Update lint-docs to default to Rust 2024

This updates the lint-docs tool to default to the 2024 edition. The lint
docs are supposed to illustrate the code with the latest edition, and I
just forgot to update this in
rust-lang#133349.

Some docs needed to add the `edition` attribute since they were assuming
a particular edition, but were missing the explicit annotation.

Author: ehuss

compiler/rustc_lint/src/builtin.rs

@@ -948,7 +948,7 @@ declare_lint! {
     ///
     /// ### Example
     ///
-    /// ```rust,compile_fail
+    /// ```rust,compile_fail,edition2021
     /// #[no_mangle]
     /// const FOO: i32 = 5;
     /// ```

compiler/rustc_lint/src/impl_trait_overcaptures.rs

@@ -41,7 +41,7 @@ declare_lint! {
     ///
     /// ### Example
     ///
-    /// ```rust,compile_fail
+    /// ```rust,compile_fail,edition2021
     /// # #![deny(impl_trait_overcaptures)]
     /// # use std::fmt::Display;
     /// let mut x = vec![];

compiler/rustc_lint_defs/src/builtin.rs

@@ -1424,7 +1424,7 @@ declare_lint! {
     ///
     /// ### Example
     ///
-    /// ```rust,compile_fail
+    /// ```rust,compile_fail,edition2021
     /// macro_rules! foo {
     ///    () => {};
     ///    ($name) => { };
@@ -4128,7 +4128,7 @@ declare_lint! {
     ///
     /// ### Example
     ///
-    /// ```rust,compile_fail
+    /// ```rust,compile_fail,edition2021
     /// #![deny(dependency_on_unit_never_type_fallback)]
     /// fn main() {
     ///     if true {

src/tools/lint-docs/src/lib.rs

@@ -449,7 +449,7 @@ impl<'a> LintExtractor<'a> {
             .filter_map(|opt| opt.strip_prefix("edition"))
             .next()
             // defaults to latest edition
-            .unwrap_or("2021");
+            .unwrap_or("2024");
         cmd.arg(format!("--edition={edition}"));
         // Just in case this is an unstable edition.
         cmd.arg("-Zunstable-options");