compiler,runtime: implement []rune to string conversion

This is used by a few packages in the standard library, at least
compress/gzip and regexp/syntax.

compiler/compiler.go

@@ -2415,6 +2415,8 @@ func (c *Compiler) parseConvert(typeFrom, typeTo types.Type, value llvm.Value, p
 				switch typeFrom.Elem().(*types.Basic).Kind() {
 				case types.Byte:
 					return c.createRuntimeCall("stringFromBytes", []llvm.Value{value}, ""), nil
+				case types.Rune:
+					return c.createRuntimeCall("stringFromRunes", []llvm.Value{value}, ""), nil
 				default:
 					return llvm.Value{}, c.makeError(pos, "todo: convert to string: "+typeFrom.String())
 				}

src/runtime/string.go

@@ -89,6 +89,30 @@ func stringToBytes(x _string) (slice struct {
 	return
 }
 
+// Convert a []rune slice to a string.
+func stringFromRunes(runeSlice []rune) (s _string) {
+	// Count the number of characters that will be in the string.
+	for _, r := range runeSlice {
+		_, numBytes := encodeUTF8(r)
+		s.length += numBytes
+	}
+
+	// Allocate memory for the string.
+	s.ptr = (*byte)(alloc(s.length))
+
+	// Encode runes to UTF-8 and store the resulting bytes in the string.
+	index := uintptr(0)
+	for _, r := range runeSlice {
+		array, numBytes := encodeUTF8(r)
+		for _, c := range array[:numBytes] {
+			*(*byte)(unsafe.Pointer(uintptr(unsafe.Pointer(s.ptr)) + index)) = c
+			index++
+		}
+	}
+
+	return
+}
+
 // Convert a string to []rune slice.
 func stringToRunes(s string) []rune {
 	var n = 0

testdata/string.go

@@ -13,7 +13,12 @@ func testStringToRunes() {
 	}
 }
 
+func testRunesToString(r []rune) {
+	println("string from runes:", string(r))
+}
+
 func main() {
 	testRangeString()
 	testStringToRunes()
+	testRunesToString([]rune{97, 98, 99, 252, 162, 8364, 66376, 176, 120})
 }

testdata/string.txt

@@ -16,3 +16,4 @@
 6 66376
 7 176
 8 120
+string from runes: abcü¢€𐍈°x