Add tests to check the situation with heterogenous spans

The edition gate is a bit stricter than the drop behaviour,
which is fine. The cases we want to avoid are the opposite:
not gated but 2021 drop behaviour.

tests/ui/rfcs/rfc-2497-if-let-chains/auxiliary/macro-in-2021.rs

@@ -0,0 +1,26 @@
+//@ edition: 2021
+
+#[macro_export]
+macro_rules! make_if {
+    (($($tt:tt)*) { $body:expr } { $else:expr }) => {{
+        if $($tt)* {
+            $body
+        } else {
+            $else
+        }
+    }};
+    (let ($expr:expr) { $body:expr } { $else:expr }) => {{
+        if let None = $expr {
+            $body
+        } else {
+            $else
+        }
+    }};
+    (let ($expr:expr) let ($expr2:expr) { $body:expr } { $else:expr }) => {{
+        if let None = $expr && let None = $expr2 {
+            $body
+        } else {
+            $else
+        }
+    }};
+}

tests/ui/rfcs/rfc-2497-if-let-chains/auxiliary/macro-in-2024.rs

@@ -0,0 +1,26 @@
+//@ edition: 2024
+
+#[macro_export]
+macro_rules! make_if {
+    (($($tt:tt)*) { $body:expr } { $else:expr }) => {{
+        if $($tt)* {
+            $body
+        } else {
+            $else
+        }
+    }};
+    (let ($expr:expr) { $body:expr } { $else:expr }) => {{
+        if let None = $expr {
+            $body
+        } else {
+            $else
+        }
+    }};
+    (let ($expr:expr) let ($expr2:expr) { $body:expr } { $else:expr }) => {{
+        if let None = $expr && let None = $expr2 {
+            $body
+        } else {
+            $else
+        }
+    }};
+}

tests/ui/rfcs/rfc-2497-if-let-chains/edition-gate-macro-error.edition2021.stderr

@@ -0,0 +1,65 @@
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/edition-gate-macro-error.rs:19:30
+   |
+LL |     macro_in_2021::make_if!((let Some(0) = None && let Some(0) = None) { never!() } { never!() });
+   |                              ^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/edition-gate-macro-error.rs:19:52
+   |
+LL |     macro_in_2021::make_if!((let Some(0) = None && let Some(0) = None) { never!() } { never!() });
+   |                                                    ^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/edition-gate-macro-error.rs:22:5
+   |
+LL |     macro_in_2021::make_if!(let (Some(0)) let (Some(0)) { never!() } { never!() });
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = note: this error originates in the macro `macro_in_2021::make_if` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/edition-gate-macro-error.rs:22:5
+   |
+LL |     macro_in_2021::make_if!(let (Some(0)) let (Some(0)) { never!() } { never!() });
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = note: this error originates in the macro `macro_in_2021::make_if` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/edition-gate-macro-error.rs:26:30
+   |
+LL |     macro_in_2024::make_if!((let Some(0) = None && let Some(0) = None) { never!() } { never!() });
+   |                              ^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/edition-gate-macro-error.rs:26:52
+   |
+LL |     macro_in_2024::make_if!((let Some(0) = None && let Some(0) = None) { never!() } { never!() });
+   |                                                    ^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/rfcs/rfc-2497-if-let-chains/edition-gate-macro-error.edition2024.stderr

@@ -0,0 +1,45 @@
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/edition-gate-macro-error.rs:19:30
+   |
+LL |     macro_in_2021::make_if!((let Some(0) = None && let Some(0) = None) { never!() } { never!() });
+   |                              ^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/edition-gate-macro-error.rs:19:52
+   |
+LL |     macro_in_2021::make_if!((let Some(0) = None && let Some(0) = None) { never!() } { never!() });
+   |                                                    ^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/edition-gate-macro-error.rs:22:5
+   |
+LL |     macro_in_2021::make_if!(let (Some(0)) let (Some(0)) { never!() } { never!() });
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = note: this error originates in the macro `macro_in_2021::make_if` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error[E0658]: `let` expressions in this position are unstable
+  --> $DIR/edition-gate-macro-error.rs:22:5
+   |
+LL |     macro_in_2021::make_if!(let (Some(0)) let (Some(0)) { never!() } { never!() });
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #53667 <https://github.com/rust-lang/rust/issues/53667> for more information
+   = help: add `#![feature(let_chains)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+   = note: this error originates in the macro `macro_in_2021::make_if` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 4 previous errors
+
+For more information about this error, try `rustc --explain E0658`.

