Add target-applies-to-host and default to true unless host-config is …

…enabled.

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

@@ -694,11 +694,21 @@ impl<'cfg> RustcTargetData<'cfg> {
         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 = config.target_cfg_triple(&rustc.host)?;
-            target_config.insert(ct, target_host_config.clone());
+            let target_host_config = if config.target_applies_to_host() {
+                let target_cfg_clone = config.target_cfg_triple(&rustc.host)?;
+                target_config.insert(ct, target_cfg_clone.clone());
+                target_cfg_clone
+            } else {
+                target_config.insert(ct, config.target_cfg_triple(&rustc.host)?);
+                config.host_cfg_triple(&rustc.host)?
+            };
             target_host_config
         } else {
-            config.host_cfg_triple(&rustc.host)?
+            if config.target_applies_to_host() {
+                config.target_cfg_triple(&rustc.host)?
+            } else {
+                config.host_cfg_triple(&rustc.host)?
+            }
         };
 
         for kind in requested_kinds {

src/cargo/util/config/mod.rs

@@ -1486,6 +1486,11 @@ impl Config {
             .try_borrow_with(|| self.get::<RustdocExternMap>("doc.extern-map"))
     }
 
+    /// Returns true if the `[target]` table should be applied to host targets.
+    pub fn target_applies_to_host(&self) -> bool {
+        target::get_target_applies_to_host(self)
+    }
+
     /// Returns the `[host]` table definition for the given target triple.
     pub fn host_cfg_triple(&self, target: &str) -> CargoResult<TargetConfig> {
         target::load_host_triple(self, target)

src/cargo/util/config/target.rs

@@ -64,6 +64,20 @@ pub(super) fn load_target_cfgs(config: &Config) -> CargoResult<Vec<(String, Targ
     Ok(result)
 }
 
+/// 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()
+    } else {
+        if config.cli_unstable().host_config {
+            false
+        } else {
+            true
+        }
+    }
+}
+
 /// Loads a single `[host]` table for the given triple.
 pub(super) fn load_host_triple(config: &Config, triple: &str) -> CargoResult<TargetConfig> {
     if config.cli_unstable().host_config {

tests/testsuite/build_script.rs

@@ -212,6 +212,7 @@ fn custom_build_env_var_rustc_linker_host_target() {
             ".cargo/config",
             &format!(
                 r#"
+                target-applies-to-host = false
                 [target.{}]
                 linker = "/path/to/linker"
                 "#,
@@ -236,6 +237,88 @@ fn custom_build_env_var_rustc_linker_host_target() {
     p.cargo("build --target").arg(&target).run();
 }
 
+#[cargo_test]
+fn custom_build_env_var_rustc_linker_host_target_env() {
+    let target = rustc_host();
+    let p = project()
+        .file(
+            ".cargo/config",
+            &format!(
+                r#"
+                [target.{}]
+                linker = "/path/to/linker"
+                "#,
+                target
+            ),
+        )
+        .file(
+            "build.rs",
+            r#"
+            use std::env;
+
+            fn main() {
+                assert!(env::var("RUSTC_LINKER").unwrap().ends_with("/path/to/linker"));
+            }
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    // 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();
+}
+
+#[cargo_test]
+fn custom_build_env_var_rustc_linker_host_target_with_bad_host_config() {
+    let target = rustc_host();
+    let p = project()
+        .file(
+            ".cargo/config",
+            &format!(
+                r#"
+                target-applies-to-host = true
+                [host]
+                linker = "/path/to/host/linker"
+                [target.{}]
+                linker = "/path/to/target/linker"
+                "#,
+                target
+            ),
+        )
+        .file(
+            "build.rs",
+            r#"
+            use std::env;
+
+            fn main() {
+                assert!(env::var("RUSTC_LINKER").unwrap().ends_with("/path/to/target/linker"));
+            }
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    // 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")
+            .arg(&target)
+            .masquerade_as_nightly_cargo()
+            .with_status(101)
+            .with_stderr_contains(
+                "\
+[COMPILING] foo v0.0.1 ([CWD])
+[RUNNING] `rustc --crate-name build_script_build build.rs [..]--crate-type bin [..]-C linker=[..]/path/to/target/linker [..]`
+[ERROR] linker `[..]/path/to/target/linker` not found
+"
+            )
+            .run();
+    }
+}
+
 #[cargo_test]
 fn custom_build_env_var_rustc_linker_bad_host() {
     let target = rustc_host();