Introduce missing ABI lint on extern blocks

compiler/rustc_lint_defs/src/builtin.rs

@@ -2917,6 +2917,7 @@ declare_lint_pass! {
         FUNCTION_ITEM_REFERENCES,
         USELESS_DEPRECATED,
         UNSUPPORTED_NAKED_FUNCTIONS,
+        MISSING_ABI,
     ]
 }
 
@@ -2944,3 +2945,28 @@ declare_lint! {
 }
 
 declare_lint_pass!(UnusedDocComment => [UNUSED_DOC_COMMENTS]);
+
+declare_lint! {
+    /// The `missing_abi` lint detects cases where the ABI is omitted from
+    /// extern declarations.
+    ///
+    /// ### Example
+    ///
+    /// ```rust,compile_fail
+    /// #![deny(missing_abi)]
+    ///
+    /// extern fn foo() {}
+    /// ```
+    ///
+    /// {{produces}}
+    ///
+    /// ### Explanation
+    ///
+    /// Historically, Rust implicitly selected C as the ABI for extern
+    /// declarations. We expect to add new ABIs, like `C-unwind`, in the future,
+    /// though this has not yet happened, and especially with their addition
+    /// seeing the ABI easily will make code review easier.
+    pub MISSING_ABI,
+    Allow,
+    "No declared ABI for extern declaration"
+}