Avoid building rustdoc for tests without doctests

src/bootstrap/src/core/builder/cargo.rs

@@ -918,7 +918,10 @@ impl Builder<'_> {
         }
 
         let rustdoc_path = match cmd_kind {
-            Kind::Doc | Kind::Test | Kind::MiriTest => self.rustdoc_for_compiler(compiler),
+            Kind::Doc => self.rustdoc_for_compiler(compiler),
+            Kind::Test | Kind::MiriTest if self.test_target.runs_doctests() => {
+                self.rustdoc_for_compiler(compiler)
+            }
             _ => PathBuf::from("/path/to/nowhere/rustdoc/not/required"),
         };
 

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

@@ -2423,6 +2423,19 @@ mod snapshot {
         }
     }
 
+    #[test]
+    fn test_library_tests_only_does_not_build_rustdoc() {
+        let ctx = TestCtx::new();
+        let host = TargetSelection::from_user(&host_target());
+        insta::assert_snapshot!(
+            ctx.config("test").args(&["--tests", "library/core"]).render_steps(),
+            @r"
+            [build] llvm <host>
+            [build] rustc 0 <host> -> rustc 1 <host>
+            [build] rustc 1 <host> -> std 1 <host>
+            ");
+    }
+
     #[test]
     fn test_cargo_stage_1() {
         let ctx = TestCtx::new();