Add support for target.'cfg(..)'.linker

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

@@ -10,7 +10,6 @@ use crate::util::errors::CargoResult;
 use crate::util::interning::InternedString;
 use crate::util::Rustc;
 use std::collections::{HashMap, HashSet};
-use std::path::PathBuf;
 
 mod target_info;
 pub use self::target_info::{
@@ -120,15 +119,6 @@ impl<'a, 'cfg> BuildContext<'a, 'cfg> {
         &self.target_data.rustc
     }
 
-    /// Gets the user-specified linker for a particular host or target.
-    pub fn linker(&self, kind: CompileKind) -> Option<PathBuf> {
-        self.target_data
-            .target_config(kind)
-            .linker
-            .as_ref()
-            .map(|l| l.val.clone().resolve_program(self.config))
-    }
-
     /// Gets the host architecture triple.
     ///
     /// For example, x86_64-unknown-linux-gnu, would be

src/cargo/core/compiler/compilation.rs

@@ -104,6 +104,8 @@ pub struct Compilation<'cfg> {
     primary_rustc_process: Option<ProcessBuilder>,
 
     target_runners: HashMap<CompileKind, Option<(PathBuf, Vec<String>)>>,
+    /// The linker to use for each host or target.
+    target_linkers: HashMap<CompileKind, Option<PathBuf>>,
 }
 
 impl<'cfg> Compilation<'cfg> {
@@ -150,6 +152,13 @@ impl<'cfg> Compilation<'cfg> {
                 .chain(Some(&CompileKind::Host))
                 .map(|kind| Ok((*kind, target_runner(bcx, *kind)?)))
                 .collect::<CargoResult<HashMap<_, _>>>()?,
+            target_linkers: bcx
+                .build_config
+                .requested_kinds
+                .iter()
+                .chain(Some(&CompileKind::Host))
+                .map(|kind| Ok((*kind, target_linker(bcx, *kind)?)))
+                .collect::<CargoResult<HashMap<_, _>>>()?,
         })
     }
 
@@ -221,6 +230,11 @@ impl<'cfg> Compilation<'cfg> {
         self.target_runners.get(&kind).and_then(|x| x.as_ref())
     }
 
+    /// Gets the user-specified linker for a particular host or target.
+    pub fn target_linker(&self, kind: CompileKind) -> Option<PathBuf> {
+        self.target_linkers.get(&kind).and_then(|x| x.clone())
+    }
+
     /// Returns a [`ProcessBuilder`] appropriate for running a process for the
     /// target platform. This is typically used for `cargo run` and `cargo
     /// test`.
@@ -441,3 +455,39 @@ fn target_runner(
         )
     }))
 }
+
+/// Gets the user-specified linker for a particular host or target from the configuration.
+fn target_linker(bcx: &BuildContext<'_, '_>, kind: CompileKind) -> CargoResult<Option<PathBuf>> {
+    // Try host.linker and target.{}.linker.
+    if let Some(path) = bcx
+        .target_data
+        .target_config(kind)
+        .linker
+        .as_ref()
+        .map(|l| l.val.clone().resolve_program(bcx.config))
+    {
+        return Ok(Some(path));
+    }
+
+    // Try target.'cfg(...)'.linker.
+    let target_cfg = bcx.target_data.info(kind).cfg();
+    let mut cfgs = bcx
+        .config
+        .target_cfgs()?
+        .iter()
+        .filter_map(|(key, cfg)| cfg.linker.as_ref().map(|linker| (key, linker)))
+        .filter(|(key, _linker)| CfgExpr::matches_key(key, target_cfg));
+    let matching_linker = cfgs.next();
+    if let Some((key, linker)) = cfgs.next() {
+        anyhow::bail!(
+            "several matching instances of `target.'cfg(..)'.linker` in configurations\n\
+             first match `{}` located in {}\n\
+             second match `{}` located in {}",
+            matching_linker.unwrap().0,
+            matching_linker.unwrap().1.definition,
+            key,
+            linker.definition
+        );
+    }
+    Ok(matching_linker.map(|(_k, linker)| linker.val.clone().resolve_program(bcx.config)))
+}

