From 042c613f227ae208f393a6401d3ce10eba7b8d1d Mon Sep 17 00:00:00 2001
From: Ross Sullivan <rosssullivan101@gmail.com>
Date: Wed, 31 Dec 2025 10:21:38 +0900
Subject: [PATCH 1/3] feat: Added any-build-script-metadata unstable feature

---
 src/cargo/core/features.rs                   |   2 +
 src/doc/src/reference/unstable.md            |  11 ++
 tests/testsuite/cargo/z_help/stdout.term.svg | 102 ++++++++++---------
 3 files changed, 65 insertions(+), 50 deletions(-)

diff --git a/src/cargo/core/features.rs b/src/cargo/core/features.rs
index edf02b00555..793ff378c78 100644
--- a/src/cargo/core/features.rs
+++ b/src/cargo/core/features.rs
@@ -844,6 +844,7 @@ unstable_cli_options!(
     // All other unstable features.
     // Please keep this list lexicographically ordered.
     advanced_env: bool,
+    any_build_script_metadata: bool = ("Allow any build script to specify env vars via cargo::metadata=key=value"),
     asymmetric_token: bool = ("Allows authenticating with asymmetric tokens"),
     avoid_dev_deps: bool = ("Avoid installing dev-dependencies if possible"),
     binary_dep_depinfo: bool = ("Track changes to dependency artifacts"),
@@ -1374,6 +1375,7 @@ impl CliUnstable {
             // Unstable features
             // Sorted alphabetically:
             "advanced-env" => self.advanced_env = parse_empty(k, v)?,
+            "any-build-script-metadata" => self.any_build_script_metadata = parse_empty(k, v)?,
             "asymmetric-token" => self.asymmetric_token = parse_empty(k, v)?,
             "avoid-dev-deps" => self.avoid_dev_deps = parse_empty(k, v)?,
             "binary-dep-depinfo" => self.binary_dep_depinfo = parse_empty(k, v)?,
diff --git a/src/doc/src/reference/unstable.md b/src/doc/src/reference/unstable.md
index 16b5260222c..9c48ac61e8d 100644
--- a/src/doc/src/reference/unstable.md
+++ b/src/doc/src/reference/unstable.md
@@ -67,6 +67,7 @@ Each new feature described below should explain how to use it.
 * Build scripts and linking
     * [Metabuild](#metabuild) --- Provides declarative build scripts.
     * [Multiple Build Scripts](#multiple-build-scripts) --- Allows use of multiple build scripts.
+    * [Any Build Script Metadata](#any-build-script-metadata) --- Allow any build script to specify env vars via `cargo::metadata=key=value`
 * Resolver and features
     * [no-index-update](#no-index-update) --- Prevents cargo from updating the index cache.
     * [avoid-dev-deps](#avoid-dev-deps) --- Prevents the resolver from including dev-dependencies during resolution.
@@ -313,6 +314,16 @@ build = ["foo.rs", "bar.rs"]
   where the `<script-name>` is the file-stem of the build script, exactly as-is.
   For example, `bar_OUT_DIR` for script at `foo/bar.rs`. (Only set during compilation, can be accessed via `env!` macro)
 
+## Any Build Script Metadata
+* Tracking Issue: [#14903](https://github.com/rust-lang/cargo/issues/3544)
+
+Allow any build script to specify env vars via `cargo::metadata=key=value`
+
+Depedant build scripts can access these key/value pair by reading the `CARGO_DEP_<dep>_<key>` env variable at runtime.
+For build scripts of crates with a `links`, both `DEP_<links>_<key>` and `CARGO_DEP_<dep>_<key>` will be set.
+
+Note that `dep` and `key` in `CARGO_DEP_<dep>_<key>` are uppercased and hyphens (`-`) replaced with underscores (`_`).
+
 ## public-dependency
 * Tracking Issue: [#44663](https://github.com/rust-lang/rust/issues/44663)
 
