Any build scripts can now use cargo::metadata=KEY=VALUE

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

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.

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)?,

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)