Add linker_static_archive_order lint for -Clink-arg archives placed before referenced dynamic libs

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -23,7 +23,7 @@ use rustc_errors::DiagCtxtHandle;
 use rustc_fs_util::{TempDirBuilder, fix_windows_verbatim_for_gcc, try_canonicalize};
 use rustc_hir::attrs::NativeLibKind;
 use rustc_hir::def_id::{CrateNum, LOCAL_CRATE};
-use rustc_lint_defs::builtin::LINKER_INFO;
+use rustc_lint_defs::builtin::{LINKER_INFO, LINKER_STATIC_ARCHIVE_ORDER};
 use rustc_macros::Diagnostic;
 use rustc_metadata::EncodedMetadata;
 use rustc_metadata::fs::{METADATA_FILENAME, copy_to_stdout, emit_wrapper_file};
@@ -1219,6 +1219,10 @@ fn link_natively(
     let (linker_path, flavor) = linker_and_flavor(sess);
     let self_contained_components = self_contained_components(sess, crate_type, &linker_path);
 
+    // Surface the `-Clink-arg` static-archive ordering issue before building the linker command
+    // line. See <https://github.com/rust-lang/rust/issues/154975>.
+    warn_static_archive_order(sess, crate_info, flavor);
+
     // On AIX, we ship all libraries as .a big_af archive
     // the expected format is lib<name>.a(libname.so) for the actual
     // dynamic library. So we link to a temporary .so file to be archived
@@ -2398,6 +2402,47 @@ fn add_user_defined_link_args(cmd: &mut dyn Linker, sess: &Session) {
     cmd.verbatim_args(&sess.opts.cg.link_args);
 }
 
+/// Detect static archives (`.a`/`.o` paths) passed via `-Clink-arg`, which rustc appends after
+/// its own native libraries. With `--as-needed`, strict left-to-right linkers like GNU `ld.bfd`
+/// may then drop a dynamic library referenced only by that archive. rustc can't know whether the
+/// back-reference exists, so this is a suppressed-by-default lint. Even with `lld` (which tolerates
+/// back-references) the risky ordering is the same, so it is flagged too — that way the lint
+/// surfaces the latent issue before a switch back to `ld.bfd`. See
+/// <https://github.com/rust-lang/rust/issues/154975>.
+fn warn_static_archive_order(sess: &Session, crate_info: &CrateInfo, flavor: LinkerFlavor) {
+    // `--as-needed` is only emitted for GNU non-Windows linkers (`GccLinker::add_as_needed`);
+    // Darwin uses a different model.
+    if !flavor.is_gnu() || sess.target.is_like_windows || sess.target.is_like_darwin {
+        return;
+    }
+
+    let static_archives: Vec<&String> = sess
+        .opts
+        .cg
+        .link_args
+        .iter()
+        .filter(|arg| {
+            let p = Path::new(arg);
+            matches!(p.extension().and_then(|e| e.to_str()), Some("a" | "o"))
+        })
+        .collect();
+
+    if static_archives.is_empty() {
+        return;
+    }
+
+    let levels = &crate_info.lint_level_specs;
+    for archive in static_archives {
+        emit_lint_base(
+            sess,
+            LINKER_STATIC_ARCHIVE_ORDER,
+            levels.linker_static_archive_order,
+            None,
+            diagnostics::LinkerStaticArchiveOrder { archive: archive.as_str() },
+        );
+    }
+}
+
 /// Add arbitrary "late link" args defined by the target spec.
 /// FIXME: Determine where exactly these args need to be inserted.
 fn add_late_link_args(

compiler/rustc_codegen_ssa/src/diagnostics.rs

@@ -1377,3 +1377,18 @@ impl<'a> MissingNativeLibrary<'a> {
 pub(crate) struct SuggestLibraryName<'a> {
     suggested_name: &'a str,
 }
+
+#[derive(Diagnostic)]
+#[diag(
+    "static archive `{$archive}` passed via `-Clink-arg` may be ordered before a dynamic \
+     library it references"
+)]
+#[note(
+    "rustc appends `-Clink-arg` arguments after its own native libraries; with `--as-needed`, \
+     linkers that resolve symbols strictly left-to-right (such as GNU `ld.bfd`) may drop a dynamic \
+     library that only a static archive to its left references"
+)]
+#[help("use `-l static={$archive}` so the archive is placed with the other native libraries")]
+pub(crate) struct LinkerStaticArchiveOrder<'a> {
+    pub archive: &'a str,
+}

