Configure hosts separately from targets when --target is specified.

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

@@ -682,10 +682,15 @@ impl<'cfg> RustcTargetData<'cfg> {
     ) -> CargoResult<RustcTargetData<'cfg>> {
         let config = ws.config();
         let rustc = config.load_global_rustc(Some(ws))?;
-        let host_config = config.target_cfg_triple(&rustc.host)?;
-        let host_info = TargetInfo::new(config, requested_kinds, &rustc, CompileKind::Host)?;
         let mut target_config = HashMap::new();
         let mut target_info = HashMap::new();
+        let target_applies_to_host = config.target_applies_to_host()?;
+        let host_info = TargetInfo::new(config, requested_kinds, &rustc, CompileKind::Host)?;
+        let host_config = if target_applies_to_host {
+            config.target_cfg_triple(&rustc.host)?
+        } else {
+            config.host_cfg_triple(&rustc.host)?
+        };
 
         // This is a hack. The unit_dependency graph builder "pretends" that
         // `CompileKind::Host` is `CompileKind::Target(host)` if the
@@ -695,8 +700,8 @@ impl<'cfg> RustcTargetData<'cfg> {
         if requested_kinds.iter().any(CompileKind::is_host) {
             let ct = CompileTarget::new(&rustc.host)?;
             target_info.insert(ct, host_info.clone());
-            target_config.insert(ct, host_config.clone());
-        }
+            target_config.insert(ct, config.target_cfg_triple(&rustc.host)?);
+        };
 
         let mut res = RustcTargetData {
             rustc,

src/cargo/core/features.rs

@@ -600,6 +600,8 @@ unstable_cli_options!(
     namespaced_features: bool = ("Allow features with `dep:` prefix"),
     no_index_update: bool = ("Do not update the registry index even if the cache is outdated"),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),
@@ -787,6 +789,8 @@ impl CliUnstable {
             "panic-abort-tests" => self.panic_abort_tests = parse_empty(k, v)?,
             "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

@@ -1486,6 +1486,16 @@ 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) -> CargoResult<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)
+    }
+
     /// Returns the `[target]` table definition for the given target triple.
     pub fn target_cfg_triple(&self, target: &str) -> CargoResult<TargetConfig> {
         target::load_target_triple(self, target)

src/cargo/util/config/target.rs

@@ -19,7 +19,7 @@ pub struct TargetCfgConfig {
     pub other: BTreeMap<String, toml::Value>,
 }
 
