Remove host parameter from step configurations

rustc is a natively cross-compiling compiler, and generally none of our steps
should care whether they are using a compiler built of triple A or B, just the
--target directive being passed to the running compiler. e.g., when building for
some target C, you don't generally want to build two stds: one with a host A
compiler and the other with a host B compiler. Just one std is sufficient.

src/bootstrap/builder.rs

@@ -87,11 +87,16 @@ pub trait Step: 'static + Clone + Debug + PartialEq + Eq + Hash {
 
 pub struct RunConfig<'a> {
     pub builder: &'a Builder<'a>,
-    pub host: TargetSelection,
     pub target: TargetSelection,
     pub path: PathBuf,
 }
 
+impl RunConfig<'_> {
+    pub fn build_triple(&self) -> TargetSelection {
+        self.builder.build.build
+    }
+}
+
 struct StepDescription {
     default: bool,
     only_hosts: bool,
@@ -165,7 +170,6 @@ impl StepDescription {
                 pathset, self.name, builder.config.exclude
             );
         }
-        let hosts = &builder.hosts;
 
         // Determine the targets participating in this rule.
         let targets = if self.only_hosts {
@@ -178,16 +182,9 @@ impl StepDescription {
             &builder.targets
         };
 
-        for host in hosts {
-            for target in targets {
-                let run = RunConfig {
-                    builder,
-                    path: pathset.path(builder),
-                    host: *host,
-                    target: *target,
-                };
-                (self.make_run)(run);
-            }
+        for target in targets {
+            let run = RunConfig { builder, path: pathset.path(builder), target: *target };
+            (self.make_run)(run);
         }
     }
 

src/bootstrap/builder/tests.rs

@@ -384,12 +384,9 @@ 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: b, 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: b, stage: 2 }, target: b },
                 compile::Std { compiler: Compiler { host: a, stage: 2 }, target: c },
-                compile::Std { compiler: Compiler { host: b, stage: 2 }, target: c },
             ]
         );
         assert!(!builder.cache.all::<compile::Assemble>().is_empty());
@@ -399,10 +396,8 @@ mod dist {
                 compile::Rustc { compiler: Compiler { host: a, stage: 0 }, target: a },
                 compile::Rustc { compiler: Compiler { host: a, stage: 1 }, target: a },
                 compile::Rustc { compiler: Compiler { host: a, stage: 2 }, target: a },
-                compile::Rustc { compiler: Compiler { host: b, stage: 2 }, target: a },
                 compile::Rustc { compiler: Compiler { host: a, stage: 1 }, target: b },
                 compile::Rustc { compiler: Compiler { host: a, stage: 2 }, target: b },
-                compile::Rustc { compiler: Compiler { host: b, stage: 2 }, target: b },
             ]
         );
     }
@@ -425,12 +420,9 @@ 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: b, 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: b, stage: 2 }, target: b },
                 compile::Std { compiler: Compiler { host: a, stage: 2 }, target: c },
-                compile::Std { compiler: Compiler { host: b, stage: 2 }, target: c },
             ]
         );
         assert_eq!(
@@ -439,7 +431,6 @@ mod dist {
                 compile::Assemble { target_compiler: Compiler { host: a, stage: 0 } },
                 compile::Assemble { target_compiler: Compiler { host: a, stage: 1 } },
                 compile::Assemble { target_compiler: Compiler { host: a, stage: 2 } },
-                compile::Assemble { target_compiler: Compiler { host: b, stage: 2 } },
             ]
         );
         assert_eq!(
@@ -447,7 +438,6 @@ mod dist {
             &[
                 compile::Rustc { compiler: Compiler { host: a, stage: 0 }, target: a },
                 compile::Rustc { compiler: Compiler { host: a, stage: 1 }, target: a },
-                compile::Rustc { compiler: Compiler { host: a, stage: 1 }, target: b },
             ]
         );
     }

