diff --git a/compiler/rustc_codegen_llvm/src/back/write.rs b/compiler/rustc_codegen_llvm/src/back/write.rs
index bead4c82a8120..0fe41f6801eef 100644
--- a/compiler/rustc_codegen_llvm/src/back/write.rs
+++ b/compiler/rustc_codegen_llvm/src/back/write.rs
@@ -827,7 +827,7 @@ pub(crate) unsafe fn codegen(
                         "LLVM_module_codegen_make_bitcode",
                         &*module.name,
                     );
-                    ThinBuffer::new(llmod, config.emit_thin_lto, false)
+                    ThinBuffer::new(llmod, cgcx.lto != Lto::Fat && config.emit_thin_lto, false)
                 };
                 let data = thin.data();
                 let _timer = cgcx
diff --git a/src/tools/run-make-support/src/external_deps/llvm.rs b/src/tools/run-make-support/src/external_deps/llvm.rs
index 9a6e35da3fe20..939160d9f41d8 100644
--- a/src/tools/run-make-support/src/external_deps/llvm.rs
+++ b/src/tools/run-make-support/src/external_deps/llvm.rs
@@ -60,6 +60,12 @@ pub fn llvm_pdbutil() -> LlvmPdbutil {
     LlvmPdbutil::new()
 }
 
+/// Construct a new `llvm-as` invocation. This assumes that `llvm-as` is available
+/// at `$LLVM_BIN_DIR/llvm-as`.
+pub fn llvm_as() -> LlvmAs {
+    LlvmAs::new()
+}
+
 /// Construct a new `llvm-dis` invocation. This assumes that `llvm-dis` is available
 /// at `$LLVM_BIN_DIR/llvm-dis`.
 pub fn llvm_dis() -> LlvmDis {
@@ -135,6 +141,13 @@ pub struct LlvmPdbutil {
     cmd: Command,
 }
 
+/// A `llvm-as` invocation builder.
+#[derive(Debug)]
+#[must_use]
+pub struct LlvmAs {
+    cmd: Command,
+}
+
 /// A `llvm-dis` invocation builder.
 #[derive(Debug)]
 #[must_use]
@@ -158,6 +171,7 @@ crate::macros::impl_common_helpers!(LlvmNm);
 crate::macros::impl_common_helpers!(LlvmBcanalyzer);
 crate::macros::impl_common_helpers!(LlvmDwarfdump);
 crate::macros::impl_common_helpers!(LlvmPdbutil);
+crate::macros::impl_common_helpers!(LlvmAs);
 crate::macros::impl_common_helpers!(LlvmDis);
 crate::macros::impl_common_helpers!(LlvmObjcopy);
 
@@ -441,6 +455,22 @@ impl LlvmObjcopy {
     }
 }
 
+impl LlvmAs {
+    /// Construct a new `llvm-as` invocation. This assumes that `llvm-as` is available
+    /// at `$LLVM_BIN_DIR/llvm-as`.
+    pub fn new() -> Self {
+        let llvm_as = llvm_bin_dir().join("llvm-as");
+        let cmd = Command::new(llvm_as);
+        Self { cmd }
+    }
+
+    /// Provide an input file.
+    pub fn input<P: AsRef<Path>>(&mut self, path: P) -> &mut Self {
+        self.cmd.arg(path.as_ref());
+        self
+    }
+}
+
 impl LlvmDis {
     /// Construct a new `llvm-dis` invocation. This assumes that `llvm-dis` is available
     /// at `$LLVM_BIN_DIR/llvm-dis`.
diff --git a/src/tools/run-make-support/src/external_deps/rustc.rs b/src/tools/run-make-support/src/external_deps/rustc.rs
index 0e2239147f122..e6ea1330e4a8b 100644
--- a/src/tools/run-make-support/src/external_deps/rustc.rs
+++ b/src/tools/run-make-support/src/external_deps/rustc.rs
@@ -171,6 +171,12 @@ impl Rustc {
         self
     }
 