diff --git a/tests/testsuite/cargo/z_help/stdout.term.svg b/tests/testsuite/cargo/z_help/stdout.term.svg
index f5a3513c066..20006653417 100644
--- a/tests/testsuite/cargo/z_help/stdout.term.svg
+++ b/tests/testsuite/cargo/z_help/stdout.term.svg
@@ -1,4 +1,4 @@
-<svg width="1255px" height="956px" xmlns="http://www.w3.org/2000/svg">
+<svg width="1255px" height="974px" xmlns="http://www.w3.org/2000/svg">
   <style>
     .fg { fill: #AAAAAA }
     .bg { fill: #000000 }
@@ -22,103 +22,105 @@
 </tspan>
     <tspan x="10px" y="64px"><tspan>    -Z allow-features              Allow *only* the listed unstable features</tspan>
 </tspan>
-    <tspan x="10px" y="82px"><tspan>    -Z asymmetric-token            Allows authenticating with asymmetric tokens</tspan>
+    <tspan x="10px" y="82px"><tspan>    -Z any-build-script-metadata   Allow any build script to specify env vars via cargo::metadata=key=value</tspan>
 </tspan>
-    <tspan x="10px" y="100px"><tspan>    -Z avoid-dev-deps              Avoid installing dev-dependencies if possible</tspan>
+    <tspan x="10px" y="100px"><tspan>    -Z asymmetric-token            Allows authenticating with asymmetric tokens</tspan>
 </tspan>
-    <tspan x="10px" y="118px"><tspan>    -Z binary-dep-depinfo          Track changes to dependency artifacts</tspan>
+    <tspan x="10px" y="118px"><tspan>    -Z avoid-dev-deps              Avoid installing dev-dependencies if possible</tspan>
 </tspan>
-    <tspan x="10px" y="136px"><tspan>    -Z bindeps                     Allow Cargo packages to depend on bin, cdylib, and staticlib crates, and use the artifacts built by those crates</tspan>
+    <tspan x="10px" y="136px"><tspan>    -Z binary-dep-depinfo          Track changes to dependency artifacts</tspan>
 </tspan>
-    <tspan x="10px" y="154px"><tspan>    -Z build-analysis              Record and persist build metrics across runs, with commands to query past builds.</tspan>
+    <tspan x="10px" y="154px"><tspan>    -Z bindeps                     Allow Cargo packages to depend on bin, cdylib, and staticlib crates, and use the artifacts built by those crates</tspan>
 </tspan>
-    <tspan x="10px" y="172px"><tspan>    -Z build-dir-new-layout        Use the new build-dir filesystem layout</tspan>
+    <tspan x="10px" y="172px"><tspan>    -Z build-analysis              Record and persist build metrics across runs, with commands to query past builds.</tspan>
 </tspan>
-    <tspan x="10px" y="190px"><tspan>    -Z build-std                   Enable Cargo to compile the standard library itself as part of a crate graph compilation</tspan>
+    <tspan x="10px" y="190px"><tspan>    -Z build-dir-new-layout        Use the new build-dir filesystem layout</tspan>
 </tspan>
-    <tspan x="10px" y="208px"><tspan>    -Z build-std-features          Configure features enabled for the standard library itself when building the standard library</tspan>
+    <tspan x="10px" y="208px"><tspan>    -Z build-std                   Enable Cargo to compile the standard library itself as part of a crate graph compilation</tspan>
 </tspan>
-    <tspan x="10px" y="226px"><tspan>    -Z cargo-lints                 Enable the `[lints.cargo]` table</tspan>
+    <tspan x="10px" y="226px"><tspan>    -Z build-std-features          Configure features enabled for the standard library itself when building the standard library</tspan>
 </tspan>
-    <tspan x="10px" y="244px"><tspan>    -Z checksum-freshness          Use a checksum to determine if output is fresh rather than filesystem mtime</tspan>
+    <tspan x="10px" y="244px"><tspan>    -Z cargo-lints                 Enable the `[lints.cargo]` table</tspan>
 </tspan>
-    <tspan x="10px" y="262px"><tspan>    -Z codegen-backend             Enable the `codegen-backend` option in profiles in .cargo/config.toml file</tspan>
+    <tspan x="10px" y="262px"><tspan>    -Z checksum-freshness          Use a checksum to determine if output is fresh rather than filesystem mtime</tspan>
 </tspan>
-    <tspan x="10px" y="280px"><tspan>    -Z direct-minimal-versions     Resolve minimal dependency versions instead of maximum (direct dependencies only)</tspan>
+    <tspan x="10px" y="280px"><tspan>    -Z codegen-backend             Enable the `codegen-backend` option in profiles in .cargo/config.toml file</tspan>
 </tspan>
-    <tspan x="10px" y="298px"><tspan>    -Z dual-proc-macros            Build proc-macros for both the host and the target</tspan>
+    <tspan x="10px" y="298px"><tspan>    -Z direct-minimal-versions     Resolve minimal dependency versions instead of maximum (direct dependencies only)</tspan>
 </tspan>
-    <tspan x="10px" y="316px"><tspan>    -Z feature-unification         Enable new feature unification modes in workspaces</tspan>
+    <tspan x="10px" y="316px"><tspan>    -Z dual-proc-macros            Build proc-macros for both the host and the target</tspan>