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

@@ -272,7 +272,7 @@ impl<'a, 'cfg> Context<'a, 'cfg> {
                     unit: unit.clone(),
                     args,
                     unstable_opts,
-                    linker: self.bcx.linker(unit.kind),
+                    linker: self.compilation.target_linker(unit.kind).clone(),
                     script_meta,
                     env: artifact::get_env(&self, self.unit_deps(unit))?,
                 });

src/cargo/core/compiler/custom_build.rs

@@ -299,11 +299,8 @@ fn build_work(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Job> {
         cmd.env(&var, value);
     }
 
-    if let Some(linker) = &bcx.target_data.target_config(unit.kind).linker {
-        cmd.env(
-            "RUSTC_LINKER",
-            linker.val.clone().resolve_program(bcx.config),
-        );
+    if let Some(linker) = &cx.compilation.target_linker(unit.kind) {
+        cmd.env("RUSTC_LINKER", linker);
     }
 
     if let Some(links) = unit.pkg.manifest().links() {

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

@@ -1426,7 +1426,7 @@ fn calculate_normal(cx: &mut Context<'_, '_>, unit: &Unit) -> CargoResult<Finger
     let m = unit.pkg.manifest().metadata();
     let metadata = util::hash_u64((&m.authors, &m.description, &m.homepage, &m.repository));
     let mut config = StableHasher::new();
-    if let Some(linker) = cx.bcx.linker(unit.kind) {
+    if let Some(linker) = cx.compilation.target_linker(unit.kind) {
         linker.hash(&mut config);
     }
     if unit.mode.is_doc() && cx.bcx.config.cli_unstable().rustdoc_map {

src/cargo/core/compiler/mod.rs

@@ -1117,7 +1117,10 @@ fn build_base_args(cx: &Context<'_, '_>, cmd: &mut ProcessBuilder, unit: &Unit)
         cmd,
         "-C",
         "linker=",
-        bcx.linker(unit.kind).as_ref().map(|s| s.as_ref()),
+        cx.compilation
+            .target_linker(unit.kind)
+            .as_ref()
+            .map(|s| s.as_ref()),
     );
     if incremental {
         let dir = cx.files().layout(unit.kind).incremental().as_os_str();

src/cargo/util/config/target.rs

@@ -12,6 +12,7 @@ use std::path::PathBuf;
 pub struct TargetCfgConfig {
     pub runner: OptValue<PathAndArgs>,
     pub rustflags: OptValue<StringList>,
+    pub linker: OptValue<ConfigRelativePath>,
     // This is here just to ignore fields from normal `TargetConfig` because
     // all `[target]` tables are getting deserialized, whether they start with
     // `cfg(` or not.

src/doc/src/reference/config.md

@@ -1098,6 +1098,12 @@ This option is deprecated and unused.
 Specifies the linker which is passed to `rustc` (via [`-C linker`]) when the
 [`<triple>`] is being compiled for. By default, the linker is not overridden.
 
+##### `target.<cfg>.linker`
+This is similar to the [target linker](#targettriplelinker), but using
+a [`cfg()` expression]. If both a [`<triple>`] and `<cfg>` runner match,
+the `<triple>` will take precedence. It is an error if more than one
+`<cfg>` runner matches the current target.
+
 ##### `target.<triple>.runner`
 * Type: string or array of strings ([program path with args])
 * Default: none

tests/testsuite/build_script.rs

@@ -453,6 +453,43 @@ fn custom_build_env_var_rustc_linker() {
     p.cargo("build --target").arg(&target).run();
 }
 
