Add a false version of build-script constraint

Author: dtolnay

constraints/BUCK

@@ -34,6 +34,10 @@ constraint(
 
 constraint(
     setting = "build-script",
+    values = [
+        "build-script=false",
+        "build-script=true",
+    ],
 )
 
 constraint(

platforms/stage1/BUCK

@@ -4,6 +4,7 @@ platform(
     name = "library",
     base = "//platforms:host",
     constraint_values = [
+        "//constraints:build-script=false",
         "//constraints:library",
         "//constraints:opt-level=3",
         "//constraints:stage1",
@@ -16,7 +17,7 @@ platform(
     name = "library-build-script",
     base = ":library",
     constraint_values = [
-        "//constraints:build-script",
+        "//constraints:build-script=true",
         "//constraints:opt-level=0",
         "//constraints:sysroot-deps=implicit",
     ],
@@ -27,6 +28,7 @@ platform(
     name = "compiler",
     base = "//platforms:host",
     constraint_values = [
+        "//constraints:build-script=false",
         "//constraints:compiler",
         "//constraints:opt-level=3",
         "//constraints:stage1",
@@ -39,7 +41,7 @@ platform(
     name = "compiler-build-script",
     base = ":compiler",
     constraint_values = [
-        "//constraints:build-script",
+        "//constraints:build-script=true",
         "//constraints:opt-level=0",
     ],
     visibility = ["PUBLIC"],

platforms/stage2/BUCK

@@ -4,6 +4,7 @@ platform(
     name = "library",
     base = "//platforms:host",
     constraint_values = [
+        "//constraints:build-script=false",
         "//constraints:library",
         "//constraints:opt-level=3",
         "//constraints:stage2",
@@ -16,7 +17,7 @@ platform(
     name = "library-build-script",
     base = ":library",
     constraint_values = [
-        "//constraints:build-script",
+        "//constraints:build-script=true",
         "//constraints:opt-level=0",
         "//constraints:sysroot-deps=implicit",
     ],
@@ -27,6 +28,7 @@ platform(
     name = "compiler",
     base = "//platforms:host",
     constraint_values = [
+        "//constraints:build-script=false",
         "//constraints:compiler",
         "//constraints:opt-level=3",
         "//constraints:stage2",
@@ -39,7 +41,7 @@ platform(
     name = "compiler-build-script",
     base = ":compiler",
     constraint_values = [
-        "//constraints:build-script",
+        "//constraints:build-script=true",
         "//constraints:opt-level=0",
     ],
     visibility = ["PUBLIC"],

toolchains/BUCK

@@ -37,8 +37,8 @@ rust_toolchain(
         "rust//constraints:stage1": "rust//stage0:rustc",
         "rust//constraints:stage2": select({
             "rust//constraints:library": select({
-                "DEFAULT": "rust//stage1:rustc",
-                "rust//constraints:build-script": "rust//stage0:rustc",
+                "rust//constraints:build-script=false": "rust//stage1:rustc",
+                "rust//constraints:build-script=true": "rust//stage0:rustc",
             }),
             "rust//constraints:compiler": "rust//stage1:rustc",
         }),
@@ -72,8 +72,8 @@ rust_toolchain(
     }),
     sysroot = select({
         "rust//constraints:library": select({
-            "DEFAULT": None,
-            "rust//constraints:build-script": "rust//stage0:sysroot",
+            "rust//constraints:build-script=false": None,
+            "rust//constraints:build-script=true": "rust//stage0:sysroot",
         }),
         "rust//constraints:compiler": select({
             "rust//constraints:stage1": dict(