compiler/rustc_codegen_ssa/src/lib.rs

@@ -24,7 +24,7 @@ use rustc_data_structures::unord::UnordMap;
 use rustc_hir::CRATE_HIR_ID;
 use rustc_hir::attrs::{CfgEntry, NativeLibKind, WindowsSubsystemKind};
 use rustc_hir::def_id::CrateNum;
-use rustc_lint_defs::builtin::LINKER_INFO;
+use rustc_lint_defs::builtin::{LINKER_INFO, LINKER_STATIC_ARCHIVE_ORDER};
 use rustc_macros::{Decodable, Encodable};
 use rustc_metadata::EncodedMetadata;
 use rustc_middle::dep_graph::WorkProduct;
@@ -421,13 +421,16 @@ impl CompiledModules {
 pub struct CodegenLintLevelSpecs {
     linker_messages: StableLevelSpec,
     linker_info: StableLevelSpec,
+    linker_static_archive_order: StableLevelSpec,
 }
 
 impl CodegenLintLevelSpecs {
     pub fn from_tcx(tcx: TyCtxt<'_>) -> Self {
         Self {
             linker_messages: tcx.lint_level_spec_at_node(LINKER_MESSAGES, CRATE_HIR_ID),
             linker_info: tcx.lint_level_spec_at_node(LINKER_INFO, CRATE_HIR_ID),
+            linker_static_archive_order: tcx
+                .lint_level_spec_at_node(LINKER_STATIC_ARCHIVE_ORDER, CRATE_HIR_ID),
         }
     }
 }

compiler/rustc_lint_defs/src/builtin.rs

@@ -66,6 +66,7 @@ pub mod hardwired {
             LEGACY_DERIVE_HELPERS,
             LINKER_INFO,
             LINKER_MESSAGES,
+            LINKER_STATIC_ARCHIVE_ORDER,
             LONG_RUNNING_CONST_EVAL,
             MACRO_EXPANDED_MACRO_EXPORTS_ACCESSED_BY_ABSOLUTE_PATHS,
             MACRO_USE_EXTERN_CRATE,
@@ -4089,6 +4090,44 @@ declare_lint! {
     "linker warnings known to be informational-only and not indicative of a problem"
 }
 
+declare_lint! {
+    /// The `linker_static_archive_order` lint detects when a static archive (`.a`/`.o`) passed via
+    /// `-Clink-arg` is placed *before* a dynamic library that it references, which can cause the
+    /// dynamic library to be dropped under `--as-needed` with linkers that resolve symbols strictly
+    /// left-to-right (such as GNU `ld.bfd`).
+    ///
+    /// ### Example
+    ///
+    /// ```rust,ignore (needs CLI args, platform-specific)
+    /// // libfoo.a references symbols from libm.so; built with the GNU `ld.bfd` linker:
+    /// // rustc -Clink-arg=/path/to/libfoo.a -Clink-arg=-lm foo.rs
+    /// fn main() {}
+    /// ```
+    ///
+    /// rustc appends `-Clink-arg` arguments after its own native libraries, so `-lm` lands after
+    /// `libfoo.a`. With `--as-needed`, GNU `ld.bfd` sees that nothing references `-lm` at the point
+    /// it appears (the reference lives inside `libfoo.a` to the left), so `-lm` is dropped, leading
+    /// to unresolved symbols. `lld` tolerates this because it back-references, masking the problem;
+    /// the lint still fires there so the latent issue surfaces before a switch back to `ld.bfd`.
+    ///
+    /// ### Explanation
+    ///
+    /// This lint flags the combination so users can use `-l static=` to place the archive with the
+    /// other native libraries. It is allowed by default because rustc cannot, without symbol
+    /// resolution, know whether the archive actually back-references the later dynamic library. See
+    /// this issue for more details:
+    /// <https://github.com/rust-lang/rust/issues/154975>.
+    pub LINKER_STATIC_ARCHIVE_ORDER,
+    Allow,
+    "static archive passed via `-Clink-arg` may be ordered before a dynamic library it references, \
+     which `--as-needed` linkers such as GNU `ld.bfd` can then drop",
+    // The lint is heuristic: rustc can't know whether a back-reference actually exists, so the
+    // diagnostic may fire on link orders that link successfully. Prevent `-D warnings` from
+    // turning a possible false positive into a hard error. `-D linker-static-archive-order` still
+    // applies.
+    ignore_deny_warnings
+}
+
 declare_lint! {
     /// The `named_arguments_used_positionally` lint detects cases where named arguments are only
     /// used positionally in format strings. This usage is valid but potentially very confusing.

