Use build-script constraint to infer sysroot deps in toolchain

Author: dtolnay

toolchains/BUCK

@@ -36,11 +36,11 @@ rust_toolchain(
     compiler = select({
         "rust//constraints:stage1": "rust//stage0:rustc",
         "rust//constraints:stage2": select({
-            "rust//constraints:sysroot-deps=explicit": "rust//stage1:rustc",
-            "rust//constraints:sysroot-deps=implicit": select({
-                "rust//constraints:library": "rust//stage0:rustc",
-                "rust//constraints:compiler": "rust//stage1:rustc",
+            "rust//constraints:library": select({
+                "DEFAULT": "rust//stage1:rustc",
+                "rust//constraints:build-script": "rust//stage0:rustc",
             }),
+            "rust//constraints:compiler": "rust//stage1:rustc",
         }),
     }),
     exec_compatible_with = [
@@ -71,33 +71,33 @@ rust_toolchain(
         "rust//constraints:stage2": "rust//stage1:rustdoc",
     }),
     sysroot = select({
-        "rust//constraints:sysroot-deps=explicit": None,
-        "rust//constraints:sysroot-deps=implicit": select({
-            "rust//constraints:library": "rust//stage0:sysroot",
-            "rust//constraints:compiler": select({
-                "rust//constraints:stage1": dict(
-                    alloc = "rust//stage1:alloc",
-                    compiler_builtins = "rust//stage1:compiler_builtins",
-                    core = "rust//stage1:core",
-                    panic_abort = "rust//stage1:panic_abort",
-                    panic_unwind = "rust//stage1:panic_unwind",
-                    proc_macro = "rust//stage1:proc_macro",
-                    rust_allocator = "rust//stage1:rust_allocator",
-                    std = "rust//stage1:std",
-                    test = "rust//stage1:test",
-                ),
-                "rust//constraints:stage2": dict(
-                    alloc = "rust//stage2:alloc",
-                    compiler_builtins = "rust//stage2:compiler_builtins",
-                    core = "rust//stage2:core",
-                    panic_abort = "rust//stage2:panic_abort",
-                    panic_unwind = "rust//stage2:panic_unwind",
-                    proc_macro = "rust//stage2:proc_macro",
-                    rust_allocator = "rust//stage2:rust_allocator",
-                    std = "rust//stage2:std",
-                    test = "rust//stage2:test",
-                ),
-            }),
+        "rust//constraints:library": select({
+            "DEFAULT": None,
+            "rust//constraints:build-script": "rust//stage0:sysroot",
+        }),
+        "rust//constraints:compiler": select({
+            "rust//constraints:stage1": dict(
+                alloc = "rust//stage1:alloc",
+                compiler_builtins = "rust//stage1:compiler_builtins",
+                core = "rust//stage1:core",
+                panic_abort = "rust//stage1:panic_abort",
+                panic_unwind = "rust//stage1:panic_unwind",
+                proc_macro = "rust//stage1:proc_macro",
+                rust_allocator = "rust//stage1:rust_allocator",
+                std = "rust//stage1:std",
+                test = "rust//stage1:test",
+            ),
+            "rust//constraints:stage2": dict(
+                alloc = "rust//stage2:alloc",
+                compiler_builtins = "rust//stage2:compiler_builtins",
+                core = "rust//stage2:core",
+                panic_abort = "rust//stage2:panic_abort",
+                panic_unwind = "rust//stage2:panic_unwind",
+                proc_macro = "rust//stage2:proc_macro",
+                rust_allocator = "rust//stage2:rust_allocator",
+                std = "rust//stage2:std",
+                test = "rust//stage2:test",
+            ),
         }),
     }),
     visibility = ["PUBLIC"],