+// Only run this test on linux, since it's difficult to construct
+// a case suitable for all platforms.
+// See:https://github.com/rust-lang/cargo/pull/12535#discussion_r1306618264
+#[cargo_test]
+#[cfg(target_os = "linux")]
+fn custom_build_env_var_rustc_linker_with_target_cfg() {
+    if cross_compile::disabled() {
+        return;
+    }
+
+    let target = cross_compile::alternate();
+    let p = project()
+        .file(
+            ".cargo/config",
+            r#"
+            [target.'cfg(target_pointer_width = "32")']
+            linker = "/path/to/linker"
+            "#,
+        )
+        .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").arg(&target).run();
+}
+
 #[cargo_test]
 fn custom_build_env_var_rustc_linker_bad_host_target() {
     let target = rustc_host();

tests/testsuite/tool_paths.rs

@@ -32,6 +32,93 @@ fn pathless_tools() {
         .run();
 }
 
+// can set a custom linker via `target.'cfg(..)'.linker`
+#[cargo_test]
+fn custom_linker_cfg() {
+    let foo = project()
+        .file("Cargo.toml", &basic_lib_manifest("foo"))
+        .file("src/lib.rs", "")
+        .file(
+            ".cargo/config",
+            r#"
+            [target.'cfg(not(target_os = "none"))']
+            linker = "nonexistent-linker"
+            "#,
+        )
+        .build();
+
+    foo.cargo("build --verbose")
+        .with_stderr(
+            "\
+[COMPILING] foo v0.5.0 ([CWD])
+[RUNNING] `rustc [..] -C linker=nonexistent-linker [..]`
+[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]
+",
+        )
+        .run();
+}
+
+// custom linker set via `target.$triple.linker` have precede over `target.'cfg(..)'.linker`
+#[cargo_test]
+fn custom_linker_cfg_precedence() {
+    let target = rustc_host();
+
+    let foo = project()
+        .file("Cargo.toml", &basic_lib_manifest("foo"))
+        .file("src/lib.rs", "")
+        .file(
+            ".cargo/config",
+            &format!(
+                r#"
+                    [target.'cfg(not(target_os = "none"))']
+                    linker = "ignored-linker"
+                    [target.{}]
+                    linker = "nonexistent-linker"
+                "#,
+                target
+            ),
+        )
+        .build();
+
+    foo.cargo("build --verbose")
+        .with_stderr(
+            "\
+[COMPILING] foo v0.5.0 ([CWD])
+[RUNNING] `rustc [..] -C linker=nonexistent-linker [..]`
+[FINISHED] dev [unoptimized + debuginfo] target(s) in [..]
+",
+        )
+        .run();
+}
+
+#[cargo_test]
+fn custom_linker_cfg_collision() {
+    let foo = project()
+        .file("Cargo.toml", &basic_lib_manifest("foo"))
+        .file("src/lib.rs", "")
+        .file(
+            ".cargo/config",
+            r#"
+            [target.'cfg(not(target_arch = "avr"))']
+            linker = "nonexistent-linker1"
+            [target.'cfg(not(target_os = "none"))']
+            linker = "nonexistent-linker2"
+            "#,
+        )
+        .build();
+
+    foo.cargo("build --verbose")
+        .with_status(101)
+        .with_stderr(&format!(
+            "\
+[ERROR] several matching instances of `target.'cfg(..)'.linker` in configurations
+first match `cfg(not(target_arch = \"avr\"))` located in [..]/foo/.cargo/config
+second match `cfg(not(target_os = \"none\"))` located in [..]/foo/.cargo/config
+",
+        ))
+        .run();
+}
+
 #[cargo_test]
 fn absolute_tools() {
     let target = rustc_host();
@@ -393,7 +480,6 @@ fn cfg_ignored_fields() {
 [WARNING] unused key `ar` in [target] config table `cfg(not(target_os = \"none\"))`
 [WARNING] unused key `foo` in [target] config table `cfg(not(target_os = \"none\"))`
 [WARNING] unused key `invalid` in [target] config table `cfg(not(target_os = \"none\"))`
-[WARNING] unused key `linker` in [target] config table `cfg(not(target_os = \"none\"))`
 [CHECKING] foo v0.0.1 ([..])
 [FINISHED] [..]
 ",