Add some tests

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_arch = "i686")']
+            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();