Rollup merge of #133753 - dingxiangfei2009:reduce-false-positive-if-l…

…et-rescope, r=jieyouxu Reduce false positives on some common cases from if-let-rescope lint r? `@jieyouxu` We would like to identify a very common case in the ecosystem in which we do not need to apply the lint suggestion for the new Edition 2024 `if let` semantics. In this patch we excluded linting from `if let`s in statements and block tail expressions. In these simple cases, new Edition 2024 drop orders are identical to those of Edition 2021 and prior. However, conservatively we should still lint for the other cases, because [this example](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=2113df5ce78f161d32a1190faf5c7469) shows that the drop order changes are very pronounced, some of which are even sensitive to runtime data.
rust-lang · Dec 3, 2024 · 9378132 · 9378132
2 parents 1ae6f47 + 2d61c09
commit 9378132
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diff --git a/compiler/rustc_lint/src/if_let_rescope.rs b/compiler/rustc_lint/src/if_let_rescope.rs
@@ -103,8 +103,11 @@ fn expr_parent_is_else(tcx: TyCtxt<'_>, hir_id: hir::HirId) -> bool {
 }
 
 fn expr_parent_is_stmt(tcx: TyCtxt<'_>, hir_id: hir::HirId) -> bool {
-    let Some((_, hir::Node::Stmt(stmt))) = tcx.hir().parent_iter(hir_id).next() else {
-        return false;
+    let mut parents = tcx.hir().parent_iter(hir_id);
+    let stmt = match parents.next() {
+        Some((_, hir::Node::Stmt(stmt))) => stmt,
+        Some((_, hir::Node::Block(_) | hir::Node::Arm(_))) => return true,
+        _ => return false,
     };
     let (hir::StmtKind::Semi(expr) | hir::StmtKind::Expr(expr)) = stmt.kind else { return false };
     expr.hir_id == hir_id

diff --git a/tests/ui/drop/lint-if-let-rescope.fixed b/tests/ui/drop/lint-if-let-rescope.fixed
@@ -14,7 +14,7 @@ impl Drop for Droppy {
     }
 }
 impl Droppy {
-    fn get(&self) -> Option<u8> {
+    const fn get(&self) -> Option<u8> {
         None
     }
 }
@@ -62,11 +62,10 @@ fn main() {
         //~| HELP: a `match` with a single arm can preserve the drop order up to Edition 2021
     }
 
-    if let () = { match Droppy.get() { 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
+    if let () = { if let Some(_value) = Droppy.get() {} } {
+        // This should not lint.
+        // This `if let` sits is a tail expression of a block.
+        // In Edition 2024, the temporaries are dropped before exiting the surrounding block.
     }
 
     #[rustfmt::skip]
@@ -94,4 +93,17 @@ fn main() {
         //~| 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]
+    match droppy().get() {
+        _ => if let Some(_value) = droppy().get() {},
+        // Should not lint
+        // There is implicitly a block surrounding the `if let`.
+        // Given that it is a tail expression, the temporaries are dropped duly before
+        // the execution is exiting the `match`.
+    }
+
+    if let Some(_value) = droppy().get() {}
 }
diff --git a/tests/ui/drop/lint-if-let-rescope.rs b/tests/ui/drop/lint-if-let-rescope.rs
@@ -14,7 +14,7 @@ impl Drop for Droppy {
     }
 }
 impl Droppy {
-    fn get(&self) -> Option<u8> {
+    const fn get(&self) -> Option<u8> {
         None
     }
 }
@@ -63,10 +63,9 @@ fn main() {
     }
 
     if let () = { if let Some(_value) = Droppy.get() {} } {
-        //~^ 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
+        // This should not lint.
+        // This `if let` sits is a tail expression of a block.
+        // In Edition 2024, the temporaries are dropped before exiting the surrounding block.
     }
 
     #[rustfmt::skip]
@@ -94,4 +93,17 @@ fn main() {
         //~| 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]
+    match droppy().get() {
+        _ => if let Some(_value) = droppy().get() {},
+        // Should not lint
+        // There is implicitly a block surrounding the `if let`.
+        // Given that it is a tail expression, the temporaries are dropped duly before
+        // the execution is exiting the `match`.
+    }
+
+    if let Some(_value) = droppy().get() {}
 }
diff --git a/tests/ui/drop/lint-if-let-rescope.stderr b/tests/ui/drop/lint-if-let-rescope.stderr
@@ -112,27 +112,7 @@ LL |     if let Some(1) = { match Droppy.get() { Some(_value) => { Some(1) } _ =
    |                        ~~~~~              +++++++++++++++++             ~~~~         +
 
