diff --git a/compiler/rustc_ast/src/ast.rs b/compiler/rustc_ast/src/ast.rs
index 9cc81f391672e..c1b26ca092538 100644
--- a/compiler/rustc_ast/src/ast.rs
+++ b/compiler/rustc_ast/src/ast.rs
@@ -2743,8 +2743,19 @@ impl Item {
 /// `extern` qualifier on a function item or function type.
 #[derive(Clone, Copy, Encodable, Decodable, Debug)]
 pub enum Extern {
+    /// No explicit extern keyword was used
+    ///
+    /// E.g. `fn foo() {}`
     None,
+    /// An explicit extern keyword was used, but with implicit ABI
+    ///
+    /// E.g. `extern fn foo() {}`
+    ///
+    /// This is just `extern "C"` (see `rustc_target::spec::abi::Abi::FALLBACK`)
     Implicit(Span),
+    /// An explicit extern keyword was used with an explicit ABI
+    ///
+    /// E.g. `extern "C" fn foo() {}`
     Explicit(StrLit, Span),
 }
 
@@ -2763,9 +2774,13 @@ impl Extern {
 /// included in this struct (e.g., `async unsafe fn` or `const extern "C" fn`).
 #[derive(Clone, Copy, Encodable, Decodable, Debug)]
 pub struct FnHeader {
+    /// The `unsafe` keyword, if any
     pub unsafety: Unsafe,
+    /// The `async` keyword, if any
     pub asyncness: Async,
+    /// The `const` keyword, if any
     pub constness: Const,
+    /// The `extern` keyword and corresponding ABI string, if any
     pub ext: Extern,
 }