Make sure search paths inside OUT_DIR precede external paths

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

@@ -301,7 +301,9 @@ impl<'a, 'gctx> BuildRunner<'a, 'gctx> {
                 .extend(output.env.iter().cloned());
 
             for dir in output.library_paths.iter() {
-                self.compilation.native_dirs.insert(dir.clone());
+                self.compilation
+                    .native_dirs
+                    .insert(dir.clone().into_path_buf());
             }
         }
         Ok(self.compilation)

src/cargo/core/compiler/custom_build.rs

@@ -31,7 +31,7 @@
 //! [`CompileMode::RunCustomBuild`]: super::CompileMode
 //! [instructions]: https://doc.rust-lang.org/cargo/reference/build-scripts.html#outputs-of-the-build-script
 
-use super::{fingerprint, BuildRunner, Job, Unit, Work};
+use super::{fingerprint, get_dynamic_search_path, BuildRunner, Job, Unit, Work};
 use crate::core::compiler::artifact;
 use crate::core::compiler::build_runner::UnitHash;
 use crate::core::compiler::fingerprint::DirtyReason;
@@ -74,11 +74,76 @@ pub enum Severity {
 
 pub type LogMessage = (Severity, String);
 
+/// Represents a path added to the library search path.
+///
+/// We need to keep track of requests to add search paths within the cargo build directory
+/// separately from paths outside of Cargo. The reason is that we want to give precedence to linking
+/// against libraries within the Cargo build directory even if a similar library exists in the
+/// system (e.g. crate A adds `/usr/lib` to the search path and then a later build of crate B adds
+/// `target/debug/...` to satisfy its request to link against the library B that it built, but B is
+/// also found in `/usr/lib`).
+///
+/// There's some nuance here because we want to preserve relative order of paths of the same type.
+/// For example, if the build process would in declaration order emit the following linker line:
+/// ```bash
+/// -L/usr/lib -Ltarget/debug/build/crate1/libs -L/lib -Ltarget/debug/build/crate2/libs)
+/// ```
+///
+/// we want the linker to actually receive:
+/// ```bash
+/// -Ltarget/debug/build/crate1/libs -Ltarget/debug/build/crate2/libs) -L/usr/lib -L/lib
+/// ```
+///
+/// so that the library search paths within the crate artifacts directory come first but retain
+/// relative ordering while the system library paths come after while still retaining relative
+/// ordering among them; ordering is the order they are emitted within the build process,
+/// not lexicographic order.
+///
+/// WARNING: Even though this type implements PartialOrd + Ord, this is a lexicographic ordering.
+/// The linker line will require an explicit sorting algorithm. PartialOrd + Ord is derived because
+/// BuildOutput requires it but that ordering is different from the one for the linker search path,
+/// at least today. It may be worth reconsidering & perhaps it's ok if BuildOutput doesn't have
+/// a lexicographic ordering for the library_paths? I'm not sure the consequence of that.
+#[derive(Clone, Debug, Hash, PartialEq, Eq, PartialOrd, Ord)]
+pub enum LibraryPath {
+    /// The path is pointing within the output folder of the crate and takes priority over
+    /// external paths when passed to the linker.
+    CargoArtifact(PathBuf),
+    /// The path is pointing outside of the crate's build location. The linker will always
+    /// receive such paths after `CargoArtifact`.
+    External(PathBuf),
+}
+
+impl LibraryPath {
+    fn new(p: PathBuf, script_out_dir: &Path) -> Self {
+        let search_path = get_dynamic_search_path(&p);
+        if search_path.starts_with(script_out_dir) {
+            Self::CargoArtifact(p)
+        } else {
+            Self::External(p)
+        }
+    }
+
+    pub fn into_path_buf(self) -> PathBuf {
+        match self {
+            LibraryPath::CargoArtifact(p) | LibraryPath::External(p) => p,
+        }
+    }
+}
+
+impl AsRef<PathBuf> for LibraryPath {
+    fn as_ref(&self) -> &PathBuf {
+        match self {
+            LibraryPath::CargoArtifact(p) | LibraryPath::External(p) => p,
+        }
+    }
+}
+
 /// Contains the parsed output of a custom build script.
 #[derive(Clone, Debug, Hash, Default, PartialEq, Eq, PartialOrd, Ord)]
 pub struct BuildOutput {
     /// Paths to pass to rustc with the `-L` flag.
-    pub library_paths: Vec<PathBuf>,
+    pub library_paths: Vec<LibraryPath>,
     /// Names and link kinds of libraries, suitable for the `-l` flag.
     pub library_links: Vec<String>,
     /// Linker arguments suitable to be passed to `-C link-arg=<args>`
@@ -237,7 +302,7 @@ fn emit_build_output(
     let library_paths = output
         .library_paths
         .iter()
-        .map(|l| l.display().to_string())
+        .map(|l| l.as_ref().display().to_string())
         .collect::<Vec<_>>();
 