 error: `if let` assigns a shorter lifetime since Edition 2024
-  --> $DIR/lint-if-let-rescope.rs:65:22
-   |
-LL |     if let () = { if let Some(_value) = Droppy.get() {} } {
-   |                      ^^^^^^^^^^^^^^^^^^^------^^^^^^
-   |                                         |
-   |                                         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 issue #124085 <https://github.com/rust-lang/rust/issues/124085>
-help: the value is now dropped here in Edition 2024
-  --> $DIR/lint-if-let-rescope.rs:65:55
-   |
-LL |     if let () = { if let Some(_value) = Droppy.get() {} } {
-   |                                                       ^
-help: a `match` with a single arm can preserve the drop order up to Edition 2021
-   |
-LL |     if let () = { match Droppy.get() { Some(_value) => {} _ => {}} } {
-   |                   ~~~~~              +++++++++++++++++    ++++++++
-
-error: `if let` assigns a shorter lifetime since Edition 2024
-  --> $DIR/lint-if-let-rescope.rs:73:12
+  --> $DIR/lint-if-let-rescope.rs:72:12
    |
 LL |     if (if let Some(_value) = droppy().get() { true } else { false }) {
    |            ^^^^^^^^^^^^^^^^^^^--------^^^^^^
@@ -142,7 +122,7 @@ LL |     if (if let Some(_value) = droppy().get() { true } else { false }) {
    = warning: this changes meaning in Rust 2024
    = note: for more information, see issue #124085 <https://github.com/rust-lang/rust/issues/124085>
 help: the value is now dropped here in Edition 2024
-  --> $DIR/lint-if-let-rescope.rs:73:53
+  --> $DIR/lint-if-let-rescope.rs:72:53
    |
 LL |     if (if let Some(_value) = droppy().get() { true } else { false }) {
    |                                                     ^
@@ -152,7 +132,7 @@ LL |     if (match droppy().get() { Some(_value) => { true } _ => { false }}) {
    |         ~~~~~                +++++++++++++++++          ~~~~          +
 
 error: `if let` assigns a shorter lifetime since Edition 2024
-  --> $DIR/lint-if-let-rescope.rs:79:21
+  --> $DIR/lint-if-let-rescope.rs:78:21
    |
 LL |     } else if (((if let Some(_value) = droppy().get() { true } else { false }))) {
    |                     ^^^^^^^^^^^^^^^^^^^--------^^^^^^
@@ -162,7 +142,7 @@ LL |     } else if (((if let Some(_value) = droppy().get() { true } else { false
    = warning: this changes meaning in Rust 2024
    = note: for more information, see issue #124085 <https://github.com/rust-lang/rust/issues/124085>
 help: the value is now dropped here in Edition 2024
-  --> $DIR/lint-if-let-rescope.rs:79:62
+  --> $DIR/lint-if-let-rescope.rs:78:62
    |
 LL |     } else if (((if let Some(_value) = droppy().get() { true } else { false }))) {
    |                                                              ^
@@ -172,7 +152,7 @@ LL |     } else if (((match droppy().get() { Some(_value) => { true } _ => { fal
    |                  ~~~~~                +++++++++++++++++          ~~~~          +
 
 error: `if let` assigns a shorter lifetime since Edition 2024
-  --> $DIR/lint-if-let-rescope.rs:91:15
+  --> $DIR/lint-if-let-rescope.rs:90:15
    |
 LL |     while (if let Some(_value) = droppy().get() { false } else { true }) {
    |               ^^^^^^^^^^^^^^^^^^^--------^^^^^^
@@ -182,7 +162,7 @@ LL |     while (if let Some(_value) = droppy().get() { false } else { true }) {
    = warning: this changes meaning in Rust 2024
    = note: for more information, see issue #124085 <https://github.com/rust-lang/rust/issues/124085>
 help: the value is now dropped here in Edition 2024
-  --> $DIR/lint-if-let-rescope.rs:91:57
+  --> $DIR/lint-if-let-rescope.rs:90:57
    |
 LL |     while (if let Some(_value) = droppy().get() { false } else { true }) {
    |                                                         ^
@@ -191,5 +171,5 @@ help: a `match` with a single arm can preserve the drop order up to Edition 2021
 LL |     while (match droppy().get() { Some(_value) => { false } _ => { true }}) {
    |            ~~~~~                +++++++++++++++++           ~~~~         +
 
-error: aborting due to 8 previous errors
+error: aborting due to 7 previous errors