Add feature gate for target-applies-to-host.

src/cargo/core/compiler/build_context/target_info.rs

@@ -691,10 +691,14 @@ impl<'cfg> RustcTargetData<'cfg> {
         // `--target` flag is not specified. Since the unit_dependency code
         // needs access to the target config data, create a copy so that it
         // can be found. See `rebuild_unit_graph_shared` for why this is done.
+        let target_applies_to_host = config.target_applies_to_host();
+        if target_applies_to_host.is_err() {
+            return Err(target_applies_to_host.unwrap_err());
+        }
         let host_config = if requested_kinds.iter().any(CompileKind::is_host) {
             let ct = CompileTarget::new(&rustc.host)?;
             target_info.insert(ct, host_info.clone());
-            let target_host_config = if config.target_applies_to_host() {
+            let target_host_config = if target_applies_to_host.unwrap() {
                 let target_cfg_clone = config.target_cfg_triple(&rustc.host)?;
                 target_config.insert(ct, target_cfg_clone.clone());
                 target_cfg_clone
@@ -704,7 +708,7 @@ impl<'cfg> RustcTargetData<'cfg> {
             };
             target_host_config
         } else {
-            if config.target_applies_to_host() {
+            if target_applies_to_host.unwrap() {
                 config.target_cfg_triple(&rustc.host)?
             } else {
                 config.host_cfg_triple(&rustc.host)?

src/cargo/core/features.rs

@@ -601,6 +601,7 @@ unstable_cli_options!(
     no_index_update: bool = ("Do not update the registry index even if the cache is outdated"),
     panic_abort_tests: bool = ("Enable support to run tests with -Cpanic=abort"),
     host_config: bool = ("Enable the [host] section in the .cargo/config.toml file"),
+    target_applies_to_host: bool = ("Enable the `target-applies-to-host` key in the .cargo/config.toml file"),
     patch_in_config: bool = ("Allow `[patch]` sections in .cargo/config.toml files"),
     rustdoc_map: bool = ("Allow passing external documentation mappings to rustdoc"),
     separate_nightlies: bool = (HIDDEN),
@@ -789,6 +790,7 @@ impl CliUnstable {
             "jobserver-per-rustc" => self.jobserver_per_rustc = parse_empty(k, v)?,
             "configurable-env" => self.configurable_env = parse_empty(k, v)?,
             "host-config" => self.host_config = parse_empty(k, v)?,
+            "target-applies-to-host" => self.target_applies_to_host = parse_empty(k, v)?,
             "patch-in-config" => self.patch_in_config = parse_empty(k, v)?,
             "features" => {
                 // For now this is still allowed (there are still some

src/cargo/util/config/mod.rs

@@ -1487,7 +1487,7 @@ impl Config {
     }
 
     /// Returns true if the `[target]` table should be applied to host targets.
-    pub fn target_applies_to_host(&self) -> bool {
+    pub fn target_applies_to_host(&self) -> CargoResult<bool> {
         target::get_target_applies_to_host(self)
     }
 

src/cargo/util/config/target.rs

@@ -65,15 +65,25 @@ pub(super) fn load_target_cfgs(config: &Config) -> CargoResult<Vec<(String, Targ
 }
 
 /// Returns true if the `[target]` table should be applied to host targets.
-pub(super) fn get_target_applies_to_host(config: &Config) -> bool {
-    let target_applies_to_host = config.get::<bool>("target-applies-to-host");
-    if target_applies_to_host.is_ok() {
-        target_applies_to_host.unwrap()
+pub(super) fn get_target_applies_to_host(config: &Config) -> CargoResult<bool> {
+    if config.cli_unstable().target_applies_to_host {
+        let target_applies_to_host = config.get::<bool>("target-applies-to-host");
+        if target_applies_to_host.is_ok() {
+            Ok(target_applies_to_host.unwrap())
+        } else {
+            if config.cli_unstable().host_config {
+                Ok(false)
+            } else {
+                Ok(true)
+            }
+        }
     } else {
         if config.cli_unstable().host_config {
-            false
+            anyhow::bail!(
+                "the -Zhost-config flag requires the -Ztarget-applies-to-host flag to be set"
+            );
         } else {
-            true
+            Ok(true)
         }
     }
 }

tests/testsuite/build_script.rs

@@ -234,7 +234,12 @@ fn custom_build_env_var_rustc_linker_host_target() {
 
     // no crate type set => linker never called => build succeeds if and
     // only if build.rs succeeds, despite linker binary not existing.
-    p.cargo("build --target").arg(&target).run();
+    if cargo_test_support::is_nightly() {
+        p.cargo("build -Z target-applies-to-host --target")
+            .arg(&target)
+            .masquerade_as_nightly_cargo()
+            .run();
+    }
 }
 
 #[cargo_test]
@@ -266,10 +271,13 @@ fn custom_build_env_var_rustc_linker_host_target_env() {
 
     // no crate type set => linker never called => build succeeds if and
     // only if build.rs succeeds, despite linker binary not existing.
-    p.cargo("build --target")
-        .env("CARGO_TARGET_APPLIES_TO_HOST", "false")
-        .arg(&target)
-        .run();
+    if cargo_test_support::is_nightly() {
+        p.cargo("build -Z target-applies-to-host --target")
+            .env("CARGO_TARGET_APPLIES_TO_HOST", "false")
+            .arg(&target)
+            .masquerade_as_nightly_cargo()
+            .run();
+    }
 }
 
 #[cargo_test]
@@ -304,7 +312,7 @@ fn custom_build_env_var_rustc_linker_host_target_with_bad_host_config() {
 
     // build.rs should fail due to bad target linker being set
     if cargo_test_support::is_nightly() {
-        p.cargo("build -Z host-config --verbose --target")
+        p.cargo("build -Z target-applies-to-host -Z host-config --verbose --target")
             .arg(&target)
             .masquerade_as_nightly_cargo()
             .with_status(101)
@@ -350,7 +358,7 @@ fn custom_build_env_var_rustc_linker_bad_host() {
 
     // build.rs should fail due to bad host linker being set
     if cargo_test_support::is_nightly() {
-        p.cargo("build -Z host-config --verbose --target")
+        p.cargo("build -Z target-applies-to-host -Z host-config --verbose --target")
             .arg(&target)
             .masquerade_as_nightly_cargo()
             .with_status(101)
@@ -398,7 +406,7 @@ fn custom_build_env_var_rustc_linker_bad_host_with_arch() {
 
     // build.rs should fail due to bad host linker being set
     if cargo_test_support::is_nightly() {
-        p.cargo("build -Z host-config --verbose --target")
+        p.cargo("build -Z target-applies-to-host -Z host-config --verbose --target")
             .arg(&target)
             .masquerade_as_nightly_cargo()
             .with_status(101)
@@ -445,7 +453,7 @@ fn custom_build_env_var_rustc_linker_cross_arch_host() {
 
     // build.rs should fail due to bad host linker being set
     if cargo_test_support::is_nightly() {
-        p.cargo("build -Z host-config --verbose --target")
+        p.cargo("build -Z target-applies-to-host -Z host-config --verbose --target")
             .arg(&target)
             .masquerade_as_nightly_cargo()
             .run();
@@ -486,7 +494,7 @@ fn custom_build_env_var_rustc_linker_bad_cross_arch_host() {
 
     // build.rs should fail due to bad host linker being set
     if cargo_test_support::is_nightly() {
-        p.cargo("build -Z host-config --verbose --target")
+        p.cargo("build -Z target-applies-to-host -Z host-config --verbose --target")
             .arg(&target)
             .masquerade_as_nightly_cargo()
             .with_status(101)