Do not explicitly build Rustc when bootstrap is invoked with the compiler path

It was confusing that the `Rustc` step was invoked twice, once with all crates and once with no crates (even though both of these mean the same thing).

Author: Kobzol

src/bootstrap/src/core/build_steps/compile.rs

@@ -1013,6 +1013,12 @@ impl Step for Rustc {
     }
 
     fn make_run(run: RunConfig<'_>) {
+        // If only `compiler` was passed, do not run this step.
+        // Instead the `Assemble` step will take care of compiling Rustc.
+        if run.builder.paths == vec![PathBuf::from("compiler")] {
+            return;
+        }
+
         let crates = run.cargo_crates_in_set();
         run.builder.ensure(Rustc {
             build_compiler: run

src/bootstrap/src/core/builder/tests.rs

@@ -1244,24 +1244,19 @@ mod staging {
         insta::assert_snapshot!(get_steps("compiler", "build", None), @r"
         [build] llvm <target1>
         [build] rustc 0 <target1> -> rustc 1 <target1>
-        [build] rustc 0 <target1> -> rustc 1 <target1>
         ");
     }
 
     #[test]
     fn build_compiler_stage_0() {
-        insta::assert_snapshot!(get_steps("compiler", "build", Some(0)), @r"
-        [build] llvm <target1>
-        [build] rustc 0 <target1> -> rustc 1 <target1>
-        ");
+        insta::assert_snapshot!(get_steps("compiler", "build", Some(0)), @"");
     }
 
     #[test]
     fn build_compiler_stage_1() {
         insta::assert_snapshot!(get_steps("compiler", "build", Some(1)), @r"
         [build] llvm <target1>
         [build] rustc 0 <target1> -> rustc 1 <target1>
-        [build] rustc 0 <target1> -> rustc 1 <target1>
         ");
     }
 
@@ -1272,12 +1267,11 @@ mod staging {
         [build] rustc 0 <target1> -> rustc 1 <target1>
         [build] rustc 1 <target1> -> std 1 <target1>
         [build] rustc 1 <target1> -> rustc 2 <target1>
-        [build] rustc 1 <target1> -> rustc 2 <target1>
         ");
     }
 
     fn get_steps(path: &str, kind: &str, stage: Option<u32>) -> String {
-        let mut args = vec![kind];
+        let mut args = vec![kind, path];
         let stage_str = stage.map(|v| v.to_string()).unwrap_or_default();
         if let Some(stage) = stage {
             args.push("--stage");