[Crater run experiment] let-chain rescoping

compiler/rustc_feature/src/unstable.rs

@@ -491,6 +491,8 @@ declare_features! (
     (unstable, half_open_range_patterns_in_slices, "1.66.0", Some(67264)),
     /// Allows `if let` guard in match arms.
     (unstable, if_let_guard, "1.47.0", Some(51114)),
+    /// Rescoping temporaries in `if let` to align with Rust 2024.
+    (unstable, if_let_rescope, "1.78.0", Some(124085)),
     /// Allows `impl Trait` to be used inside associated types (RFC 2515).
     (unstable, impl_trait_in_assoc_type, "1.70.0", Some(63063)),
     /// Allows `impl Trait` as output type in `Fn` traits in return position of functions.

compiler/rustc_hir_typeck/src/rvalue_scopes.rs

@@ -70,7 +70,7 @@ pub fn resolve_rvalue_scopes<'a, 'tcx>(
     def_id: DefId,
 ) -> RvalueScopes {
     let tcx = &fcx.tcx;
-    let mut rvalue_scopes = RvalueScopes::new();
+    let mut rvalue_scopes = RvalueScopes::new(true);
     debug!("start resolving rvalue scopes, def_id={def_id:?}");
     debug!("rvalue_scope: rvalue_candidates={:?}", scope_tree.rvalue_candidates);
     for (&hir_id, candidate) in &scope_tree.rvalue_candidates {

compiler/rustc_middle/src/ty/rvalue_scopes.rs

@@ -7,12 +7,13 @@ use rustc_macros::{HashStable, TyDecodable, TyEncodable};
 /// rules laid out in `rustc_hir_analysis::check::rvalue_scopes`.
 #[derive(TyEncodable, TyDecodable, Clone, Debug, Default, Eq, PartialEq, HashStable)]
 pub struct RvalueScopes {
+    rescope_if_let: bool,
     map: ItemLocalMap<Option<Scope>>,
 }
 
 impl RvalueScopes {
-    pub fn new() -> Self {
-        Self { map: <_>::default() }
+    pub fn new(rescope_if_let: bool) -> Self {
+        Self { rescope_if_let, map: <_>::default() }
     }
 
     /// Returns the scope when the temp created by `expr_id` will be cleaned up.
@@ -39,7 +40,17 @@ impl RvalueScopes {
                     debug!("temporary_scope({expr_id:?}) = {id:?} [enclosing]");
                     return Some(id);
                 }
-                _ => id = p,
+                ScopeData::IfThen => {
+                    if self.rescope_if_let {
+                        debug!("temporary_scope({expr_id:?}) = {p:?} [enclosing]");
+                        return Some(p);
+                    }
+                    id = p;
+                }
+                ScopeData::Node
+                | ScopeData::CallSite
+                | ScopeData::Arguments
+                | ScopeData::Remainder(_) => id = p,
             }
         }
 

compiler/rustc_span/src/symbol.rs

@@ -978,6 +978,7 @@ symbols! {
         ident,
         if_let,
         if_let_guard,
+        if_let_rescope,
         if_while_or_patterns,
         ignore,
         impl_header_lifetime_elision,