 </tspan>
-    <tspan x="10px" y="334px"><tspan>    -Z fine-grain-locking          Use fine grain locking instead of locking the entire build cache</tspan>
+    <tspan x="10px" y="334px"><tspan>    -Z feature-unification         Enable new feature unification modes in workspaces</tspan>
 </tspan>
-    <tspan x="10px" y="352px"><tspan>    -Z fix-edition                 Permanently unstable edition migration helper</tspan>
+    <tspan x="10px" y="352px"><tspan>    -Z fine-grain-locking          Use fine grain locking instead of locking the entire build cache</tspan>
 </tspan>
-    <tspan x="10px" y="370px"><tspan>    -Z gc                          Track cache usage and "garbage collect" unused files</tspan>
+    <tspan x="10px" y="370px"><tspan>    -Z fix-edition                 Permanently unstable edition migration helper</tspan>
 </tspan>
-    <tspan x="10px" y="388px"><tspan>    -Z git                         Enable support for shallow git fetch operations</tspan>
+    <tspan x="10px" y="388px"><tspan>    -Z gc                          Track cache usage and "garbage collect" unused files</tspan>
 </tspan>
-    <tspan x="10px" y="406px"><tspan>    -Z gitoxide                    Use gitoxide for the given git interactions, or all of them if no argument is given</tspan>
+    <tspan x="10px" y="406px"><tspan>    -Z git                         Enable support for shallow git fetch operations</tspan>
 </tspan>
-    <tspan x="10px" y="424px"><tspan>    -Z host-config                 Enable the `[host]` section in the .cargo/config.toml file</tspan>
+    <tspan x="10px" y="424px"><tspan>    -Z gitoxide                    Use gitoxide for the given git interactions, or all of them if no argument is given</tspan>
 </tspan>
-    <tspan x="10px" y="442px"><tspan>    -Z minimal-versions            Resolve minimal dependency versions instead of maximum</tspan>
+    <tspan x="10px" y="442px"><tspan>    -Z host-config                 Enable the `[host]` section in the .cargo/config.toml file</tspan>
 </tspan>
-    <tspan x="10px" y="460px"><tspan>    -Z msrv-policy                 Enable rust-version aware policy within cargo</tspan>
+    <tspan x="10px" y="460px"><tspan>    -Z minimal-versions            Resolve minimal dependency versions instead of maximum</tspan>
 </tspan>
-    <tspan x="10px" y="478px"><tspan>    -Z mtime-on-use                Configure Cargo to update the mtime of used files</tspan>
+    <tspan x="10px" y="478px"><tspan>    -Z msrv-policy                 Enable rust-version aware policy within cargo</tspan>
 </tspan>
-    <tspan x="10px" y="496px"><tspan>    -Z no-embed-metadata           Avoid embedding metadata in library artifacts</tspan>
+    <tspan x="10px" y="496px"><tspan>    -Z mtime-on-use                Configure Cargo to update the mtime of used files</tspan>
 </tspan>
-    <tspan x="10px" y="514px"><tspan>    -Z no-index-update             Do not update the registry index even if the cache is outdated</tspan>
+    <tspan x="10px" y="514px"><tspan>    -Z no-embed-metadata           Avoid embedding metadata in library artifacts</tspan>
 </tspan>
-    <tspan x="10px" y="532px"><tspan>    -Z panic-abort-tests           Enable support to run tests with -Cpanic=abort</tspan>
+    <tspan x="10px" y="532px"><tspan>    -Z no-index-update             Do not update the registry index even if the cache is outdated</tspan>
 </tspan>
-    <tspan x="10px" y="550px"><tspan>    -Z panic-immediate-abort       Enable setting `panic = "immediate-abort"` in profiles</tspan>
+    <tspan x="10px" y="550px"><tspan>    -Z panic-abort-tests           Enable support to run tests with -Cpanic=abort</tspan>
 </tspan>
-    <tspan x="10px" y="568px"><tspan>    -Z profile-hint-mostly-unused  Enable the `hint-mostly-unused` setting in profiles to mark a crate as mostly unused.</tspan>
+    <tspan x="10px" y="568px"><tspan>    -Z panic-immediate-abort       Enable setting `panic = "immediate-abort"` in profiles</tspan>
 </tspan>
-    <tspan x="10px" y="586px"><tspan>    -Z profile-rustflags           Enable the `rustflags` option in profiles in .cargo/config.toml file</tspan>
+    <tspan x="10px" y="586px"><tspan>    -Z profile-hint-mostly-unused  Enable the `hint-mostly-unused` setting in profiles to mark a crate as mostly unused.</tspan>
 </tspan>
-    <tspan x="10px" y="604px"><tspan>    -Z public-dependency           Respect a dependency's `public` field in Cargo.toml to control public/private dependencies</tspan>
+    <tspan x="10px" y="604px"><tspan>    -Z profile-rustflags           Enable the `rustflags` option in profiles in .cargo/config.toml file</tspan>
 </tspan>
-    <tspan x="10px" y="622px"><tspan>    -Z publish-timeout             Enable the `publish.timeout` key in .cargo/config.toml file</tspan>
