Auto merge of #143214 - camsteffen:remove-let-chains-feature, r=est31

Remove let_chains unstable feature

Per https://github.com/rust-lang/rust/issues/53667#issuecomment-3016742982 (but then I also noticed rust-lang/rust#140722)

This replaces the feature gate with a parser error that says let chains require 2024.

A lot of tests were using the unstable feature. I either added edition:2024 to the test or split out the parts that require 2024.

compiler/rustc_feature/src/accepted.rs

@@ -265,6 +265,8 @@ declare_features! (
     (accepted, keylocker_x86, "1.89.0", Some(134813)),
     /// Allows `'a: { break 'a; }`.
     (accepted, label_break_value, "1.65.0", Some(48594)),
+    /// Allows `if/while p && let q = r && ...` chains.
+    (accepted, let_chains, "1.88.0", Some(53667)),
     /// Allows `let...else` statements.
     (accepted, let_else, "1.65.0", Some(87335)),
     /// Allows using `reason` in lint attributes and the `#[expect(lint)]` lint check.

compiler/rustc_feature/src/unstable.rs

@@ -550,8 +550,6 @@ declare_features! (
     (unstable, large_assignments, "1.52.0", Some(83518)),
     /// Allow to have type alias types for inter-crate use.
     (incomplete, lazy_type_alias, "1.72.0", Some(112792)),
-    /// Allows `if/while p && let q = r && ...` chains.
-    (unstable, let_chains, "1.37.0", Some(53667)),
     /// Allows using `#[link(kind = "link-arg", name = "...")]`
     /// to pass custom arguments to the linker.
     (unstable, link_arg_attribute, "1.76.0", Some(99427)),