-/// Config definition of a `[target]` table.
+/// Config definition of a `[target]` table or `[host]`.
 #[derive(Debug, Clone)]
 pub struct TargetConfig {
     /// Process to run as a wrapper for `cargo run`, `test`, and `bench` commands.
@@ -64,18 +64,62 @@ 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) -> CargoResult<bool> {
+    if config.cli_unstable().target_applies_to_host {
+        if let Ok(target_applies_to_host) = config.get::<bool>("target-applies-to-host") {
+            Ok(target_applies_to_host)
+        } else {
+            Ok(!config.cli_unstable().host_config)
+        }
+    } else {
+        if config.cli_unstable().host_config {
+            anyhow::bail!(
+                "the -Zhost-config flag requires the -Ztarget-applies-to-host flag to be set"
+            );
+        } else {
+            Ok(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 {
+        let host_triple_prefix = format!("host.{}", triple);
+        let host_triple_key = ConfigKey::from_str(&host_triple_prefix);
+        let host_prefix = match config.get_cv(&host_triple_key)? {
+            Some(_) => host_triple_prefix,
+            None => "host".to_string(),
+        };
+        load_config_table(config, &host_prefix)
+    } else {
+        Ok(TargetConfig {
+            runner: None,
+            rustflags: None,
+            linker: None,
+            links_overrides: BTreeMap::new(),
+        })
+    }
+}
+
 /// Loads a single `[target]` table for the given triple.
 pub(super) fn load_target_triple(config: &Config, triple: &str) -> CargoResult<TargetConfig> {
+    load_config_table(config, &format!("target.{}", triple))
+}
+
+/// Loads a single table for the given prefix.
+fn load_config_table(config: &Config, prefix: &str) -> CargoResult<TargetConfig> {
     // This needs to get each field individually because it cannot fetch the
     // struct all at once due to `links_overrides`. Can't use `serde(flatten)`
     // because it causes serde to use `deserialize_map` which means the config
     // deserializer does not know which keys to deserialize, which means
     // environment variables would not work.
-    let runner: OptValue<PathAndArgs> = config.get(&format!("target.{}.runner", triple))?;
-    let rustflags: OptValue<StringList> = config.get(&format!("target.{}.rustflags", triple))?;
-    let linker: OptValue<ConfigRelativePath> = config.get(&format!("target.{}.linker", triple))?;
+    let runner: OptValue<PathAndArgs> = config.get(&format!("{}.runner", prefix))?;
+    let rustflags: OptValue<StringList> = config.get(&format!("{}.rustflags", prefix))?;
+    let linker: OptValue<ConfigRelativePath> = config.get(&format!("{}.linker", prefix))?;
     // Links do not support environment variables.
-    let target_key = ConfigKey::from_str(&format!("target.{}", triple));
+    let target_key = ConfigKey::from_str(prefix);
     let links_overrides = match config.get_table(&target_key)? {
         Some(links) => parse_links_overrides(&target_key, links.val, config)?,
         None => BTreeMap::new(),

src/doc/src/reference/unstable.md

@@ -641,6 +641,61 @@ cargo +nightly -Zunstable-options -Zconfig-include --config somefile.toml build
 
 CLI paths are relative to the current working directory.
 
+### target-applies-to-host
+* Original Pull Request: [#9322](https://github.com/rust-lang/cargo/pull/9322)
+* Tracking Issue: [#9453](https://github.com/rust-lang/cargo/issues/9453)
+
+The `target-applies-to-host` key in a config file can be used set the desired
+behavior for passing target config flags to build scripts.
+
+It requires the `-Ztarget-applies-to-host` command-line option.
+
+The current default for `target-applies-to-host` is `true`, which will be
+changed to `false` in the future, if `-Zhost-config` is used the new `false`
+default will be set for `target-applies-to-host`.
+
+```toml
+# config.toml
+target-applies-to-host = false
+```
+
+```console
+cargo +nightly -Ztarget-applies-to-host build --target x86_64-unknown-linux-gnu
+```
+
+### host-config
+* Original Pull Request: [#9322](https://github.com/rust-lang/cargo/pull/9322)
+* Tracking Issue: [#9452](https://github.com/rust-lang/cargo/issues/9452)
+
+The `host` key in a config file can be used pass flags to host build targets
+such as build scripts that must run on the host system instead of the target
+system when cross compiling. It supports both generic and host arch specific
+tables. Matching host arch tables take precedence over generic host tables.
+
+It requires the `-Zhost-config` and `-Ztarget-applies-to-host` command-line
+options to be set.
+
+```toml
+# config.toml
+[host]
+linker = "/path/to/host/linker"
+[host.x86_64-unknown-linux-gnu]
+linker = "/path/to/host/arch/linker"
+[target.x86_64-unknown-linux-gnu]
+linker = "/path/to/target/linker"
+```
+
+The generic `host` table above will be entirely ignored when building on a
+`x86_64-unknown-linux-gnu` host as the `host.x86_64-unknown-linux-gnu` table
+takes precedence.
+
+Setting `-Zhost-config` changes the default for `target-applies-to-host` to
+`false` from `true`.
+
+```console
+cargo +nightly -Ztarget-applies-to-host -Zhost-config build --target x86_64-unknown-linux-gnu
+```
+
 ### unit-graph
 * Tracking Issue: [#8002](https://github.com/rust-lang/cargo/issues/8002)