+    /// This flag enables LTO in the specified form.
+    pub fn lto(&mut self, option: &str) -> &mut Self {
+        self.cmd.arg(format!("-Clto={option}"));
+        self
+    }
+
     /// This flag defers LTO optimizations to the linker.
     pub fn linker_plugin_lto(&mut self, option: &str) -> &mut Self {
         self.cmd.arg(format!("-Clinker-plugin-lto={option}"));
diff --git a/src/tools/run-make-support/src/lib.rs b/src/tools/run-make-support/src/lib.rs
index e0ad3ee9bedaa..f7cffea82cb46 100644
--- a/src/tools/run-make-support/src/lib.rs
+++ b/src/tools/run-make-support/src/lib.rs
@@ -63,7 +63,7 @@ pub use cargo::cargo;
 pub use clang::{clang, Clang};
 pub use htmldocck::htmldocck;
 pub use llvm::{
-    llvm_ar, llvm_bcanalyzer, llvm_dis, llvm_dwarfdump, llvm_filecheck, llvm_nm, llvm_objcopy,
+    llvm_ar, llvm_bcanalyzer, llvm_as, llvm_dis, llvm_dwarfdump, llvm_filecheck, llvm_nm, llvm_objcopy,
     llvm_objdump, llvm_profdata, llvm_readobj, LlvmAr, LlvmBcanalyzer, LlvmDis, LlvmDwarfdump,
     LlvmFilecheck, LlvmNm, LlvmObjcopy, LlvmObjdump, LlvmProfdata, LlvmReadobj,
 };
diff --git a/tests/run-make/cross-lang-lto-clang/rmake.rs b/tests/run-make/cross-lang-lto-clang/rmake.rs
index 3fed6ea20667a..0c4383e2cd815 100644
--- a/tests/run-make/cross-lang-lto-clang/rmake.rs
+++ b/tests/run-make/cross-lang-lto-clang/rmake.rs
@@ -28,7 +28,16 @@ static C_NEVER_INLINED_PATTERN: &'static str = "bl.*<c_never_inlined>";
 static C_NEVER_INLINED_PATTERN: &'static str = "call.*c_never_inlined";
 