     let msg = machine_message::BuildScript {
@@ -884,10 +949,16 @@ impl BuildOutput {
                 "rustc-flags" => {
                     let (paths, links) = BuildOutput::parse_rustc_flags(&value, &whence)?;
                     library_links.extend(links.into_iter());
-                    library_paths.extend(paths.into_iter());
+                    library_paths.extend(
+                        paths
+                            .into_iter()
+                            .map(|p| LibraryPath::new(p, script_out_dir)),
+                    );
                 }
                 "rustc-link-lib" => library_links.push(value.to_string()),
-                "rustc-link-search" => library_paths.push(PathBuf::from(value)),
+                "rustc-link-search" => {
+                    library_paths.push(LibraryPath::new(PathBuf::from(value), script_out_dir))
+                }
                 "rustc-link-arg-cdylib" | "rustc-cdylib-link-arg" => {
                     if !targets.iter().any(|target| target.is_cdylib()) {
                         log_messages.push((

src/cargo/core/compiler/mod.rs

@@ -79,7 +79,7 @@ pub use self::compilation::{Compilation, Doctest, UnitOutput};
 pub use self::compile_kind::{CompileKind, CompileTarget};
 pub use self::crate_type::CrateType;
 pub use self::custom_build::LinkArgTarget;
-pub use self::custom_build::{BuildOutput, BuildScriptOutputs, BuildScripts};
+pub use self::custom_build::{BuildOutput, BuildScriptOutputs, BuildScripts, LibraryPath};
 pub(crate) use self::fingerprint::DirtyReason;
 pub use self::job_queue::Freshness;
 use self::job_queue::{Job, JobQueue, JobState, Work};
@@ -495,16 +495,39 @@ fn rustc(
         target: &Target,
         current_id: PackageId,
     ) -> CargoResult<()> {
+        let mut library_paths = vec![];
+
+        for key in build_scripts.to_link.iter() {
+            let output = build_script_outputs.get(key.1).ok_or_else(|| {
+                internal(format!(
+                    "couldn't find build script output for {}/{}",
+                    key.0, key.1
+                ))
+            })?;
+            library_paths.extend(output.library_paths.iter());
+        }
+
+        // NOTE: This very intentionally does not use the derived ord from LibraryPath because we need to
+        // retain relative ordering within the same type (i.e. not lexicographic). The use of a stable sort
+        // is also important here because it ensures that paths of the same type retain the same relative
+        // ordering (for an unstable sort to work here, the list would need to retain the idx of each element
+        // and then sort by that idx when the type is equivalent.
+        library_paths.sort_by_key(|p| match p {
+            LibraryPath::CargoArtifact(_) => 0,
+            LibraryPath::External(_) => 1,
+        });
+
+        for path in library_paths.iter() {
+            rustc.arg("-L").arg(path.as_ref());
+        }
+
         for key in build_scripts.to_link.iter() {
             let output = build_script_outputs.get(key.1).ok_or_else(|| {
                 internal(format!(
                     "couldn't find build script output for {}/{}",
                     key.0, key.1
                 ))
             })?;
-            for path in output.library_paths.iter() {
-                rustc.arg("-L").arg(path);
-            }
 
             if key.0 == current_id {
                 if pass_l_flag {
@@ -650,7 +673,7 @@ fn add_plugin_deps(
             .get(*metadata)
             .ok_or_else(|| internal(format!("couldn't find libs for plugin dep {}", pkg_id)))?;
         search_path.append(&mut filter_dynamic_search_path(
-            output.library_paths.iter(),
+            output.library_paths.iter().map(AsRef::as_ref),
             root_output,
         ));
     }
@@ -659,6 +682,13 @@ fn add_plugin_deps(
     Ok(())
 }
 
+fn get_dynamic_search_path(path: &Path) -> &Path {
+    match path.to_str().and_then(|s| s.split_once("=")) {
+        Some(("native" | "crate" | "dependency" | "framework" | "all", path)) => Path::new(path),
+        _ => path,
+    }
+}
+
 // Determine paths to add to the dynamic search path from -L entries
 //
 // Strip off prefixes like "native=" or "framework=" and filter out directories
@@ -670,12 +700,9 @@ where
 {
     let mut search_path = vec![];
     for dir in paths {
-        let dir = match dir.to_str().and_then(|s| s.split_once("=")) {
-            Some(("native" | "crate" | "dependency" | "framework" | "all", path)) => path.into(),
-            _ => dir.clone(),
-        };
+        let dir = get_dynamic_search_path(dir);
         if dir.starts_with(&root_output) {
-            search_path.push(dir);
+            search_path.push(dir.to_path_buf());
         } else {
             debug!(
                 "Not including path {} in runtime library search path because it is \