+    <tspan x="10px" y="622px"><tspan>    -Z public-dependency           Respect a dependency's `public` field in Cargo.toml to control public/private dependencies</tspan>
 </tspan>
-    <tspan x="10px" y="640px"><tspan>    -Z root-dir                    Set the root directory relative to which paths are printed (defaults to workspace root)</tspan>
+    <tspan x="10px" y="640px"><tspan>    -Z publish-timeout             Enable the `publish.timeout` key in .cargo/config.toml file</tspan>
 </tspan>
-    <tspan x="10px" y="658px"><tspan>    -Z rustc-unicode               Enable `rustc`'s unicode error format in Cargo's error messages</tspan>
+    <tspan x="10px" y="658px"><tspan>    -Z root-dir                    Set the root directory relative to which paths are printed (defaults to workspace root)</tspan>
 </tspan>
-    <tspan x="10px" y="676px"><tspan>    -Z rustdoc-depinfo             Use dep-info files in rustdoc rebuild detection</tspan>
+    <tspan x="10px" y="676px"><tspan>    -Z rustc-unicode               Enable `rustc`'s unicode error format in Cargo's error messages</tspan>
 </tspan>
-    <tspan x="10px" y="694px"><tspan>    -Z rustdoc-map                 Allow passing external documentation mappings to rustdoc</tspan>
+    <tspan x="10px" y="694px"><tspan>    -Z rustdoc-depinfo             Use dep-info files in rustdoc rebuild detection</tspan>
 </tspan>
-    <tspan x="10px" y="712px"><tspan>    -Z rustdoc-mergeable-info      Use rustdoc mergeable cross-crate-info files</tspan>
+    <tspan x="10px" y="712px"><tspan>    -Z rustdoc-map                 Allow passing external documentation mappings to rustdoc</tspan>
 </tspan>
-    <tspan x="10px" y="730px"><tspan>    -Z rustdoc-scrape-examples     Allows Rustdoc to scrape code examples from reverse-dependencies</tspan>
+    <tspan x="10px" y="730px"><tspan>    -Z rustdoc-mergeable-info      Use rustdoc mergeable cross-crate-info files</tspan>
 </tspan>
-    <tspan x="10px" y="748px"><tspan>    -Z sbom                        Enable the `sbom` option in build config in .cargo/config.toml file</tspan>
+    <tspan x="10px" y="748px"><tspan>    -Z rustdoc-scrape-examples     Allows Rustdoc to scrape code examples from reverse-dependencies</tspan>
 </tspan>
-    <tspan x="10px" y="766px"><tspan>    -Z script                      Enable support for single-file, `.rs` packages</tspan>
+    <tspan x="10px" y="766px"><tspan>    -Z sbom                        Enable the `sbom` option in build config in .cargo/config.toml file</tspan>
 </tspan>
-    <tspan x="10px" y="784px"><tspan>    -Z section-timings             Enable support for extended compilation sections in --timings output</tspan>
+    <tspan x="10px" y="784px"><tspan>    -Z script                      Enable support for single-file, `.rs` packages</tspan>
 </tspan>
-    <tspan x="10px" y="802px"><tspan>    -Z target-applies-to-host      Enable the `target-applies-to-host` key in the .cargo/config.toml file</tspan>
+    <tspan x="10px" y="802px"><tspan>    -Z section-timings             Enable support for extended compilation sections in --timings output</tspan>
 </tspan>
-    <tspan x="10px" y="820px"><tspan>    -Z trim-paths                  Enable the `trim-paths` option in profiles</tspan>
+    <tspan x="10px" y="820px"><tspan>    -Z target-applies-to-host      Enable the `target-applies-to-host` key in the .cargo/config.toml file</tspan>
 </tspan>
-    <tspan x="10px" y="838px"><tspan>    -Z unstable-options            Allow the usage of unstable options</tspan>
+    <tspan x="10px" y="838px"><tspan>    -Z trim-paths                  Enable the `trim-paths` option in profiles</tspan>
 </tspan>
-    <tspan x="10px" y="856px"><tspan>    -Z warnings                    Allow use of the build.warnings config key</tspan>
+    <tspan x="10px" y="856px"><tspan>    -Z unstable-options            Allow the usage of unstable options</tspan>
 </tspan>
-    <tspan x="10px" y="874px">
+    <tspan x="10px" y="874px"><tspan>    -Z warnings                    Allow use of the build.warnings config key</tspan>
 </tspan>
-    <tspan x="10px" y="892px"><tspan>Run with `cargo -Z [FLAG] [COMMAND]`</tspan>
+    <tspan x="10px" y="892px">
 </tspan>
-    <tspan x="10px" y="910px">
+    <tspan x="10px" y="910px"><tspan>Run with `cargo -Z [FLAG] [COMMAND]`</tspan>
 </tspan>
-    <tspan x="10px" y="928px"><tspan>See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html for more information about these flags.</tspan>
+    <tspan x="10px" y="928px">
 </tspan>
-    <tspan x="10px" y="946px">
+    <tspan x="10px" y="946px"><tspan>See https://doc.rust-lang.org/nightly/cargo/reference/unstable.html for more information about these flags.</tspan>
+</tspan>
+    <tspan x="10px" y="964px">
 </tspan>
   </text>
 

