Consider lvalues of field and index as possibly temporary places

(cherry picked from commit bad8e98)

Author: compiler-errors

compiler/rustc_lint/src/if_let_rescope.rs

@@ -436,6 +436,14 @@ impl<'tcx> Visitor<'tcx> for FindSignificantDropper<'_, 'tcx> {
                 self.check_promoted_temp_with_drop(expr)?;
                 intravisit::walk_expr(self, expr)
             }
+            // `(Drop, ()).1` introduces a temporary and then moves out of
+            // part of it, therefore we should check it for temporaries.
+            // FIXME: This may have false positives if we move the part
+            // that actually has drop, but oh well.
+            hir::ExprKind::Index(expr, _, _) | hir::ExprKind::Field(expr, _) => {
+                self.check_promoted_temp_with_drop(expr)?;
+                intravisit::walk_expr(self, expr)
+            }
             // If always introduces a temporary terminating scope for its cond and arms,
             // so don't visit them.
             hir::ExprKind::If(..) => ControlFlow::Continue(()),

tests/ui/drop/lint-if-let-rescope-false-positives.rs

@@ -26,4 +26,8 @@ fn main() {
 
     // Make sure we don't lint if we consume the droppy value.
     if let None = consume(Drop) {} else {}
+
+    // Make sure we don't lint on field exprs of place exprs.
+    let tup_place = (Drop, ());
+    if let None = consume(tup_place.1) {} else {}
 }

tests/ui/drop/lint-if-let-rescope.fixed

@@ -94,6 +94,12 @@ fn main() {
         //~| HELP: a `match` with a single arm can preserve the drop order up to Edition 2021
     }
 
+    match Some((droppy(), ()).1) { Some(_value) => {} _ => {}}
+    //~^ ERROR: `if let` assigns a shorter lifetime since Edition 2024
+    //~| WARN: this changes meaning in Rust 2024
+    //~| HELP: the value is now dropped here in Edition 2024
+    //~| HELP: a `match` with a single arm can preserve the drop order up to Edition 2021
+
     // We want to keep the `if let`s below as direct descendents of match arms,
     // so the formatting is suppressed.
     #[rustfmt::skip]

tests/ui/drop/lint-if-let-rescope.rs

@@ -94,6 +94,12 @@ fn main() {
         //~| HELP: a `match` with a single arm can preserve the drop order up to Edition 2021
     }
 
+    if let Some(_value) = Some((droppy(), ()).1) {} else {}
+    //~^ ERROR: `if let` assigns a shorter lifetime since Edition 2024
+    //~| WARN: this changes meaning in Rust 2024
+    //~| HELP: the value is now dropped here in Edition 2024
+    //~| HELP: a `match` with a single arm can preserve the drop order up to Edition 2021
+
     // We want to keep the `if let`s below as direct descendents of match arms,
     // so the formatting is suppressed.
     #[rustfmt::skip]

tests/ui/drop/lint-if-let-rescope.stderr

@@ -175,5 +175,26 @@ LL -     while (if let Some(_value) = droppy().get() { false } else { true }) {
 LL +     while (match droppy().get() { Some(_value) => { false } _ => { true }}) {
    |
 
-error: aborting due to 7 previous errors
+error: `if let` assigns a shorter lifetime since Edition 2024
+  --> $DIR/lint-if-let-rescope.rs:97:8
+   |
+LL |     if let Some(_value) = Some((droppy(), ()).1) {} else {}
+   |        ^^^^^^^^^^^^^^^^^^^^^^^^--------------^^^
+   |                                |
+   |                                this value has a significant drop implementation which may observe a major change in drop order and requires your discretion
+   |
+   = warning: this changes meaning in Rust 2024
+   = note: for more information, see <https://doc.rust-lang.org/nightly/edition-guide/rust-2024/temporary-if-let-scope.html>
+help: the value is now dropped here in Edition 2024
+  --> $DIR/lint-if-let-rescope.rs:97:51
+   |
+LL |     if let Some(_value) = Some((droppy(), ()).1) {} else {}
+   |                                                   ^
+help: a `match` with a single arm can preserve the drop order up to Edition 2021
+   |
+LL -     if let Some(_value) = Some((droppy(), ()).1) {} else {}
+LL +     match Some((droppy(), ()).1) { Some(_value) => {} _ => {}}
+   |
+
+error: aborting due to 8 previous errors