fix: dont apply host-config gating to stable behavior

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

@@ -21,7 +21,6 @@ pub use self::target_info::FileFlavor;
 pub use self::target_info::FileType;
 pub use self::target_info::RustcTargetData;
 pub use self::target_info::TargetInfo;
-pub(crate) use self::target_info::host_artifact_uses_only_host_config;
 
 /// The build context, containing complete information needed for a build task
 /// before it gets started.

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

@@ -909,7 +909,7 @@ fn rustflags_from_build(gctx: &GlobalContext, flag: Flags) -> CargoResult<Option
 }
 
 /// Whether a host artifact must take its configuration solely from `[host]` and ignore `[target]`.
-pub(crate) fn host_artifact_uses_only_host_config(
+fn host_artifact_uses_only_host_config(
     gctx: &GlobalContext,
     requested_kinds: &[CompileKind],
     kind: CompileKind,

src/cargo/core/compiler/compilation.rs

@@ -14,7 +14,6 @@ use crate::core::compiler::BuildContext;
 use crate::core::compiler::CompileTarget;
 use crate::core::compiler::RustdocFingerprint;
 use crate::core::compiler::apply_env_config;
-use crate::core::compiler::build_context::host_artifact_uses_only_host_config;
 use crate::core::compiler::{CompileKind, Unit, UnitHash};
 use crate::util::{CargoResult, GlobalContext};
 
@@ -509,8 +508,10 @@ fn target_runner(
         return Ok(Some((path, runner.val.args.clone())));
     }
 
-    // Host artifacts should not pick up a runner from `[target.'cfg(...)']`.
-    if host_artifact_uses_only_host_config(bcx.gctx, &bcx.build_config.requested_kinds, kind)? {
+    // With `target-applies-to-host = true`,
+    // host artifacts must fall through to pick up from [target]
+    // since this is the stable behavior
+    if kind.is_host() && !bcx.gctx.target_applies_to_host()? {
         return Ok(None);
     }
 
@@ -555,8 +556,10 @@ fn target_linker(bcx: &BuildContext<'_, '_>, kind: CompileKind) -> CargoResult<O
         return Ok(Some(path));
     }
 
-    // Host artifacts should not pick up a linker from `[target.'cfg(...)']`.
-    if host_artifact_uses_only_host_config(bcx.gctx, &bcx.build_config.requested_kinds, kind)? {
+    // With `target-applies-to-host = true`,
+    // host artifacts must fall through to pick up from [target]
+    // since this is the stable behavior
+    if kind.is_host() && !bcx.gctx.target_applies_to_host()? {
         return Ok(None);
     }
 

tests/testsuite/build_script.rs

@@ -1038,6 +1038,208 @@ fn target_runner_does_not_apply_to_build_script() {
         .run();
 }
 