From a69a2831dff926e36779f7c29e86bbd737e8d115 Mon Sep 17 00:00:00 2001
From: Ross Sullivan <rosssullivan101@gmail.com>
Date: Sat, 27 Dec 2025 00:51:19 +0900
Subject: [PATCH 2/3] test: Added test to verify build scripts cannot pass env
 vars without links

---
 tests/testsuite/build_script.rs | 357 ++++++++++++++++++++++++++++++++
 1 file changed, 357 insertions(+)

diff --git a/tests/testsuite/build_script.rs b/tests/testsuite/build_script.rs
index e925f9ecfbb..ed6d2f0aaa4 100644
--- a/tests/testsuite/build_script.rs
+++ b/tests/testsuite/build_script.rs
@@ -1343,6 +1343,363 @@ fn links_passes_env_vars() {
     p.cargo("build -v").run();
 }
 
+#[cargo_test]
+fn metadata_from_dep_kinds() {
+    let set_metadata = r#"
+fn main() {
+    println!("cargo::metadata=key=value");
+}
+"#;
+
+    let get_metadata = r#"
+fn warn_print_env_var(key: &str) {
+    println!("cargo::warning={key}={:?}", std::env::var(key));
+}
+fn main() {
+    warn_print_env_var("DEP_FOO_KEY");
+}
+"#;
+
+    // Create a project with a crate with `links` and 3 test crates that imports this crate as a
+    // normal dependency (n), dev-dependency (d), and build-dependency (b).
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [workspace]
+                resolver = "3"
+                members = ["links", "n", "b", "d"]
+            "#,
+        )
+        .file(
+            "links/Cargo.toml",
+            r#"
+[package]
+name = "links"
+edition = "2024"
+links = "foo"
+"#,
+        )
+        .file("links/src/lib.rs", "")
+        .file("links/build.rs", set_metadata)
+        .file(
+            "n/Cargo.toml",
+            r#"
+[package]
+name = "n"
+edition = "2024"
+
+[dependencies]
+links.path = "../links"
+"#,
+        )
+        .file("n/src/lib.rs", "")
+        .file("n/build.rs", get_metadata)
+        .file(
+            "b/Cargo.toml",
+            r#"
+[package]
+name = "b"
+edition = "2024"
+
+[build-dependencies]
+links.path = "../links"
+"#,
+        )
+        .file("b/src/lib.rs", "")
+        .file("b/build.rs", get_metadata)
+        .file(
+            "d/Cargo.toml",
+            r#"
+[package]
+name = "d"
+edition = "2024"
+
+[dev-dependencies]
+links.path = "../links"
+"#,
+        )
+        .file("d/src/lib.rs", "")
+        .file("d/build.rs", get_metadata)
+        .build();
+
+    p.cargo("check --all-targets")
+        .with_stderr_data(
+            str![[r#"
+[COMPILING] links v0.0.0 ([ROOT]/foo/links)
+[COMPILING] n v0.0.0 ([ROOT]/foo/n)
+[COMPILING] b v0.0.0 ([ROOT]/foo/b)
+[COMPILING] d v0.0.0 ([ROOT]/foo/d)
+[FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
+[WARNING] n@0.0.0: DEP_FOO_KEY=Ok("value")
+[WARNING] d@0.0.0: DEP_FOO_KEY=Err(NotPresent)
+[WARNING] b@0.0.0: DEP_FOO_KEY=Err(NotPresent)
+
+"#]]
+            .unordered(),
+        )
+        .run();
+}
+
+#[cargo_test]
+fn links_passes_env_vars_with_any_build_script_unstable_feature() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.5.0"
+                edition = "2015"
+                authors = []
+                build = "build.rs"
+
+                [dependencies.a]
+                path = "a"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "build.rs",
+            r#"
+                use std::env;
+                fn main() {
+                    assert_eq!(env::var("DEP_FOO_FOO").unwrap(), "bar");
+                    assert_eq!(env::var("DEP_FOO_BAR").unwrap(), "baz");
+                    assert!(env::var("CARGO_DEP_A_FOO").is_err());
+                    assert!(env::var("CARGO_DEP_A_BAR").is_err());
+                }
+            "#,
+        )
+        .file(
+            "a/Cargo.toml",
+            r#"
+                [package]
+                name = "a"
+                version = "0.5.0"
+                edition = "2015"
+                authors = []
+                links = "foo"
+                build = "build.rs"
+            "#,
+        )
+        .file("a/src/lib.rs", "")
+        .file(
+            "a/build.rs",
+            r#"
+                use std::env;
+                fn main() {
+                    let lib = env::var("CARGO_MANIFEST_LINKS").unwrap();
+                    assert_eq!(lib, "foo");
+
+                    println!("cargo::metadata=foo=bar");
+                    println!("cargo::metadata=bar=baz");
+                }
+            "#,
+        )
+        .build();
+
+    p.cargo("check -v").run();
+}
+
+#[cargo_test]
+fn non_links_cannot_pass_env_vars() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.5.0"
+                edition = "2015"
+                authors = []
+                build = "build.rs"
+
+                [dependencies.a]
+                path = "a"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "build.rs",
+            r#"
+                use std::env;