 fn main() {
+    test_lto(false);
+    test_lto(true);
+}
+
+fn test_lto(fat_lto: bool) {
+    let lto = if fat_lto { "fat" } else { "thin" };
+    let clang_lto = if fat_lto { "full" } else { "thin" };
+
     rustc()
+        .lto(lto)
         .linker_plugin_lto("on")
         .output(static_lib_name("rustlib-xlto"))
         .opt_level("2")
@@ -36,7 +45,7 @@ fn main() {
         .input("rustlib.rs")
         .run();
     clang()
-        .lto("thin")
+        .lto(clang_lto)
         .use_ld("lld")
         .arg("-lrustlib-xlto")
         .out_exe("cmain")
@@ -57,9 +66,10 @@ fn main() {
         .input("cmain")
         .run()
         .assert_stdout_contains_regex(RUST_NEVER_INLINED_PATTERN);
-    clang().input("clib.c").lto("thin").arg("-c").out_exe("clib.o").arg("-O2").run();
+    clang().input("clib.c").lto(clang_lto).arg("-c").out_exe("clib.o").arg("-O2").run();
     llvm_ar().obj_to_ar().output_input(static_lib_name("xyz"), "clib.o").run();
     rustc()
+        .lto(lto)
         .linker_plugin_lto("on")
         .opt_level("2")
         .linker(&env_var("CLANG"))
@@ -72,9 +82,12 @@ fn main() {
         .input("rsmain")
         .run()
         .assert_stdout_not_contains_regex(C_ALWAYS_INLINED_PATTERN);
-    llvm_objdump()
-        .disassemble()
-        .input("rsmain")
-        .run()
-        .assert_stdout_contains_regex(C_NEVER_INLINED_PATTERN);
+
+    let dump = llvm_objdump().disassemble().input("rsmain").run();
+    if !fat_lto {
+        dump.assert_stdout_contains_regex(C_NEVER_INLINED_PATTERN);
+    } else {
+        // fat lto inlines this anyway
+        dump.assert_stdout_not_contains_regex(C_NEVER_INLINED_PATTERN);
+    }
 }
diff --git a/tests/run-make/fat-then-thin-lto/lib.rs b/tests/run-make/fat-then-thin-lto/lib.rs
new file mode 100644
index 0000000000000..3091988368628
--- /dev/null
+++ b/tests/run-make/fat-then-thin-lto/lib.rs
@@ -0,0 +1,8 @@
+#![feature(no_core, lang_items)]
+#![no_core]
+#![crate_type = "rlib"]
+
+#[lang = "sized"]
+trait Sized {}
+
+pub fn foo() {}
diff --git a/tests/run-make/fat-then-thin-lto/main.rs b/tests/run-make/fat-then-thin-lto/main.rs
new file mode 100644
index 0000000000000..a3f2e18158bc0
--- /dev/null
+++ b/tests/run-make/fat-then-thin-lto/main.rs
@@ -0,0 +1,11 @@
+#![allow(internal_features)]
+#![feature(no_core, lang_items)]
+#![no_core]
+#![crate_type = "cdylib"]
+
+extern crate lib;
+
+#[unsafe(no_mangle)]
+pub fn bar() {
+    lib::foo();
+}
diff --git a/tests/run-make/fat-then-thin-lto/rmake.rs b/tests/run-make/fat-then-thin-lto/rmake.rs
new file mode 100644
index 0000000000000..ef4f26689d4e8
--- /dev/null
+++ b/tests/run-make/fat-then-thin-lto/rmake.rs
@@ -0,0 +1,25 @@
+// Compile a library with lto=fat, then compile a binary with lto=thin
+// and check that lto is applied with the library.
+// The goal is to mimic the standard library being build with lto=fat
+// and allowing users to build with lto=thin.
+
+//@ only-x86_64-unknown-linux-gnu
+
+use run_make_support::{dynamic_lib_name, llvm_objdump, rustc};
+
+fn main() {
+    rustc().input("lib.rs").opt_level("3").lto("fat").run();
+    rustc().input("main.rs").panic("abort").opt_level("3").lto("thin").run();
+
+    llvm_objdump()
+        .input(dynamic_lib_name("main"))
+        .arg("--disassemble-symbols=bar")
+        .run()
+        // The called function should be inlined.
+        // Check that we have a ret (to detect tail
+        // calls with a jmp) and no call.
+        .assert_stdout_contains("bar")
+        .assert_stdout_contains("ret")
+        .assert_stdout_not_contains("foo")
+        .assert_stdout_not_contains("call");
+}
diff --git a/tests/run-make/linker-plugin-lto-fat/ir.ll b/tests/run-make/linker-plugin-lto-fat/ir.ll
new file mode 100644
index 0000000000000..fa3dbdd4e088d
--- /dev/null
+++ b/tests/run-make/linker-plugin-lto-fat/ir.ll
@@ -0,0 +1,6 @@
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define void @ir_callee() {
+  ret void
+}
diff --git a/tests/run-make/linker-plugin-lto-fat/main.rs b/tests/run-make/linker-plugin-lto-fat/main.rs
new file mode 100644
index 0000000000000..3a7b02f4b92e3
--- /dev/null
+++ b/tests/run-make/linker-plugin-lto-fat/main.rs
@@ -0,0 +1,17 @@
+#![feature(no_core, lang_items)]
+#![no_core]
+#![crate_type = "cdylib"]
+
+#[lang = "sized"]
+trait Sized {}
+
+extern "C" {
+    fn ir_callee();
+}
+
+#[no_mangle]
+extern "C" fn rs_foo() {
+    unsafe {
+        ir_callee();
+    }
+}
diff --git a/tests/run-make/linker-plugin-lto-fat/rmake.rs b/tests/run-make/linker-plugin-lto-fat/rmake.rs
new file mode 100644
index 0000000000000..0cfc799d2aaa1
--- /dev/null
+++ b/tests/run-make/linker-plugin-lto-fat/rmake.rs
@@ -0,0 +1,32 @@
+// Check that -C lto=fat with -C linker-plugin-lto actually works and can inline functions.
+// A library is created from LLVM IR, defining a single function. Then a dylib is compiled,
+// linking to the library and calling the function from the library.
+// The function from the library should end up inlined and disappear from the output.
+
+//@ only-x86_64-unknown-linux-gnu
+//@ needs-rust-lld
+
+use run_make_support::{dynamic_lib_name, llvm_as, llvm_objdump, rustc};
+
+fn main() {
+    llvm_as().input("ir.ll").run();
+    rustc()
+        .input("main.rs")
+        .opt_level("3")
+        .lto("fat")
+        .linker_plugin_lto("on")
+        .link_arg("ir.bc")
+        .arg("-Zlinker-features=+lld")
+        .run();
+
+    llvm_objdump()
+        .input(dynamic_lib_name("main"))
+        .arg("--disassemble-symbols=rs_foo")
+        .run()
+        // The called function should be inlined.
+        // Check that we have a ret (to detect tail
+        // calls with a jmp) and no call.
+        .assert_stdout_contains("foo")
+        .assert_stdout_contains("ret")
+        .assert_stdout_not_contains("call");
+}