show linker warnings even if it returns 0

compiler/rustc_codegen_ssa/messages.ftl

@@ -160,6 +160,8 @@ codegen_ssa_linker_file_stem = couldn't extract file stem from specified linker
 codegen_ssa_linker_not_found = linker `{$linker_path}` not found
     .note = {$error}
 
+codegen_ssa_linker_output = {$inner}
+
 codegen_ssa_linker_unsupported_modifier = `as-needed` modifier not supported for current linker
 
 codegen_ssa_linking_failed = linking with `{$linker_path}` failed: {$exit_status}

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -721,6 +721,15 @@ fn link_dwarf_object<'a>(
     }
 }
 
+#[derive(Diagnostic)]
+#[diag(codegen_ssa_linker_output)]
+/// Translating this is kind of useless. We don't pass translation flags to the linker, so we'd just
+/// end up with inconsistent languages within the same diagnostic. Ideally we'd accept strings in
+/// `emit_warning`, but I didn't feel like doing that much of a refactor.
+struct LinkerOutput {
+    inner: String,
+}
+
 /// Create a dynamic library or executable.
 ///
 /// This will invoke the system linker/cc to create the resulting file. This links to all upstream
@@ -972,8 +981,20 @@ fn link_natively<'a>(
 
                 sess.abort_if_errors();
             }
-            info!("linker stderr:\n{}", escape_string(&prog.stderr));
-            info!("linker stdout:\n{}", escape_string(&prog.stdout));
+
+            if !prog.stderr.is_empty() {
+                // We already print `warning:` at the start of the diagnostic. Remove it from the linker output if present.
+                let stderr = escape_string(&prog.stderr);
+                debug!("original stderr: {stderr}");
+                let stderr =
+                    stderr.strip_prefix("warning: ").unwrap_or(&stderr).replace(" warning: ", " ");
+                sess.emit_warning(LinkerOutput { inner: format!("linker stderr: {stderr}") });
+            }
+            if !prog.stdout.is_empty() && sess.opts.verbose {
+                sess.emit_warning(LinkerOutput {
+                    inner: format!("linker stdout: {}", escape_string(&prog.stdout)),
+                });
+            }
         }
         Err(e) => {
             let linker_not_found = e.kind() == io::ErrorKind::NotFound;

tests/run-make/linker-warning/Makefile

@@ -0,0 +1,17 @@
+include ../tools.mk
+
+RUN_RUSTC := $(RUSTC_ORIGINAL) main.rs -o $(TMPDIR)/main -C linker=./fake-linker.sh
+
+all:
+	# Run rustc with our fake linker, and make sure it shows warnings
+	$(RUN_RUSTC) -C link-arg=run_make_warn 2>&1 | $(CGREP) "warning: linker stderr: bar"
+
+	# Make sure it shows stdout, but only when --verbose is passed
+	$(RUN_RUSTC) -C link-arg=run_make_info --verbose 2>&1 | $(CGREP) "warning: linker stdout: foo"
+	$(RUN_RUSTC) -C link-arg=run_make_info 2>&1 | $(CGREP) -v "warning: linker stdout: foo"
+
+	# Make sure we short-circuit this new path if the linker exits with an error (so the diagnostic is less verbose)
+	rm -f $(TMPDIR)/main
+	$(RUN_RUSTC) -C link-arg=run_make_error 2>&1 | $(CGREP) "note: error: baz"
+	! [ -e $(TMPDIR)/main ]
+

tests/run-make/linker-warning/fake-linker.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+
+code=0
+while ! [ $# = 0 ]; do
+    case "$1" in
+        run_make_info) echo "foo"
+            ;;
+        run_make_warn) echo "warning: bar" >&2
+            ;;
+        run_make_error) echo "error: baz" >&2; code=1
+            ;;
+        *) ;;   # rustc passes lots of args we don't care about
+    esac
+    shift
+done
+
+exit $code

tests/run-make/linker-warning/main.rs

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