Switch Steps from crates to crate_or_deps where possible

and document why the single remaining place can't switch

Author: jyn514

src/bootstrap/builder.rs

@@ -442,6 +442,8 @@ impl<'a> ShouldRun<'a> {
     /// Indicates it should run if the command-line selects any of the given crates.
     ///
     /// `make_run` will be called a single time with all matching command-line paths.
+    ///
+    /// Prefer [`ShouldRun::crate_or_deps`] to this function where possible.
     pub(crate) fn crates(mut self, crates: Vec<&Crate>) -> Self {
         for krate in crates {
             let path = krate.local_path(self.builder);

src/bootstrap/check.rs

@@ -85,8 +85,7 @@ impl Step for Std {
     const DEFAULT: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        let crates = run.builder.in_tree_crates("sysroot", None);
-        run.crates(crates).path("library")
+        run.crate_or_deps("sysroot").path("library")
     }
 
     fn make_run(run: RunConfig<'_>) {
@@ -215,8 +214,7 @@ impl Step for Rustc {
     const DEFAULT: bool = true;
 
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
-        let crates = run.builder.in_tree_crates("rustc-main", None);
-        run.crates(crates).path("compiler")
+        run.crate_or_deps("rustc-main").path("compiler")
     }
 
     fn make_run(run: RunConfig<'_>) {

src/bootstrap/compile.rs

@@ -620,6 +620,8 @@ impl Step for Rustc {
     fn should_run(run: ShouldRun<'_>) -> ShouldRun<'_> {
         let mut crates = run.builder.in_tree_crates("rustc-main", None);
         for (i, krate) in crates.iter().enumerate() {
+            // We can't allow `build rustc` as an alias for this Step, because that's reserved by `Assemble`.
+            // Ideally Assemble would use `build compiler` instead, but that seems too confusing to be worth the breaking change.
             if krate.name == "rustc-main" {
                 crates.swap_remove(i);
                 break;