diff --git a/compiler/rustc_codegen_llvm/src/lib.rs b/compiler/rustc_codegen_llvm/src/lib.rs
index 7f26bbd7f8757..6a303e1e6024b 100644
--- a/compiler/rustc_codegen_llvm/src/lib.rs
+++ b/compiler/rustc_codegen_llvm/src/lib.rs
@@ -6,6 +6,7 @@
 
 // tidy-alphabetical-start
 #![allow(internal_features)]
+#![cfg_attr(bootstrap, feature(unsafe_extern_blocks))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(assert_matches)]
diff --git a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
index 7a495174ff1aa..5e0d7418993cc 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
@@ -564,12 +564,12 @@ pub enum ArchiveKind {
 }
 
 // LLVMRustThinLTOData
-extern "C" {
+unsafe extern "C" {
     pub type ThinLTOData;
 }
 
 // LLVMRustThinLTOBuffer
-extern "C" {
+unsafe extern "C" {
     pub type ThinLTOBuffer;
 }
 
@@ -621,7 +621,7 @@ pub enum MemoryEffects {
     InaccessibleMemOnly,
 }
 
-extern "C" {
+unsafe extern "C" {
     type Opaque;
 }
 #[repr(C)]
@@ -631,54 +631,54 @@ struct InvariantOpaque<'a> {
 }
 
 // Opaque pointer types
-extern "C" {
+unsafe extern "C" {
     pub type Module;
 }
-extern "C" {
+unsafe extern "C" {
     pub type Context;
 }
-extern "C" {
+unsafe extern "C" {
     pub type Type;
 }
-extern "C" {
+unsafe extern "C" {
     pub type Value;
 }
-extern "C" {
+unsafe extern "C" {
     pub type ConstantInt;
 }
-extern "C" {
+unsafe extern "C" {
     pub type Attribute;
 }
-extern "C" {
+unsafe extern "C" {
     pub type Metadata;
 }
-extern "C" {
+unsafe extern "C" {
     pub type BasicBlock;
 }
 #[repr(C)]
 pub struct Builder<'a>(InvariantOpaque<'a>);
 #[repr(C)]
 pub struct PassManager<'a>(InvariantOpaque<'a>);
-extern "C" {
+unsafe extern "C" {
     pub type Pass;
 }
-extern "C" {
+unsafe extern "C" {
     pub type TargetMachine;
 }
-extern "C" {
+unsafe extern "C" {
     pub type Archive;
 }
 #[repr(C)]
 pub struct ArchiveIterator<'a>(InvariantOpaque<'a>);
 #[repr(C)]
 pub struct ArchiveChild<'a>(InvariantOpaque<'a>);
-extern "C" {
+unsafe extern "C" {
     pub type Twine;
 }
-extern "C" {
+unsafe extern "C" {
     pub type DiagnosticInfo;
 }
-extern "C" {
+unsafe extern "C" {
     pub type SMDiagnostic;
 }
 #[repr(C)]
@@ -688,7 +688,7 @@ pub struct OperandBundleDef<'a>(InvariantOpaque<'a>);
 #[repr(C)]
 pub struct Linker<'a>(InvariantOpaque<'a>);
 
-extern "C" {
+unsafe extern "C" {
     pub type DiagnosticHandler;
 }
 
@@ -823,7 +823,7 @@ bitflags! {
     }
 }
 
-extern "C" {
+unsafe extern "C" {
     pub type ModuleBuffer;
 }
 
@@ -834,7 +834,7 @@ pub type SelfProfileAfterPassCallback = unsafe extern "C" fn(*mut c_void);
 pub type GetSymbolsCallback = unsafe extern "C" fn(*mut c_void, *const c_char) -> *mut c_void;
 pub type GetSymbolsErrorCallback = unsafe extern "C" fn(*const c_char) -> *mut c_void;
 
-extern "C" {
+unsafe extern "C" {
     // Create and destroy contexts.
     pub fn LLVMContextDispose(C: &'static mut Context);
     pub fn LLVMGetMDKindIDInContext(C: &Context, Name: *const c_char, SLen: c_uint) -> c_uint;
@@ -1518,7 +1518,7 @@ extern "C" {
 }
 
