Add step metadata and a simple test for codegen backends

Author: Kobzol

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

@@ -1705,6 +1705,13 @@ impl Step for CodegenBackend {
         let codegen_backend = codegen_backend.to_str().unwrap();
         t!(stamp.add_stamp(codegen_backend).write());
     }
+
+    fn metadata(&self) -> Option<StepMetadata> {
+        Some(
+            StepMetadata::build(&format!("rustc_codegen_{}", self.backend), self.target)
+                .built_by(self.compiler),
+        )
+    }
 }
 
 /// Creates the `codegen-backends` folder for a compiler that's about to be

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

@@ -719,6 +719,26 @@ mod snapshot {
         ");
     }
 
+    #[test]
+    fn build_compiler_codegen_backend() {
+        let ctx = TestCtx::new();
+        insta::assert_snapshot!(
+            ctx
+                .config("build")
+                .args(&["--set", "rust.codegen-backends=['llvm', 'cranelift']"])
+                .render_steps(), @r"
+        [build] llvm <host>
+        [build] rustc 0 <host> -> rustc 1 <host>
+        [build] rustc 0 <host> -> rustc_codegen_cranelift 1 <host>
+        [build] rustc 1 <host> -> std 1 <host>
+        [build] rustc 1 <host> -> std 1 <host>
+        [build] rustc 1 <host> -> rustc 2 <host>
+        [build] rustc 1 <host> -> rustc_codegen_cranelift 2 <host>
+        [build] rustdoc 1 <host>
+        "
+        );
+    }
+
     #[test]
     fn build_compiler_tools() {
         let ctx = TestCtx::new();