tests/ui/drop/drop_order_if_let_rescope.rs

@@ -0,0 +1,122 @@
+//@ run-pass
+//@ edition:2024
+//@ compile-flags: -Z validate-mir -Zunstable-options
+
+#![feature(let_chains)]
+#![feature(if_let_rescope)]
+
+use std::cell::RefCell;
+use std::convert::TryInto;
+
+#[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 option_loud_drop(&self, n: u32) -> Option<LoudDrop> {
+        Some(LoudDrop(self, n))
+    }
+
+    fn print(&self, n: u32) {
+        println!("{}", n);
+        self.0.borrow_mut().push(n)
+    }
+
+    fn assert_sorted(self) {
+        assert!(
+            self.0
+                .into_inner()
+                .into_iter()
+                .enumerate()
+                .all(|(idx, item)| idx + 1 == item.try_into().unwrap())
+        );
+    }
+
+    fn if_let(&self) {
+        if let None = self.option_loud_drop(1) {
+            unreachable!();
+        } else {
+            self.print(2);
+        }
+
+        if let Some(_) = self.option_loud_drop(4) {
+            self.print(3);
+        }
+
+        if let Some(_d) = self.option_loud_drop(6) {
+            self.print(5);
+        }
+    }
+
+    fn let_chain(&self) {
+        // take the "then" branch
+        if self.option_loud_drop(1).is_some() // 1
+            && self.option_loud_drop(2).is_some() // 2
+            && let Some(_d) = self.option_loud_drop(4)
+        // 4
+        {
+            self.print(3); // 3
+        }
+
+        // take the "else" branch
+        if self.option_loud_drop(5).is_some() // 1
+            && self.option_loud_drop(6).is_some() // 2
+            && let None = self.option_loud_drop(7)
+        // 3
+        {
+            unreachable!();
+        } else {
+            self.print(8); // 4
+        }
+
+        // let exprs interspersed
+        if self.option_loud_drop(9).is_some() // 1
+            && let Some(_d) = self.option_loud_drop(13) // 5
+            && self.option_loud_drop(10).is_some() // 2
+            && let Some(_e) = self.option_loud_drop(12)
+        // 4
+        {
+            self.print(11); // 3
+        }
+
+        // let exprs first
+        if let Some(_d) = self.option_loud_drop(18) // 5
+            && let Some(_e) = self.option_loud_drop(17) // 4
+            && self.option_loud_drop(14).is_some() // 1
+            && self.option_loud_drop(15).is_some()
+        // 2
+        {
+            self.print(16); // 3
+        }
+
+        // let exprs last
+        if self.option_loud_drop(19).is_some() // 1
+            && self.option_loud_drop(20).is_some() // 2
+            && let Some(_d) = self.option_loud_drop(23) // 5
+            && let Some(_e) = self.option_loud_drop(22)
+        // 4
+        {
+            self.print(21); // 3
+        }
+    }
+}
+
+fn main() {
+    println!("-- if let --");
+    let collector = DropOrderCollector::default();
+    collector.if_let();
+    collector.assert_sorted();
+
+    println!("-- let chain --");
+    let collector = DropOrderCollector::default();
+    collector.let_chain();
+    collector.assert_sorted();
+}

tests/ui/feature-gates/feature-gate-if-let-rescope.rs

@@ -0,0 +1,25 @@
+struct A;
+struct B<'a, T>(&'a mut T);
+
+impl A {
+    fn f(&mut self) -> Option<B<'_, Self>> {
+        Some(B(self))
+    }
+}
+
+impl<'a, T> Drop for B<'a, T> {
+    fn drop(&mut self) {
+        // this is needed to keep NLL's hands off and to ensure
+        // the inner mutable borrow stays alive
+    }
+}
+
+fn main() {
+    let mut a = A;
+    if let None = a.f().as_ref() {
+        unreachable!()
+    } else {
+        a.f().unwrap();
+        //~^ ERROR cannot borrow `a` as mutable more than once at a time
+    };
+}

tests/ui/feature-gates/feature-gate-if-let-rescope.stderr

@@ -0,0 +1,18 @@
+error[E0499]: cannot borrow `a` as mutable more than once at a time
+  --> $DIR/feature-gate-if-let-rescope.rs:22:9
+   |
+LL |     if let None = a.f().as_ref() {
+   |                   -----
+   |                   |
+   |                   first mutable borrow occurs here
+   |                   a temporary with access to the first borrow is created here ...
+...
+LL |         a.f().unwrap();
+   |         ^ second mutable borrow occurs here
+LL |
+LL |     };
+   |      - ... and the first borrow might be used here, when that temporary is dropped and runs the destructor for type `Option<B<'_, A>>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0499`.

tests/ui/nll/issue-54556-niconii.rs

@@ -1,3 +1,4 @@
+//@ check-pass
 // This is a reduction of a concrete test illustrating a case that was
 // annoying to Rust developer niconii (see comment thread on #21114).
 //
@@ -19,7 +20,7 @@ impl Mutex {
 fn main() {
     let counter = Mutex;
 
-    if let Ok(_) = counter.lock() { } //~ ERROR does not live long enough
+    if let Ok(_) = counter.lock() { }
 
     // With this code as written, the dynamic semantics here implies
     // that `Mutex::drop` for `counter` runs *before*
@@ -28,4 +29,5 @@ fn main() {
     //
     // The goal of #54556 is to explain that within a compiler
     // diagnostic.
+    ()
 }