bootstrap: Always build for host, even when target is given

This changes the behavior from *not* building for host whenever an
explicit target is specified. I find this much less confusing.

You can still disable host steps by passing an explicit empty list for
host.

Fixes rust-lang#76990.

Author: tmandry

src/bootstrap/builder/tests.rs

@@ -342,31 +342,29 @@ mod dist {
     }
 
     #[test]
-    fn dist_with_target_flag() {
-        let mut config = configure(&["B"], &["C"]);
-        config.skip_only_host_steps = true; // as-if --target=C was passed
+    fn dist_with_empty_host() {
+        let mut config = configure(&[], &["C"]);
+        config.skip_only_host_steps = true;
         let build = Build::new(config);
         let mut builder = Builder::new(&build);
         builder.run_step_descriptions(&Builder::get_step_descriptions(Kind::Dist), &[]);
 
         let a = TargetSelection::from_user("A");
-        let b = TargetSelection::from_user("B");
         let c = TargetSelection::from_user("C");
 
         assert_eq!(
             first(builder.cache.all::<dist::Docs>()),
-            &[dist::Docs { host: a }, dist::Docs { host: b }, dist::Docs { host: c },]
+            &[dist::Docs { host: a }, dist::Docs { host: c },]
         );
         assert_eq!(
             first(builder.cache.all::<dist::Mingw>()),
-            &[dist::Mingw { host: a }, dist::Mingw { host: b }, dist::Mingw { host: c },]
+            &[dist::Mingw { host: a }, dist::Mingw { host: c },]
         );
         assert_eq!(first(builder.cache.all::<dist::Rustc>()), &[]);
         assert_eq!(
             first(builder.cache.all::<dist::Std>()),
             &[
                 dist::Std { compiler: Compiler { host: a, stage: 1 }, target: a },
-                dist::Std { compiler: Compiler { host: a, stage: 1 }, target: b },
                 dist::Std { compiler: Compiler { host: a, stage: 2 }, target: c },
             ]
         );
@@ -464,15 +462,14 @@ mod dist {
     }
 
     #[test]
-    fn build_with_target_flag() {
-        let mut config = configure(&["B"], &["C"]);
+    fn build_with_empty_host() {
+        let mut config = configure(&[], &["C"]);
         config.skip_only_host_steps = true;
         let build = Build::new(config);
         let mut builder = Builder::new(&build);
         builder.run_step_descriptions(&Builder::get_step_descriptions(Kind::Build), &[]);
 
         let a = TargetSelection::from_user("A");
-        let b = TargetSelection::from_user("B");
         let c = TargetSelection::from_user("C");
 
         assert_eq!(
@@ -481,8 +478,6 @@ mod dist {
                 compile::Std { compiler: Compiler { host: a, stage: 0 }, target: a },
                 compile::Std { compiler: Compiler { host: a, stage: 1 }, target: a },
                 compile::Std { compiler: Compiler { host: a, stage: 2 }, target: a },
-                compile::Std { compiler: Compiler { host: a, stage: 1 }, target: b },
-                compile::Std { compiler: Compiler { host: a, stage: 2 }, target: b },
                 compile::Std { compiler: Compiler { host: a, stage: 2 }, target: c },
             ]
         );

src/bootstrap/config.rs

@@ -586,18 +586,15 @@ impl Config {
 
         let build = toml.build.unwrap_or_default();
 
-        // If --target was specified but --host wasn't specified, don't run any host-only tests.
-        let has_hosts = build.host.is_some() || flags.host.is_some();
-        let has_targets = build.target.is_some() || flags.target.is_some();
-        config.skip_only_host_steps = !has_hosts && has_targets;
-
         config.hosts = if let Some(arg_host) = flags.host {
             arg_host
         } else if let Some(file_host) = build.host {
             file_host.iter().map(|h| TargetSelection::from_user(h)).collect()
         } else {
             vec![config.build]
         };
+        // If host was explicitly given an empty list, don't run any host-only steps.
+        config.skip_only_host_steps = config.hosts.is_empty();
         config.targets = if let Some(arg_target) = flags.target {
             arg_target
         } else if let Some(file_target) = build.target {