+                fn main() {
+                    assert!(env::var("DEP_FOO_FOO").is_err());
+                    assert!(env::var("DEP_FOO_BAR").is_err());
+
+                    assert!(env::var("CARGO_DEP_A_FOO").is_err());
+                    assert!(env::var("CARGO_DEP_A_BAR").is_err());
+                }
+            "#,
+        )
+        .file(
+            "a/Cargo.toml",
+            r#"
+                [package]
+                name = "a"
+                version = "0.5.0"
+                edition = "2015"
+                authors = []
+                build = "build.rs"
+            "#,
+        )
+        .file("a/src/lib.rs", "")
+        .file(
+            "a/build.rs",
+            r#"
+                fn main() {
+                    println!("cargo::metadata=foo=bar");
+                    println!("cargo::metadata=bar=baz");
+                }
+            "#,
+        )
+        .build();
+
+    p.cargo("check -v").run();
+}
+
+#[cargo_test]
+fn non_links_cannot_pass_env_vars_with_dep_renamed() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.5.0"
+                edition = "2015"
+                authors = []
+                build = "build.rs"
+
+                [dependencies]
+                my-renamed-package = { package = "a", path = "a" }
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "build.rs",
+            r#"
+                use std::env;
+                fn main() {
+                    assert!(env::var("DEP_A_FOO").is_err());
+                    assert!(env::var("DEP_A_BAR").is_err());
+                    assert!(env::var("DEP_MY_RENAMED_PACKAGE_FOO").is_err());
+                    assert!(env::var("DEP_MY_RENAMED_PACKAGE_BAR").is_err());
+
+                    assert!(env::var("CARGO_DEP_MY_RENAMED_PACKAGE_FOO").is_err());
+                    assert!(env::var("CARGO_DEP_MY_RENAMED_PACKAGE_BAR").is_err());
+                }
+            "#,
+        )
+        .file(
+            "a/Cargo.toml",
+            r#"
+                [package]
+                name = "a"
+                version = "0.5.0"
+                edition = "2015"
+                authors = []
+                build = "build.rs"
+            "#,
+        )
+        .file("a/src/lib.rs", "")
+        .file(
+            "a/build.rs",
+            r#"
+                fn main() {
+                    println!("cargo::metadata=foo=bar");
+                    println!("cargo::metadata=bar=baz");
+                }
+            "#,
+        )
+        .build();
+
+    p.cargo("check -v").run();
+}
+
+#[cargo_test]
+fn non_links_can_pass_env_vars_direct_deps_only() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.5.0"
+                edition = "2015"
+                authors = []
+                build = "build.rs"
+
+                [dependencies]
+                direct = { path = "direct" }
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .file(
+            "build.rs",
+            r#"
+                use std::env;
+                fn main() {
+                    assert!(env::var("DEP_DIRECT_FOO").is_err());
+                    assert!(env::var("DEP_TRANSITIVE_FOO").is_err());
+
+                    assert!(env::var("CARGO_DEP_DIRECT_FOO").is_err());
+                    assert!(env::var("CARGO_DEP_TRANSITIVE_FOO").is_err());
+                }
+            "#,
+        )
+        .file(
+            "direct/Cargo.toml",
+            r#"
+                [package]
+                name = "direct"
+                version = "0.5.0"
+                edition = "2015"
+                authors = []
+                build = "build.rs"
+
+                [dependencies]
+                transitive = { path = "../transitive" }
+            "#,
+        )
+        .file("direct/src/lib.rs", "")
+        .file(
+            "direct/build.rs",
+            r#"
+                use std::env;
+                fn main() {
+                    println!("cargo::metadata=foo=direct");
+
+                    assert!(env::var("CARGO_DEP_TRANSITIVE_FOO").is_err());
+                }
+            "#,
+        )
+        .file(
+            "transitive/Cargo.toml",
+            r#"
+                [package]
+                name = "transitive"
+                version = "0.5.0"
+                edition = "2015"
+                authors = []
+                build = "build.rs"
+            "#,
+        )
+        .file("transitive/src/lib.rs", "")
+        .file(
+            "transitive/build.rs",
+            r#"
+                fn main() {
+                    println!("cargo::metadata=foo=transitive");
+                }
+            "#,
+        )
+        .build();
+
+    p.cargo("check -v").run();
+}
+
 #[cargo_test]
 fn only_rerun_build_script() {
     let p = project()

