targets: let specific targets inherit more general targets

This avoids a ton of duplication and makes it easier to change a generic
target (for example, the "cortex-m" target) for all boards that use it.

Also, by making it possible to inherit properties from a parent target
specification, it is easier to support out-of-tree boards that don't
have to be updated so often. A target specification for a
special-purpose board can simply inherit the specification of a
supported chip and override the properites it needs to override (like
the programming interface).

target.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"encoding/json"
+	"io"
 	"os"
 	"os/user"
 	"path/filepath"
@@ -17,6 +18,7 @@ import (
 // https://doc.rust-lang.org/nightly/nightly-rustc/rustc_target/spec/struct.TargetOptions.html
 // https://github.com/shepmaster/rust-arduino-blink-led-no-core-with-cargo/blob/master/blink/arduino.json
 type TargetSpec struct {
+	Inherits    []string `json:"inherits"`
 	Triple      string   `json:"llvm-target"`
 	BuildTags   []string `json:"build-tags"`
 	Linker      string   `json:"linker"`
@@ -30,40 +32,127 @@ type TargetSpec struct {
 	GDBCmds     []string `json:"gdb-initial-cmds"`
 }
 
-// Load a target specification
+// copyProperties copies all properties that are set in spec2 into itself.
+func (spec *TargetSpec) copyProperties(spec2 *TargetSpec) {
+	// TODO: simplify this using reflection? Inherits and BuildTags are special
+	// cases, but the rest can simply be copied if set.
+	spec.Inherits = append(spec.Inherits, spec2.Inherits...)
+	if spec2.Triple != "" {
+		spec.Triple = spec2.Triple
+	}
+	spec.BuildTags = append(spec.BuildTags, spec2.BuildTags...)
+	if spec2.Linker != "" {
+		spec.Linker = spec2.Linker
+	}
+	if spec2.RTLib != "" {
+		spec.RTLib = spec2.RTLib
+	}
+	if len(spec2.PreLinkArgs) != 0 {
+		spec.PreLinkArgs = spec2.PreLinkArgs
+	}
+	if spec2.Objcopy != "" {
+		spec.Objcopy = spec2.Objcopy
+	}
+	if len(spec2.Emulator) != 0 {
+		spec.Emulator = spec2.Emulator
+	}
+	if spec2.Flasher != "" {
+		spec.Flasher = spec2.Flasher
+	}
+	if len(spec2.OCDDaemon) != 0 {
+		spec.OCDDaemon = spec2.OCDDaemon
+	}
+	if spec2.GDB != "" {
+		spec.GDB = spec2.GDB
+	}
+	if len(spec2.GDBCmds) != 0 {
+		spec.GDBCmds = spec2.GDBCmds
+	}
+}
+
+// load reads a target specification from the JSON in the given io.Reader. It
+// may load more targets specified using the "inherits" property.
+func (spec *TargetSpec) load(r io.Reader) error {
+	err := json.NewDecoder(r).Decode(spec)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// loadFromName loads the given target from the targets/ directory inside the
+// compiler sources.
+func (spec *TargetSpec) loadFromName(name string) error {
+	path := filepath.Join(sourceDir(), "targets", strings.ToLower(name)+".json")
+	fp, err := os.Open(path)
+	if err != nil {
+		return err
+	}
+	defer fp.Close()
+	return spec.load(fp)
+}
+
+// resolveInherits loads inherited targets, recursively.
+func (spec *TargetSpec) resolveInherits() error {
+	// First create a new spec with all the inherited properties.
+	newSpec := &TargetSpec{}
+	for _, name := range spec.Inherits {
+		subtarget := &TargetSpec{}
+		err := subtarget.loadFromName(name)
+		if err != nil {
+			return err
+		}
+		err = subtarget.resolveInherits()
+		if err != nil {
+			return err
+		}
+		newSpec.copyProperties(subtarget)
+	}
+
+	// When all properties are loaded, make sure they are properly inherited.
+	newSpec.copyProperties(spec)
+	*spec = *newSpec
+
+	return nil
+}
+
+// Load a target specification.
 func LoadTarget(target string) (*TargetSpec, error) {
 	if target == "" {
 		target = llvm.DefaultTargetTriple()
 	}
 
-	spec := &TargetSpec{
-		Triple:      target,
-		BuildTags:   []string{runtime.GOOS, runtime.GOARCH},
-		Linker:      "cc",
-		PreLinkArgs: []string{"-no-pie"}, // WARNING: clang < 5.0 requires -nopie
-		Objcopy:     "objcopy",
-		GDB:         "gdb",
-		GDBCmds:     []string{"run"},
-	}
-
 	// See whether there is a target specification for this target (e.g.
 	// Arduino).
-	path := filepath.Join(sourceDir(), "targets", strings.ToLower(target)+".json")
-	if fp, err := os.Open(path); err == nil {
-		defer fp.Close()
-		*spec = TargetSpec{} // reset all fields
-		err := json.NewDecoder(fp).Decode(spec)
+	spec := &TargetSpec{}
+	err := spec.loadFromName(target)
+	if err == nil {
+		// Successfully loaded this target from a built-in .json file. Make sure
+		// it includes all parents as specified in the "inherits" key.
+		err = spec.resolveInherits()
 		if err != nil {
 			return nil, err
 		}
+		return spec, nil
 	} else if !os.IsNotExist(err) {
-		// Expected a 'file not found' error, got something else.
+		// Expected a 'file not found' error, got something else. Report it as
+		// an error.
 		return nil, err
 	} else {
-		// No target spec available. Use the default one.
+		// No target spec available. Use the default one, useful on most systems
+		// with a regular OS.
+		*spec = TargetSpec{
+			Triple:      target,
+			BuildTags:   []string{runtime.GOOS, runtime.GOARCH},
+			Linker:      "cc",
+			PreLinkArgs: []string{"-no-pie"}, // WARNING: clang < 5.0 requires -nopie
+			Objcopy:     "objcopy",
+			GDB:         "gdb",
+			GDBCmds:     []string{"run"},
+		}
+		return spec, nil
 	}
-
-	return spec, nil
 }
 
 // Return the source directory of this package, or "." when it cannot be