cgo: implement rudimentary C array decaying

This is just a first step. It's not complete, but it gets some real
world C code to parse.

This signature, from the ESP-IDF:

    esp_err_t esp_wifi_get_mac(wifi_interface_t ifx, uint8_t mac[6]);

Was previously converted to something like this (pseudocode):

    C.esp_err_t esp_wifi_get_mac(ifx C.wifi_interface_t, mac [6]uint8)

But this is not correct. C array parameters will decay. The array is
passed by reference instead of by value. Instead, this would be the
correct signature:

    C.esp_err_t esp_wifi_get_mac(ifx C.wifi_interface_t, mac *uint8)

So that it can be called like this (using CGo):

    var mac [6]byte
    errCode := C.esp_wifi_get_mac(C.ESP_IF_WIFI_AP, &mac[0])

This stores the result in the 6-element array mac.

cgo/libclang.go

@@ -196,7 +196,7 @@ func tinygo_clang_globals_visitor(c, parent C.GoCXCursor, client_data C.CXClient
 			}
 			fn.args = append(fn.args, paramInfo{
 				name:     argName,
-				typeExpr: p.makeASTType(argType, pos),
+				typeExpr: p.makeDecayingASTType(argType, pos),
 			})
 		}
 		resultType := C.tinygo_clang_getCursorResultType(c)
@@ -391,6 +391,41 @@ func (p *cgoPackage) addErrorAt(position token.Position, msg string) {
 	})
 }
 
+// makeDecayingASTType does the same as makeASTType but takes care of decaying
+// types (arrays in function parameters, etc). It is otherwise identical to
+// makeASTType.
+func (p *cgoPackage) makeDecayingASTType(typ C.CXType, pos token.Pos) ast.Expr {
+	// Strip typedefs, if any.
+	underlyingType := typ
+	if underlyingType.kind == C.CXType_Typedef {
+		c := C.tinygo_clang_getTypeDeclaration(typ)
+		underlyingType = C.tinygo_clang_getTypedefDeclUnderlyingType(c)
+		// TODO: support a chain of typedefs. At the moment, it seems to get
+		// stuck in an endless loop when trying to get to the most underlying
+		// type.
+	}
+	// Check for decaying type. An example would be an array type in a
+	// parameter. This declaration:
+	//   void foo(char buf[6]);
+	// is the same as this one:
+	//   void foo(char *buf);
+	// But this one:
+	//   void bar(char buf[6][4]);
+	// equals this:
+	//   void bar(char *buf[4]);
+	// so not all array dimensions should be stripped, just the first one.
+	// TODO: there are more kinds of decaying types.
+	if underlyingType.kind == C.CXType_ConstantArray {
+		// Apply type decaying.
+		pointeeType := C.clang_getElementType(underlyingType)
+		return &ast.StarExpr{
+			Star: pos,
+			X:    p.makeASTType(pointeeType, pos),
+		}
+	}
+	return p.makeASTType(typ, pos)
+}
+
 // makeASTType return the ast.Expr for the given libclang type. In other words,
 // it converts a libclang type to a type in the Go AST.
 func (p *cgoPackage) makeASTType(typ C.CXType, pos token.Pos) ast.Expr {

testdata/cgo/main.c

@@ -61,3 +61,7 @@ void unionSetData(short f0, short f1, short f2) {
 	globalUnion.data[1] = 8;
 	globalUnion.data[2] = 1;
 }
+
+void arraydecay(int buf1[5], int buf2[3][8], int buf3[4][7][2]) {
+	// Do nothing.
+}

testdata/cgo/main.go

@@ -128,6 +128,12 @@ func main() {
 	// Check whether CFLAGS are correctly passed on to compiled C files.
 	println("CFLAGS value:", C.cflagsConstant)
 
+	// Check array-to-pointer decaying. This signature:
+	//   void arraydecay(int buf1[5], int buf2[3][8], int buf3[4][7][2]);
+	// decays to:
+	//   void arraydecay(int *buf1, int *buf2[8], int *buf3[7][2]);
+	C.arraydecay((*C.int)(nil), (*[8]C.int)(nil), (*[7][2]C.int)(nil))
+
 	// libc: test whether C functions work at all.
 	buf1 := []byte("foobar\x00")
 	buf2 := make([]byte, len(buf1))

testdata/cgo/main.h

@@ -144,3 +144,7 @@ extern int cflagsConstant;
 // test duplicate definitions
 int add(int a, int b);
 extern int global;
+
+// Test array decaying into a pointer.
+typedef int arraydecay_buf3[4][7][2];
+void arraydecay(int buf1[5], int buf2[3][8], arraydecay_buf3 buf3);