From a919c32c25961d136ba8d433164680c12ed07aa6 Mon Sep 17 00:00:00 2001
From: Ross Sullivan <rosssullivan101@gmail.com>
Date: Tue, 30 Dec 2025 13:32:41 +0900
Subject: [PATCH 3/3] feat: any build scripts can now use
 cargo::metadata=KEY=VALUE

---
 src/cargo/core/compiler/custom_build.rs      | 40 ++++++++++++---
 src/cargo/core/compiler/unit_dependencies.rs |  8 +--
 tests/testsuite/build_script.rs              | 52 ++++++++++++++------
 3 files changed, 76 insertions(+), 24 deletions(-)

diff --git a/src/cargo/core/compiler/custom_build.rs b/src/cargo/core/compiler/custom_build.rs
index db9bc9fce07..6193e09c92f 100644
--- a/src/cargo/core/compiler/custom_build.rs
+++ b/src/cargo/core/compiler/custom_build.rs
@@ -451,6 +451,8 @@ fn build_work(build_runner: &mut BuildRunner<'_, '_>, unit: &Unit) -> CargoResul
         cmd.display_env_vars();
     }
 
+    let any_build_script_metadata = bcx.gctx.cli_unstable().any_build_script_metadata;
+
     // Gather the set of native dependencies that this package has along with
     // some other variables to close over.
     //
@@ -461,8 +463,26 @@ fn build_work(build_runner: &mut BuildRunner<'_, '_>, unit: &Unit) -> CargoResul
         .filter_map(|dep| {
             if dep.unit.mode.is_run_custom_build() {
                 let dep_metadata = build_runner.get_run_build_script_metadata(&dep.unit);
+
+                let Some(dependency) = unit.pkg.dependencies().iter().find(|d| {
+                    d.package_name() == dep.unit.pkg.name()
+                        && d.source_id() == dep.unit.pkg.package_id().source_id()
+                        && d.version_req().matches(unit.pkg.version())
+                }) else {
+                    panic!(
+                        "Dependency `{}` not found in `{}`s dependencies",
+                        dep.unit.pkg.name(),
+                        unit.pkg.name()
+                    )
+                };
+
                 Some((
-                    dep.unit.pkg.manifest().links().unwrap().to_string(),
+                    dependency.name_in_toml(),
+                    dep.unit
+                        .pkg
+                        .manifest()
+                        .links()
+                        .map(|links| links.to_string()),
                     dep.unit.pkg.package_id(),
                     dep_metadata,
                 ))
@@ -531,7 +551,7 @@ fn build_work(build_runner: &mut BuildRunner<'_, '_>, unit: &Unit) -> CargoResul
         // native dynamic libraries.
         {
             let build_script_outputs = build_script_outputs.lock().unwrap();
-            for (name, dep_id, dep_metadata) in lib_deps {
+            for (name, links, dep_id, dep_metadata) in lib_deps {
                 let script_output = build_script_outputs.get(dep_metadata).ok_or_else(|| {
                     internal(format!(
                         "failed to locate build state for env vars: {}/{}",
@@ -540,10 +560,18 @@ fn build_work(build_runner: &mut BuildRunner<'_, '_>, unit: &Unit) -> CargoResul
                 })?;
                 let data = &script_output.metadata;
                 for (key, value) in data.iter() {
-                    cmd.env(
-                        &format!("DEP_{}_{}", super::envify(&name), super::envify(key)),
-                        value,
-                    );
+                    if let Some(ref links) = links {
+                        cmd.env(
+                            &format!("DEP_{}_{}", super::envify(&links), super::envify(key)),
+                            value,
+                        );
+                    }
+                    if any_build_script_metadata {
+                        cmd.env(
+                            &format!("CARGO_DEP_{}_{}", super::envify(&name), super::envify(key)),
+                            value,
+                        );
+                    }
                 }
             }
             if let Some(build_scripts) = build_scripts
diff --git a/src/cargo/core/compiler/unit_dependencies.rs b/src/cargo/core/compiler/unit_dependencies.rs
index 947fb17e841..7b9e6426ab5 100644
--- a/src/cargo/core/compiler/unit_dependencies.rs
+++ b/src/cargo/core/compiler/unit_dependencies.rs
@@ -970,11 +970,13 @@ fn connect_run_custom_build_deps(state: &mut State<'_, '_>) {
                         .iter()
                         .map(move |a| (reverse_dep, a))
                 })
+                // Exclude ourself
+                .filter(|(_parent, other)| other.unit.pkg != unit.pkg)
                 // Only deps with `links`.
                 .filter(|(_parent, other)| {
-                    other.unit.pkg != unit.pkg
-                        && other.unit.target.is_linkable()
-                        && other.unit.pkg.manifest().links().is_some()
+                    state.gctx.cli_unstable().any_build_script_metadata
+                        || (other.unit.target.is_linkable()
+                            && other.unit.pkg.manifest().links().is_some())
                 })
                 // Avoid cycles when using the doc --scrape-examples feature:
                 // Say a workspace has crates A and B where A has a build-dependency on B.
diff --git a/tests/testsuite/build_script.rs b/tests/testsuite/build_script.rs
index ed6d2f0aaa4..190158a3078 100644
--- a/tests/testsuite/build_script.rs
+++ b/tests/testsuite/build_script.rs
@@ -1357,6 +1357,7 @@ fn warn_print_env_var(key: &str) {
 }
 fn main() {
     warn_print_env_var("DEP_FOO_KEY");
+    warn_print_env_var("CARGO_DEP_LINKS_KEY");
 }
 "#;
 
@@ -1423,7 +1424,8 @@ links.path = "../links"
         .file("d/build.rs", get_metadata)
         .build();
 
-    p.cargo("check --all-targets")
+    p.cargo("check --all-targets -Zany-build-script-metadata")
+        .masquerade_as_nightly_cargo(&["any-build-script-metadata"])
         .with_stderr_data(
             str![[r#"
 [COMPILING] links v0.0.0 ([ROOT]/foo/links)
@@ -1432,8 +1434,11 @@ links.path = "../links"
 [COMPILING] d v0.0.0 ([ROOT]/foo/d)
 [FINISHED] `dev` profile [unoptimized + debuginfo] target(s) in [ELAPSED]s
 [WARNING] n@0.0.0: DEP_FOO_KEY=Ok("value")
+[WARNING] n@0.0.0: CARGO_DEP_LINKS_KEY=Ok("value")
 [WARNING] d@0.0.0: DEP_FOO_KEY=Err(NotPresent)
+[WARNING] d@0.0.0: CARGO_DEP_LINKS_KEY=Err(NotPresent)
 [WARNING] b@0.0.0: DEP_FOO_KEY=Err(NotPresent)
+[WARNING] b@0.0.0: CARGO_DEP_LINKS_KEY=Err(NotPresent)
 
 "#]]
             .unordered(),
@@ -1466,8 +1471,8 @@ fn links_passes_env_vars_with_any_build_script_unstable_feature() {
                 fn main() {
                     assert_eq!(env::var("DEP_FOO_FOO").unwrap(), "bar");
                     assert_eq!(env::var("DEP_FOO_BAR").unwrap(), "baz");
-                    assert!(env::var("CARGO_DEP_A_FOO").is_err());
-                    assert!(env::var("CARGO_DEP_A_BAR").is_err());
+                    assert_eq!(env::var("CARGO_DEP_A_FOO").unwrap(), "bar");
+                    assert_eq!(env::var("CARGO_DEP_A_BAR").unwrap(), "baz");
                 }
             "#,
         )
@@ -1499,11 +1504,13 @@ fn links_passes_env_vars_with_any_build_script_unstable_feature() {
         )
         .build();
 