src/cargo/util/context/target.rs

@@ -1,5 +1,5 @@
 use super::{ConfigKey, ConfigRelativePath, GlobalContext, OptValue, PathAndArgs, StringList, CV};
-use crate::core::compiler::{BuildOutput, LinkArgTarget};
+use crate::core::compiler::{BuildOutput, LibraryPath, LinkArgTarget};
 use crate::util::CargoResult;
 use serde::Deserialize;
 use std::collections::{BTreeMap, HashMap};
@@ -167,7 +167,9 @@ fn parse_links_overrides(
                     let flags = value.string(key)?;
                     let whence = format!("target config `{}.{}` (in {})", target_key, key, flags.1);
                     let (paths, links) = BuildOutput::parse_rustc_flags(flags.0, &whence)?;
-                    output.library_paths.extend(paths);
+                    output
+                        .library_paths
+                        .extend(paths.into_iter().map(LibraryPath::External));
                     output.library_links.extend(links);
                 }
                 "rustc-link-lib" => {
@@ -178,9 +180,11 @@ fn parse_links_overrides(
                 }
                 "rustc-link-search" => {
                     let list = value.list(key)?;
-                    output
-                        .library_paths
-                        .extend(list.iter().map(|v| PathBuf::from(&v.0)));
+                    output.library_paths.extend(
+                        list.iter()
+                            .map(|v| PathBuf::from(&v.0))
+                            .map(LibraryPath::External),
+                    );
                 }
                 "rustc-link-arg-cdylib" | "rustc-cdylib-link-arg" => {
                     let args = extra_link_args(LinkArgTarget::Cdylib, key, value)?;

tests/testsuite/build_script.rs

@@ -6087,3 +6087,90 @@ fn directory_with_leading_underscore() {
         .with_status(0)
         .run();
 }
+
+#[cargo_test]
+fn linker_search_path_preference() {
+    // This isn't strictly the exact scenario that causes the issue, but it's the shortest demonstration
+    // of the issue.
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+                [package]
+                name = "foo"
+                version = "0.1.0"
+                edition = "2024"
+                build = "build.rs"
+
+                [dependencies]
+                a = { path = "a" }
+                b = { path = "b" }
+            "#,
+        )
+        .file(
+            "build.rs",
+            r#"
+                fn main() {
+                    let out_dir = std::env::var("OUT_DIR").unwrap();
+                    println!("cargo::rustc-link-search=/usr/lib");
+                    println!("cargo::rustc-link-search={}/libs2", out_dir);
+                    println!("cargo::rustc-link-search=/lib");
+                    println!("cargo::rustc-link-search={}/libs1", out_dir);
+                }
+            "#,
+        )
+        .file("src/main.rs", "fn main() {}")
+        .file(
+            "a/Cargo.toml",
+            r#"
+                [package]
+                name = "a"
+                version = "0.1.0"
+                edition = "2024"
+                build = "build.rs"
+            "#,
+        )
+        .file("a/src/lib.rs", "")
+        .file(
+            "a/build.rs",
+            r#"
+                fn main() {
+                    let out_dir = std::env::var("OUT_DIR").unwrap();
+                    println!("cargo::rustc-link-search=/usr/lib3");
+                    println!("cargo::rustc-link-search={}/libsA.2", out_dir);
+                    println!("cargo::rustc-link-search=/lib3");
+                    println!("cargo::rustc-link-search={}/libsA.1", out_dir);
+                }
+            "#,
+        )
+        .file(
+            "b/Cargo.toml",
+            r#"
+                [package]
+                name = "b"
+                version = "0.1.0"
+                edition = "2024"
+                build = "build.rs"
+            "#,
+        )
+        .file("b/src/lib.rs", "")
+        .file(
+            "b/build.rs",
+            r#"
+                fn main() {
+                    let out_dir = std::env::var("OUT_DIR").unwrap();
+                    println!("cargo::rustc-link-search=/usr/lib2");
+                    println!("cargo::rustc-link-search={}/libsB.1", out_dir);
+                    println!("cargo::rustc-link-search=/lib2");
+                    println!("cargo::rustc-link-search={}/libsB.2", out_dir);
+                }
+            "#,
+        )
+        .build();
+
+    p.cargo("build -v").with_stderr_data(str![[r#"
+...
+[RUNNING] `rustc --crate-name foo [..] -L [ROOT]/foo/target/debug/build/foo-[HASH]/out/libs2 -L [ROOT]/foo/target/debug/build/foo-[HASH]/out/libs1 -L [ROOT]/foo/target/debug/build/a-[HASH]/out/libsA.2 -L [ROOT]/foo/target/debug/build/a-[HASH]/out/libsA.1 -L [ROOT]/foo/target/debug/build/b-[HASH]/out/libsB.1 -L [ROOT]/foo/target/debug/build/b-[HASH]/out/libsB.2 -L /usr/lib -L /lib -L /usr/lib3 -L /lib3 -L /usr/lib2 -L /lib2`
+...
+"#]]).run();
+}