 #[link(name = "llvm-wrapper", kind = "static")]
-extern "C" {
+unsafe extern "C" {
     pub fn LLVMRustInstallErrorHandlers();
     pub fn LLVMRustDisableSystemDialogsOnCrash();
 
diff --git a/compiler/rustc_driver_impl/src/lib.rs b/compiler/rustc_driver_impl/src/lib.rs
index 2b7dc040f6411..ed072cd10b7a0 100644
--- a/compiler/rustc_driver_impl/src/lib.rs
+++ b/compiler/rustc_driver_impl/src/lib.rs
@@ -7,6 +7,7 @@
 // tidy-alphabetical-start
 #![allow(internal_features)]
 #![allow(rustc::untranslatable_diagnostic)] // FIXME: make this translatable
+#![cfg_attr(bootstrap, feature(unsafe_extern_blocks))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(decl_macro)]
diff --git a/compiler/rustc_driver_impl/src/signal_handler.rs b/compiler/rustc_driver_impl/src/signal_handler.rs
index 51f2a508cf90c..e1f868c2522d9 100644
--- a/compiler/rustc_driver_impl/src/signal_handler.rs
+++ b/compiler/rustc_driver_impl/src/signal_handler.rs
@@ -6,7 +6,7 @@ use std::{fmt, mem, ptr};
 
 use rustc_interface::util::{DEFAULT_STACK_SIZE, STACK_SIZE};
 
-extern "C" {
+unsafe extern "C" {
     fn backtrace_symbols_fd(buffer: *const *mut libc::c_void, size: libc::c_int, fd: libc::c_int);
 }
 
diff --git a/compiler/rustc_llvm/src/lib.rs b/compiler/rustc_llvm/src/lib.rs
index 939e5e4dbd4ff..96e370c5b2648 100644
--- a/compiler/rustc_llvm/src/lib.rs
+++ b/compiler/rustc_llvm/src/lib.rs
@@ -1,5 +1,6 @@
 // tidy-alphabetical-start
 #![allow(internal_features)]
+#![cfg_attr(bootstrap, feature(unsafe_attributes, unsafe_extern_blocks))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(rustdoc_internals)]
@@ -28,7 +29,7 @@ impl RustString {
 }
 
 /// Appending to a Rust string -- used by RawRustStringOstream.
-#[no_mangle]
+#[unsafe(no_mangle)]
 pub unsafe extern "C" fn LLVMRustStringWriteImpl(
     sr: &RustString,
     ptr: *const c_char,
@@ -46,7 +47,7 @@ pub fn initialize_available_targets() {
         ($cfg:meta, $($method:ident),*) => { {
             #[cfg($cfg)]
             fn init() {
-                extern "C" {
+                unsafe extern "C" {
                     $(fn $method();)*
                 }
                 unsafe {
diff --git a/compiler/rustc_middle/src/lib.rs b/compiler/rustc_middle/src/lib.rs
index 6886562d9b76d..ce330b4744bf6 100644
--- a/compiler/rustc_middle/src/lib.rs
+++ b/compiler/rustc_middle/src/lib.rs
@@ -28,7 +28,7 @@
 #![allow(rustc::diagnostic_outside_of_impl)]
 #![allow(rustc::potential_query_instability)]
 #![allow(rustc::untranslatable_diagnostic)]
-#![cfg_attr(bootstrap, feature(min_exhaustive_patterns))]
+#![cfg_attr(bootstrap, feature(min_exhaustive_patterns, unsafe_extern_blocks))]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![doc(rust_logo)]
 #![feature(allocator_api)]
diff --git a/compiler/rustc_middle/src/ty/list.rs b/compiler/rustc_middle/src/ty/list.rs
index 1a1acf36d77a4..af76d5b7d9267 100644
--- a/compiler/rustc_middle/src/ty/list.rs
+++ b/compiler/rustc_middle/src/ty/list.rs
@@ -58,7 +58,7 @@ impl<T> Default for &List<T> {
     }
 }
 
-extern "C" {
+unsafe extern "C" {
     /// A dummy type used to force `List` to be unsized while not requiring
     /// references to it be wide pointers.
     type OpaqueListContents;