-    p.cargo("check -v").run();
+    p.cargo("check -v -Zany-build-script-metadata")
+        .masquerade_as_nightly_cargo(&["any-build-script-metadata"])
+        .run();
 }
 
 #[cargo_test]
-fn non_links_cannot_pass_env_vars() {
+fn non_links_can_pass_env_vars() {
     let p = project()
         .file(
             "Cargo.toml",
@@ -1525,11 +1532,16 @@ fn non_links_cannot_pass_env_vars() {
             r#"
                 use std::env;
                 fn main() {
+                    // DEP_<links>_<key> is only allowed for `links` crates
                     assert!(env::var("DEP_FOO_FOO").is_err());
                     assert!(env::var("DEP_FOO_BAR").is_err());
+                    // Make sure DEP_<name-in-toml>_<key> is not present.
+                    // This is not a valid scenario but verify these are not present just incase.
+                    assert!(env::var("DEP_A_FOO").is_err());
+                    assert!(env::var("DEP_A_BAR").is_err());
 
-                    assert!(env::var("CARGO_DEP_A_FOO").is_err());
-                    assert!(env::var("CARGO_DEP_A_BAR").is_err());
+                    assert_eq!(env::var("CARGO_DEP_A_FOO").unwrap(), "bar");
+                    assert_eq!(env::var("CARGO_DEP_A_BAR").unwrap(), "baz");
                 }
             "#,
         )
@@ -1556,11 +1568,13 @@ fn non_links_cannot_pass_env_vars() {
         )
         .build();
 
-    p.cargo("check -v").run();
+    p.cargo("check -v -Zany-build-script-metadata")
+        .masquerade_as_nightly_cargo(&["any-build-script-metadata"])
+        .run();
 }
 
 #[cargo_test]
-fn non_links_cannot_pass_env_vars_with_dep_renamed() {
+fn non_links_can_pass_env_vars_with_dep_renamed() {
     let p = project()
         .file(
             "Cargo.toml",
@@ -1587,8 +1601,12 @@ fn non_links_cannot_pass_env_vars_with_dep_renamed() {
                     assert!(env::var("DEP_MY_RENAMED_PACKAGE_FOO").is_err());
                     assert!(env::var("DEP_MY_RENAMED_PACKAGE_BAR").is_err());
 
-                    assert!(env::var("CARGO_DEP_MY_RENAMED_PACKAGE_FOO").is_err());
-                    assert!(env::var("CARGO_DEP_MY_RENAMED_PACKAGE_BAR").is_err());
+                    // If dep was renamed, we should not add env vars with the original name
+                    // and env vars with the renamed package should be added
+                    assert!(env::var("CARGO_DEP_A_FOO").is_err());
+                    assert!(env::var("CARGO_DEP_A_BAR").is_err());
+                    assert_eq!(env::var("CARGO_DEP_MY_RENAMED_PACKAGE_FOO").unwrap(), "bar");
+                    assert_eq!(env::var("CARGO_DEP_MY_RENAMED_PACKAGE_BAR").unwrap(), "baz");
                 }
             "#,
         )
@@ -1615,7 +1633,9 @@ fn non_links_cannot_pass_env_vars_with_dep_renamed() {
         )
         .build();
 
-    p.cargo("check -v").run();
+    p.cargo("check -v -Zany-build-script-metadata")
+        .masquerade_as_nightly_cargo(&["any-build-script-metadata"])
+        .run();
 }
 
 #[cargo_test]
@@ -1644,7 +1664,7 @@ fn non_links_can_pass_env_vars_direct_deps_only() {
                     assert!(env::var("DEP_DIRECT_FOO").is_err());
                     assert!(env::var("DEP_TRANSITIVE_FOO").is_err());
 
-                    assert!(env::var("CARGO_DEP_DIRECT_FOO").is_err());
+                    assert_eq!(env::var("CARGO_DEP_DIRECT_FOO").unwrap(), "direct");
                     assert!(env::var("CARGO_DEP_TRANSITIVE_FOO").is_err());
                 }
             "#,
@@ -1671,7 +1691,7 @@ fn non_links_can_pass_env_vars_direct_deps_only() {
                 fn main() {
                     println!("cargo::metadata=foo=direct");
 
-                    assert!(env::var("CARGO_DEP_TRANSITIVE_FOO").is_err());
+                    assert_eq!(env::var("CARGO_DEP_TRANSITIVE_FOO").unwrap(), "transitive");
                 }
             "#,
         )
@@ -1697,7 +1717,9 @@ fn non_links_can_pass_env_vars_direct_deps_only() {
         )
         .build();
 
-    p.cargo("check -v").run();
+    p.cargo("check -v -Zany-build-script-metadata")
+        .masquerade_as_nightly_cargo(&["any-build-script-metadata"])
+        .run();
 }
 
 #[cargo_test]