+#[cargo_test]
+fn target_runner_build_script_with_target() {
+    let target = rustc_host();
+    let p = project()
+        .file(
+            ".cargo/config.toml",
+            &format!(
+                r#"
+                [target.{target}]
+                runner = "nonexistent-runner"
+                [target.'cfg(all())']
+                runner = "nonexistent-cfg-runner"
+                "#,
+            ),
+        )
+        .file("build.rs", "fn main() {}")
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check -v --target")
+        .arg(&target)
+        .with_stderr_data(str![[r#"
+[COMPILING] foo v0.0.1 ([ROOT]/foo)
+[RUNNING] `rustc --crate-name build_script_build [..]`
+[RUNNING] `[ROOT]/foo/target/debug/build/foo-[HASH]/build-script-build`
+[RUNNING] `rustc --crate-name foo [..]`
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+}
+
+#[cargo_test]
+fn target_cfg_runner_build_script_with_host_config_and_target() {
+    let target = rustc_host();
+    let p = project()
+        .file(
+            ".cargo/config.toml",
+            r#"
+            [target.'cfg(all())']
+            runner = "nonexistent-cfg-runner"
+            "#,
+        )
+        .file("build.rs", "fn main() {}")
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check -v -Ztarget-applies-to-host -Zhost-config --target")
+        .arg(&target)
+        .masquerade_as_nightly_cargo(&["target-applies-to-host", "host-config"])
+        .with_stderr_data(str![[r#"
+[COMPILING] foo v0.0.1 ([ROOT]/foo)
+[RUNNING] `rustc --crate-name build_script_build [..]`
+[RUNNING] `[ROOT]/foo/target/debug/build/foo-[HASH]/build-script-build`
+[RUNNING] `rustc --crate-name foo [..]`
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+}
+
+#[cargo_test]
+fn host_runner_build_script_without_target() {
+    let p = project()
+        .file(
+            ".cargo/config.toml",
+            r#"
+            [host]
+            runner = "nonexistent-host-runner"
+            "#,
+        )
+        .file("build.rs", "fn main() {}")
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("check -Ztarget-applies-to-host -Zhost-config")
+        .masquerade_as_nightly_cargo(&["target-applies-to-host", "host-config"])
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[COMPILING] foo v0.0.1 ([ROOT]/foo)
+[ERROR] failed to run custom build command for `foo v0.0.1 ([ROOT]/foo)`
+
+Caused by:
+  could not execute process `nonexistent-host-runner [ROOT]/foo/target/debug/build/foo-[HASH]/build-script-build` (never executed)
+
+Caused by:
+  [NOT_FOUND]
+
+"#]])
+        .run();
+}
+
+#[cargo_test]
+fn target_cfg_linker_build_script() {
+    let p = project()
+        .file(
+            ".cargo/config.toml",
+            r#"
+            [target.'cfg(all())']
+            linker = "/path/to/cfg/linker"
+            "#,
+        )
+        .file("build.rs", "fn main() {}")
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("build -v")
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[COMPILING] foo v0.0.1 ([ROOT]/foo)
+[RUNNING] `rustc --crate-name build_script_build [..]--crate-type bin [..]-C linker=[..]/path/to/cfg/linker [..]`
+[ERROR] linker `[..]/path/to/cfg/linker` not found
+...
+"#]])
+        .run();
+}
+
+#[cargo_test]
+fn target_cfg_linker_build_script_with_target() {
+    let target = rustc_host();
+    let p = project()
+        .file(
+            ".cargo/config.toml",
+            r#"
+            [target.'cfg(all())']
+            linker = "/path/to/cfg/linker"
+            "#,
+        )
+        .file("build.rs", "fn main() {}")
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("build -v --target")
+        .arg(&target)
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[COMPILING] foo v0.0.1 ([ROOT]/foo)
+[RUNNING] `rustc --crate-name build_script_build [..]--crate-type bin [..]-C linker=[..]/path/to/cfg/linker [..]`
+[ERROR] linker `[..]/path/to/cfg/linker` not found
+...
+"#]])
+        .run();
+}
+
+#[cargo_test]
+fn target_linker_build_script_with_target() {
+    let target = rustc_host();
+    let p = project()
+        .file(
+            ".cargo/config.toml",
+            &format!(
+                r#"
+                [target.{target}]
+                linker = "/path/to/target/linker"
+                "#,
+            ),
+        )
+        .file("build.rs", "fn main() {}")
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("build -v --target")
+        .arg(&target)
+        .with_status(101)
+        .with_stderr_data(str![[r#"
+[COMPILING] foo v0.0.1 ([ROOT]/foo)
+[RUNNING] `rustc --crate-name build_script_build [..]--crate-type bin [..]-C linker=[..]/path/to/target/linker [..]`
+[ERROR] linker `[..]/path/to/target/linker` not found
+...
+"#]])
+        .run();
+}
+
+#[cargo_test]
+fn target_cfg_linker_build_script_with_host_config_and_target() {
+    let target = rustc_host();
+    let p = project()
+        .file(
+            ".cargo/config.toml",
+            r#"
+            [target.'cfg(all())']
+            linker = "/path/to/cfg/linker"
+            "#,
+        )
+        .file("build.rs", "fn main() {}")
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("build -v -Ztarget-applies-to-host -Zhost-config --target")
+        .arg(&target)
+        .masquerade_as_nightly_cargo(&["target-applies-to-host", "host-config"])
+        .with_stderr_data(str![[r#"
+[COMPILING] foo v0.0.1 ([ROOT]/foo)
+[RUNNING] `rustc --crate-name build_script_build [..]--crate-type bin [..]`
+[RUNNING] `[ROOT]/foo/target/debug/build/foo-[HASH]/build-script-build`
+[RUNNING] `rustc --crate-name foo [..]-C linker=[..]/path/to/cfg/linker [..]`
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+
+"#]])
+        .run();
+}
+
 #[cargo_test]
 fn custom_build_script_wrong_rustc_flags() {
     let p = project()