tests/run-make/link-arg-static-archive-order/empty.rs

@@ -0,0 +1 @@
+fn main() {}

tests/run-make/link-arg-static-archive-order/rmake.rs

@@ -0,0 +1,117 @@
+// Tests for the `linker_static_archive_order` lint.
+// See <https://github.com/rust-lang/rust/issues/154975>.
+
+//@ only-linux
+// The lint applies to GNU `ld`-family linkers (including `lld`) on non-Windows, non-Darwin.
+
+use run_make_support::{cwd, is_darwin, is_windows, rustc};
+
+fn main() {
+    // The diagnostic carries the full archive path, so assert on the stable filename substring.
+    let archive = cwd().join("libdoesnotexist.a");
+    let archive_str = archive.to_str().unwrap();
+
+    // 1. A `.a` path via `-Clink-arg` on a bare `ld` flavor fires the lint.
+    rustc()
+        .input("empty.rs")
+        .linker_flavor("ld")
+        .link_arg(archive_str)
+        .arg("-Wlinker-static-archive-order")
+        .print("link-args")
+        .run_unchecked()
+        .assert_stderr_contains("warning: static archive")
+        .assert_stderr_contains("libdoesnotexist.a");
+
+    // 2. A `.o` path is flagged the same way.
+    let object = cwd().join("doesnotexist.o");
+    rustc()
+        .input("empty.rs")
+        .linker_flavor("ld")
+        .link_arg(object.to_str().unwrap())
+        .arg("-Wlinker-static-archive-order")
+        .print("link-args")
+        .run_unchecked()
+        .assert_stderr_contains("warning: static archive")
+        .assert_stderr_contains("doesnotexist.o");
+
+    // 3. A non-archive arg like `-lm` does not fire.
+    rustc()
+        .input("empty.rs")
+        .linker_flavor("ld")
+        .link_arg("-lm")
+        .arg("-Wlinker-static-archive-order")
+        .print("link-args")
+        .run_unchecked()
+        .assert_stderr_not_contains("linker_static_archive_order");
+
+    // 4. `Allow` by default: no `-W`, no output.
+    rustc()
+        .input("empty.rs")
+        .linker_flavor("ld")
+        .link_arg(archive_str)
+        .print("link-args")
+        .run_unchecked()
+        .assert_stderr_not_contains("linker_static_archive_order");
+
+    // 5. `ignore_deny_warnings`: `-Dwarnings` does not promote the `-W` to an error.
+    rustc()
+        .input("empty.rs")
+        .linker_flavor("ld")
+        .link_arg(archive_str)
+        .arg("-Wlinker-static-archive-order")
+        .arg("-Dwarnings")
+        .print("link-args")
+        .run_unchecked()
+        .assert_stderr_contains("warning: static archive")
+        .assert_stderr_contains("the `linker_static_archive_order` lint ignores `-D warnings`");
+
+    // 6. `-Dlinker-static-archive-order` (specific) does promote to an error.
+    rustc()
+        .input("empty.rs")
+        .linker_flavor("ld")
+        .link_arg(archive_str)
+        .arg("-Dlinker-static-archive-order")
+        .print("link-args")
+        .run_fail()
+        .assert_stderr_contains("error: static archive")
+        .assert_stderr_contains("libdoesnotexist.a");
+
+    // 7. `#![allow]` suppresses even an explicit `-W`.
+    rustc()
+        .input("-")
+        .linker_flavor("ld")
+        .link_arg(archive_str)
+        .arg("-Wlinker-static-archive-order")
+        .arg("--crate-type=lib")
+        .stdin_buf("#![allow(linker_static_archive_order)] fn main() {}")
+        .print("link-args")
+        .run_unchecked()
+        .assert_stderr_not_contains("static archive");
+
+    // 8. The `gnu-cc` flavor (cc-driven, `Gnu(Cc::Yes, Lld::No)`) also fires: it uses real `ld`
+    //    behind `cc` and the same `--as-needed` + left-to-right interaction applies.
+    rustc()
+        .input("empty.rs")
+        .arg("-Zunstable-options")
+        .linker_flavor("gnu-cc")
+        .link_arg(archive_str)
+        .arg("-Wlinker-static-archive-order")
+        .print("link-args")
+        .run_unchecked()
+        .assert_stderr_contains("warning: static archive")
+        .assert_stderr_contains("libdoesnotexist.a");
+
+    // 9. The lint also fires with `lld` (`Gnu(.., Lld::Yes)`): the risky ordering is the same, so
+    //    the latent issue surfaces before a switch back to `ld.bfd`.
+    rustc()
+        .input("empty.rs")
+        .linker_flavor("ld.lld")
+        .link_arg(archive_str)
+        .arg("-Wlinker-static-archive-order")
+        .print("link-args")
+        .run_unchecked()
+        .assert_stderr_contains("warning: static archive")
+        .assert_stderr_contains("libdoesnotexist.a");
+
+    assert!(!is_windows() && !is_darwin());
+}