src/bootstrap/compile.rs

@@ -45,7 +45,7 @@ impl Step for Std {
 
     fn make_run(run: RunConfig<'_>) {
         run.builder.ensure(Std {
-            compiler: run.builder.compiler(run.builder.top_stage, run.host),
+            compiler: run.builder.compiler(run.builder.top_stage, run.build_triple()),
             target: run.target,
         });
     }
@@ -385,7 +385,7 @@ impl Step for StartupObjects {
 
     fn make_run(run: RunConfig<'_>) {
         run.builder.ensure(StartupObjects {
-            compiler: run.builder.compiler(run.builder.top_stage, run.host),
+            compiler: run.builder.compiler(run.builder.top_stage, run.build_triple()),
             target: run.target,
         });
     }
@@ -454,7 +454,7 @@ impl Step for Rustc {
 
     fn make_run(run: RunConfig<'_>) {
         run.builder.ensure(Rustc {
-            compiler: run.builder.compiler(run.builder.top_stage, run.host),
+            compiler: run.builder.compiler(run.builder.top_stage, run.build_triple()),
             target: run.target,
         });
     }

src/bootstrap/dist.rs

@@ -605,7 +605,9 @@ impl Step for DebuggerScripts {
 
     fn make_run(run: RunConfig<'_>) {
         run.builder.ensure(DebuggerScripts {
-            sysroot: run.builder.sysroot(run.builder.compiler(run.builder.top_stage, run.host)),
+            sysroot: run
+                .builder
+                .sysroot(run.builder.compiler(run.builder.top_stage, run.build_triple())),
             host: run.target,
         });
     }

src/bootstrap/test.rs

@@ -584,7 +584,7 @@ impl Step for RustdocTheme {
     }
 
     fn make_run(run: RunConfig<'_>) {
-        let compiler = run.builder.compiler(run.builder.top_stage, run.host);
+        let compiler = run.builder.compiler(run.builder.top_stage, run.target);
 
         run.builder.ensure(RustdocTheme { compiler });
     }
@@ -651,7 +651,6 @@ impl Step for RustdocJSStd {
 
 #[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
 pub struct RustdocJSNotStd {
-    pub host: TargetSelection,
     pub target: TargetSelection,
     pub compiler: Compiler,
 }
@@ -666,8 +665,8 @@ impl Step for RustdocJSNotStd {
     }
 
     fn make_run(run: RunConfig<'_>) {
-        let compiler = run.builder.compiler(run.builder.top_stage, run.host);
-        run.builder.ensure(RustdocJSNotStd { host: run.host, target: run.target, compiler });
+        let compiler = run.builder.compiler(run.builder.top_stage, run.build_triple());
+        run.builder.ensure(RustdocJSNotStd { target: run.target, compiler });
     }
 
     fn run(self, builder: &Builder<'_>) {
@@ -688,7 +687,6 @@ impl Step for RustdocJSNotStd {
 
 #[derive(Debug, Copy, Clone, Hash, PartialEq, Eq)]
 pub struct RustdocUi {
-    pub host: TargetSelection,
     pub target: TargetSelection,
     pub compiler: Compiler,
 }
@@ -703,8 +701,8 @@ impl Step for RustdocUi {
     }
 
     fn make_run(run: RunConfig<'_>) {
-        let compiler = run.builder.compiler(run.builder.top_stage, run.host);
-        run.builder.ensure(RustdocUi { host: run.host, target: run.target, compiler });
+        let compiler = run.builder.compiler(run.builder.top_stage, run.build_triple());
+        run.builder.ensure(RustdocUi { target: run.target, compiler });
     }
 
     fn run(self, builder: &Builder<'_>) {
@@ -873,7 +871,7 @@ macro_rules! test_definitions {
             }
 
             fn make_run(run: RunConfig<'_>) {
-                let compiler = run.builder.compiler(run.builder.top_stage, run.host);
+                let compiler = run.builder.compiler(run.builder.top_stage, run.build_triple());
 
                 run.builder.ensure($name { compiler, target: run.target });
             }
@@ -1422,7 +1420,7 @@ macro_rules! test_book {
 
                 fn make_run(run: RunConfig<'_>) {
                     run.builder.ensure($name {
-                        compiler: run.builder.compiler(run.builder.top_stage, run.host),
+                        compiler: run.builder.compiler(run.builder.top_stage, run.target),
                     });
                 }
 
