i++ {
+		if value[i] < '0' || value[i] > '9' {
+			nonnum = value[i]
+			break
+		}
+	}
+	// determine number type based on the first non-number
+	switch nonnum {
+	case 0:
+		// no non-number found, must be an integer
+		return &ast.BasicLit{ValuePos: pos, Kind: token.INT, Value: value}
+	case 'x', 'X':
+		// hex integer constant
+		// TODO: may also be a floating point number per C++17.
+		return &ast.BasicLit{ValuePos: pos, Kind: token.INT, Value: value}
+	case '.', 'e':
+		// float constant
+		value = strings.TrimRight(value, "fFlL")
+		return &ast.BasicLit{ValuePos: pos, Kind: token.FLOAT, Value: value}
+	default:
+		// unknown type, ignore
+	}
+	return nil
+}

cgo/const_test.go

@@ -0,0 +1,45 @@
+package cgo
+
+import (
+	"bytes"
+	"go/format"
+	"go/token"
+	"testing"
+)
+
+func TestParseConst(t *testing.T) {
+	// Test converting a C constant to a Go constant.
+	for _, tc := range []struct {
+		C  string
+		Go string
+	}{
+		{`5`, `5`},
+		{`(5)`, `5`},
+		{`(((5)))`, `5`},
+		{`5.8f`, `5.8`},
+		{`foo`, `<invalid>`}, // identifiers unimplemented
+		{``, `<invalid>`},    // empty constants not allowed in Go
+		{`"foo"`, `"foo"`},
+		{`'a'`, `'a'`},
+		{`0b10`, `<invalid>`}, // binary number literals unimplemented
+	} {
+		fset := token.NewFileSet()
+		startPos := fset.AddFile("test.c", -1, 1000).Pos(0)
+		expr := parseConst(startPos, tc.C)
+		s := "<invalid>"
+		if expr != nil {
+			// Serialize the Go constant to a string, for more readable test
+			// cases.
+			buf := &bytes.Buffer{}
+			err := format.Node(buf, fset, expr)
+			if err != nil {
+				t.Errorf("could not format expr from C constant %#v: %v", tc.C, err)
+				continue
+			}
+			s = buf.String()
+		}
+		if s != tc.Go {
+			t.Errorf("C constant %#v was parsed to %#v while expecting %#v", tc.C, s, tc.Go)
+		}
+	}
+}

cgo/libclang.go

@@ -246,51 +246,11 @@ func tinygo_clang_globals_visitor(c, parent C.GoCXCursor, client_data C.CXClient
 			break
 		}
 		value := strings.TrimSpace(source[len(name):])
-		for len(value) != 0 && value[0] == '(' && value[len(value)-1] == ')' {
-			value = strings.TrimSpace(value[1 : len(value)-1])
-		}
-		if len(value) == 0 {
-			// Pretend it doesn't exist at all.
-			return C.CXChildVisit_Continue
-		}
-		// For information about integer literals:
-		// https://en.cppreference.com/w/cpp/language/integer_literal
-		if value[0] == '"' {
-			// string constant
-			p.constants[name] = constantInfo{&ast.BasicLit{pos, token.STRING, value}, pos}
-			return C.CXChildVisit_Continue
-		}
-		if value[0] == '\'' {
-			// char constant
-			p.constants[name] = constantInfo{&ast.BasicLit{pos, token.CHAR, value}, pos}
-			return C.CXChildVisit_Continue
-		}
-		// assume it's a number (int or float)
-		value = strings.Replace(value, "'", "", -1) // remove ' chars
-		value = strings.TrimRight(value, "lu")      // remove llu suffixes etc.
-		// find the first non-number
-		nonnum := byte(0)
-		for i := 0; i < len(value); i++ {
-			if value[i] < '0' || value[i] > '9' {
-				nonnum = value[i]
-				break
-			}
-		}
-		// determine number type based on the first non-number
-		switch nonnum {
-		case 0:
-			// no non-number found, must be an integer
-			p.constants[name] = constantInfo{&ast.BasicLit{pos, token.INT, value}, pos}
-		case 'x', 'X':
-			// hex integer constant
-			// TODO: may also be a floating point number per C++17.
-			p.constants[name] = constantInfo{&ast.BasicLit{pos, token.INT, value}, pos}
-		case '.', 'e':
-			// float constant
-			value = strings.TrimRight(value, "fFlL")
-			p.constants[name] = constantInfo{&ast.BasicLit{pos, token.FLOAT, value}, pos}
-		default:
-			// unknown type, ignore
+		// Try to convert this #define into a Go constant expression.
+		expr := parseConst(pos, value)
+		if expr != nil {
+			// Parsing was successful.
+			p.constants[name] = constantInfo{expr, pos}
 		}
 	}
 	return C.CXChildVisit_Continue