tests/ui/rfcs/rfc-2497-if-let-chains/edition-gate-macro-error.rs

@@ -0,0 +1,30 @@
+//@ revisions: edition2021 edition2024
+//@ compile-flags: -Z lint-mir -Z validate-mir
+//@ [edition2021] edition: 2021
+//@ [edition2024] edition: 2024
+//@ aux-build:macro-in-2021.rs
+//@ aux-build:macro-in-2024.rs
+
+use std::unreachable as never;
+
+// Compiletest doesn't specify the needed --extern flags to make `extern crate` unneccessary
+extern crate macro_in_2021;
+extern crate macro_in_2024;
+
+fn main() {
+    // Gated on both 2021 and 2024 if the `if` comes from a 2021 macro
+    // Gated only on 2021 if the `if` comes from a 2024 macro
+    // No gating if both the `if` and the chain are from a 2024 macro
+
+    macro_in_2021::make_if!((let Some(0) = None && let Some(0) = None) { never!() } { never!() });
+    //~^ ERROR `let` expressions in this position are unstable
+    //~| ERROR `let` expressions in this position are unstable
+    macro_in_2021::make_if!(let (Some(0)) let (Some(0)) { never!() } { never!() });
+    //~^ ERROR `let` expressions in this position are unstable
+    //~| ERROR `let` expressions in this position are unstable
+
+    macro_in_2024::make_if!((let Some(0) = None && let Some(0) = None) { never!() } { never!() });
+    //[edition2021]~^ ERROR `let` expressions in this position are unstable
+    //[edition2021]~| ERROR `let` expressions in this position are unstable
+    macro_in_2024::make_if!(let (Some(0)) let (Some(0)) { never!() } { never!() });
+}

tests/ui/rfcs/rfc-2497-if-let-chains/edition-gate-macro.rs

@@ -0,0 +1,76 @@
+//@ revisions: edition2021 edition2024
+//@ compile-flags: -Z lint-mir -Z validate-mir
+//@ [edition2021] edition: 2021
+//@ [edition2024] edition: 2024
+//@ aux-build:macro-in-2021.rs
+//@ aux-build:macro-in-2024.rs
+//@ run-pass
+
+#![allow(dead_code)]
+
+use std::unreachable as never;
+use std::cell::RefCell;
+use std::convert::TryInto;
+
+// Compiletest doesn't specify the needed --extern flags to make `extern crate` unneccessary
+extern crate macro_in_2021;
+extern crate macro_in_2024;
+
+#[derive(Default)]
+struct DropOrderCollector(RefCell<Vec<u32>>);
+
+struct LoudDrop<'a>(&'a DropOrderCollector, u32);
+
+impl Drop for LoudDrop<'_> {
+    fn drop(&mut self) {
+        println!("{}", self.1);
+        self.0.0.borrow_mut().push(self.1);
+    }
+}
+
+impl DropOrderCollector {
+    fn print(&self, n: u32) {
+        println!("{n}");
+        self.0.borrow_mut().push(n)
+    }
+    fn some_loud(&self, n: u32) -> Option<LoudDrop> {
+        Some(LoudDrop(self, n))
+    }
+
+    #[track_caller]
+    fn validate(self) {
+        assert!(
+            self.0
+                .into_inner()
+                .into_iter()
+                .enumerate()
+                .all(|(idx, item)| idx + 1 == item.try_into().unwrap())
+        );
+    }
+    fn with_macro_2021(self) {
+        // Edition 2021 drop behaviour
+        macro_in_2021::make_if!((let None = self.some_loud(2)) { never!() } {self.print(1) });
+        macro_in_2021::make_if!(let (self.some_loud(4)) { never!() } { self.print(3) });
+        self.validate();
+    }
+    fn with_macro_2024(self) {
+        // Edition 2024 drop behaviour
+        macro_in_2024::make_if!((let None = self.some_loud(1)) { never!() } { self.print(2) });
+        macro_in_2024::make_if!(let (self.some_loud(3)) { never!() } { self.print(4) });
+        self.validate();
+    }
+}
+
+fn main() {
+    // 2021 drop order if it's a 2021 macro creating the `if`
+    // 2024 drop order if it's a 2024 macro creating the `if`
+
+    // Compare this with edition-gate-macro-error.rs: We want to avoid exposing 2021 drop order,
+    // because it can create bad MIR (issue #104843)
+    // This test doesn't contain any let chains at all: it should be understood
+    // in combination with `edition-gate-macro-error.rs`
+
+    DropOrderCollector::default().with_macro_2021();
+    DropOrderCollector::default().with_macro_2024();
+
+}