@@ -1469,7 +1467,7 @@ impl Step for ErrorIndex {
         // error_index_generator depends on librustdoc. Use the compiler that
         // is normally used to build rustdoc for other tests (like compiletest
         // tests in src/test/rustdoc) so that it shares the same artifacts.
-        let compiler = run.builder.compiler_for(run.builder.top_stage, run.host, run.host);
+        let compiler = run.builder.compiler_for(run.builder.top_stage, run.target, run.target);
         run.builder.ensure(ErrorIndex { compiler });
     }
 
@@ -1573,7 +1571,7 @@ impl Step for CrateLibrustc {
 
     fn make_run(run: RunConfig<'_>) {
         let builder = run.builder;
-        let compiler = builder.compiler(builder.top_stage, run.host);
+        let compiler = builder.compiler(builder.top_stage, run.build_triple());
 
         for krate in builder.in_tree_crates("rustc-main") {
             if krate.path.ends_with(&run.path) {
@@ -1620,7 +1618,7 @@ impl Step for CrateNotDefault {
 
     fn make_run(run: RunConfig<'_>) {
         let builder = run.builder;
-        let compiler = builder.compiler(builder.top_stage, run.host);
+        let compiler = builder.compiler(builder.top_stage, run.build_triple());
 
         let test_kind = builder.kind.into();
 
@@ -1668,7 +1666,7 @@ impl Step for Crate {
 
     fn make_run(run: RunConfig<'_>) {
         let builder = run.builder;
-        let compiler = builder.compiler(builder.top_stage, run.host);
+        let compiler = builder.compiler(builder.top_stage, run.build_triple());
 
         let make = |mode: Mode, krate: &CargoCrate| {
             let test_kind = builder.kind.into();
@@ -1808,7 +1806,7 @@ impl Step for CrateRustdoc {
 
         let test_kind = builder.kind.into();
 
-        builder.ensure(CrateRustdoc { host: run.host, test_kind });
+        builder.ensure(CrateRustdoc { host: run.target, test_kind });
     }
 
     fn run(self, builder: &Builder<'_>) {
@@ -2054,7 +2052,6 @@ impl Step for Bootstrap {
 #[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
 pub struct TierCheck {
     pub compiler: Compiler,
-    target: TargetSelection,
 }
 
 impl Step for TierCheck {
@@ -2067,18 +2064,19 @@ impl Step for TierCheck {
     }
 
     fn make_run(run: RunConfig<'_>) {
-        let compiler = run.builder.compiler_for(run.builder.top_stage, run.host, run.host);
-        run.builder.ensure(TierCheck { compiler, target: run.host });
+        let compiler =
+            run.builder.compiler_for(run.builder.top_stage, run.builder.build.build, run.target);
+        run.builder.ensure(TierCheck { compiler });
     }
 
     /// Tests the Platform Support page in the rustc book.
     fn run(self, builder: &Builder<'_>) {
-        builder.ensure(compile::Std { compiler: self.compiler, target: self.target });
+        builder.ensure(compile::Std { compiler: self.compiler, target: self.compiler.host });
         let mut cargo = tool::prepare_tool_cargo(
             builder,
             self.compiler,
-            Mode::ToolRustc,
-            self.target,
+            Mode::ToolStd,
+            self.compiler.host,
             "run",
             "src/tools/tier-check",
             SourceType::InTree,

src/bootstrap/tool.rs

@@ -469,8 +469,9 @@ impl Step for Rustdoc {
     }
 
     fn make_run(run: RunConfig<'_>) {
-        run.builder
-            .ensure(Rustdoc { compiler: run.builder.compiler(run.builder.top_stage, run.host) });
+        run.builder.ensure(Rustdoc {
+            compiler: run.builder.compiler(run.builder.top_stage, run.build_triple()),
+        });
     }
 
     fn run(self, builder: &Builder<'_>) -> PathBuf {