Require should_run to be implemented.

src/bootstrap/builder.rs

@@ -70,7 +70,7 @@ pub trait Step: 'static + Clone + Debug + PartialEq + Eq + Hash {
     /// will execute. However, it does not get called in a "default" context
     /// when we are not passed any paths; in that case, make_run is called
     /// directly.
-    fn should_run(_builder: &Builder, _path: &Path) -> bool { false }
+    fn should_run(builder: &Builder, path: &Path) -> bool;
 
     /// Build up a "root" rule, either as a default rule or from a path passed
     /// to us.
@@ -83,7 +83,13 @@ pub trait Step: 'static + Clone + Debug + PartialEq + Eq + Hash {
         _path: Option<&Path>,
         _host: Interned<String>,
         _target: Interned<String>,
-    ) { unimplemented!() }
+    ) {
+        // It is reasonable to not have an implementation of make_run for rules
+        // who do not want to get called from the root context. This means that
+        // they are likely dependencies (e.g., sysroot creation) or similar, and
+        // as such calling them from ./x.py isn't logical.
+        unimplemented!()
+    }
 }
 
 #[derive(Copy, Clone, PartialEq, Eq, Debug)]
@@ -188,6 +194,11 @@ impl<'a> Builder<'a> {
         }
         impl Step for Libdir {
             type Output = Interned<PathBuf>;
+
+            fn should_run(_builder: &Builder, _path: &Path) -> bool {
+                false
+            }
+
             fn run(self, builder: &Builder) -> Interned<PathBuf> {
                 let compiler = self.compiler;
                 let lib = if compiler.stage >= 2 && builder.build.config.libdir_relative.is_some() {

src/bootstrap/check.rs

@@ -1360,6 +1360,10 @@ pub struct RemoteCopyLibs {
 impl Step for RemoteCopyLibs {
     type Output = ();
 
+    fn should_run(_builder: &Builder, _path: &Path) -> bool {
+        false
+    }
+
     fn run(self, builder: &Builder) {
         let build = builder.build;
         let compiler = self.compiler;
@@ -1411,6 +1415,10 @@ pub struct Distcheck;
 impl Step for Distcheck {
     type Output = ();
 
+    fn should_run(_builder: &Builder, path: &Path) -> bool {
+        path.ends_with("distcheck")
+    }
+
     /// Run "distcheck", a 'make check' from a tarball
     fn run(self, builder: &Builder) {
         let build = builder.build;

src/bootstrap/compile.rs

@@ -276,6 +276,10 @@ struct StdLink {
 impl Step for StdLink {
     type Output = ();
 
+    fn should_run(_builder: &Builder, _path: &Path) -> bool {
+        false
+    }
+
     /// Link all libstd rlibs/dylibs into the sysroot location.
     ///
     /// Links those artifacts generated by `compiler` to a the `stage` compiler's
@@ -503,6 +507,10 @@ pub struct TestLink {
 impl Step for TestLink {
     type Output = ();
 
+    fn should_run(_builder: &Builder, _path: &Path) -> bool {
+        false
+    }
+
     /// Same as `std_link`, only for libtest
     fn run(self, builder: &Builder) {
         let build = builder.build;
@@ -691,6 +699,10 @@ struct RustcLink {
 impl Step for RustcLink {
     type Output = ();
 
+    fn should_run(_builder: &Builder, _path: &Path) -> bool {
+        false
+    }
+
     /// Same as `std_link`, only for librustc
     fn run(self, builder: &Builder) {
         let build = builder.build;
@@ -743,6 +755,10 @@ pub struct Sysroot {
 impl Step for Sysroot {
     type Output = Interned<PathBuf>;
 
+    fn should_run(_builder: &Builder, _path: &Path) -> bool {
+        false
+    }
+
     /// Returns the sysroot for the `compiler` specified that *this build system
     /// generates*.
     ///
@@ -789,6 +805,10 @@ pub struct Assemble {
 impl Step for Assemble {
     type Output = Compiler;
 
+    fn should_run(_builder: &Builder, path: &Path) -> bool {
+        path.ends_with("src/rustc")
+    }
+
     /// Prepare a new compiler from the artifacts in `stage`
     ///
     /// This will assemble a compiler in `build/$host/stage$stage`. The compiler

src/bootstrap/doc.rs

@@ -107,6 +107,12 @@ pub struct Rustbook {
 impl Step for Rustbook {
     type Output = ();
 
+    // rustbook is never directly called, and only serves as a shim for the nomicon and the
+    // reference.
+    fn should_run(_builder: &Builder, _path: &Path) -> bool {
+        false
+    }
+
     /// Invoke `rustbook` for `target` for the doc book `name`.
     ///
     /// This will not actually generate any documentation if the documentation has
@@ -182,6 +188,11 @@ pub struct RustbookSrc {
 impl Step for RustbookSrc {
     type Output = ();
 
+    fn should_run(_builder: &Builder, _path: &Path) -> bool {
+        // RustbookSrc is also never run directly, only as a helper to other rules
+        false
+    }
+
     /// Invoke `rustbook` for `target` for the doc book `name` from the `src` path.
     ///
     /// This will not actually generate any documentation if the documentation has

src/bootstrap/native.rs

@@ -55,6 +55,10 @@ impl Step for Llvm {
     type Output = ();
     const ONLY_HOSTS: bool = true;
 
+    fn should_run(_builder: &Builder, path: &Path) -> bool {
+        path.ends_with("src/llvm")
+    }
+
     /// Compile LLVM for `target`.
     fn run(self, builder: &Builder) {
         let build = builder.build;

src/bootstrap/tool.rs

@@ -55,6 +55,10 @@ pub struct CleanTools {
 impl Step for CleanTools {
     type Output = ();
 
+    fn should_run(_builder: &Builder, _path: &Path) -> bool {
+        false
+    }
+
     /// Build a tool in `src/tools`
     ///
     /// This will build the specified tool with the specified `host` compiler in
@@ -89,6 +93,10 @@ pub struct ToolBuild {
 impl Step for ToolBuild {
     type Output = PathBuf;
 
+    fn should_run(_builder: &Builder, _path: &Path) -> bool {
+        false
+    }
+
     /// Build a tool in `src/tools`
     ///
     /// This will build the specified tool with the specified `host` compiler in