Auto merge of #131326 - dingxiangfei2009:issue-130836-attempt-2, r=nikomatsakis

Reduce false positives of tail-expr-drop-order from consumed values (attempt #2)

r? `@nikomatsakis`

Tracked by #123739.

Related to #129864 but not replacing, yet.

Related to #130836.

This is an implementation of the approach suggested in the [Zulip stream](https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/temporary.20drop.20order.20changes). A new MIR statement `BackwardsIncompatibleDrop` is added to the MIR syntax. The lint now works by inspecting possibly live move paths before at the `BackwardsIncompatibleDrop` location and the actual drop under the current edition, which should be one before Edition 2024 in practice.

1 files changed, 22 insertions, 0 deletions

diff --git a/compiler/rustc_middle/src/query/mod.rs b/compiler/rustc_middle/src/query/mod.rs
index 9ae519dfe7e..3fdb38a433e 100644
--- a/compiler/rustc_middle/src/query/mod.rs
+++ b/compiler/rustc_middle/src/query/mod.rs
@@ -1448,6 +1448,28 @@ rustc_queries! {
         cache_on_disk_if { false }
     }
 
+    /// Returns a list of types which (a) have a potentially significant destructor
+    /// and (b) may be dropped as a result of dropping a value of some type `ty`
+    /// (in the given environment).
+    ///
+    /// The idea of "significant" drop is somewhat informal and is used only for
+    /// diagnostics and edition migrations. The idea is that a significant drop may have
+    /// some visible side-effect on execution; freeing memory is NOT considered a side-effect.
+    /// The rules are as follows:
+    /// * Type with no explicit drop impl do not have significant drop.
+    /// * Types with a drop impl are assumed to have significant drop unless they have a `#[rustc_insignificant_dtor]` annotation.
+    ///
+    /// Note that insignificant drop is a "shallow" property. A type like `Vec<LockGuard>` does not
+    /// have significant drop but the type `LockGuard` does, and so if `ty  = Vec<LockGuard>`
+    /// then the return value would be `&[LockGuard]`.
+    /// *IMPORTANT*: *DO NOT* run this query before promoted MIR body is constructed,
+    /// because this query partially depends on that query.
+    /// Otherwise, there is a risk of query cycles.
+    query list_significant_drop_tys(ty: ty::ParamEnvAnd<'tcx, Ty<'tcx>>) -> &'tcx ty::List<Ty<'tcx>> {
+        desc { |tcx| "computing when `{}` has a significant destructor", ty.value }
+        cache_on_disk_if { false }
+    }
+
     /// Computes the layout of a type. Note that this implicitly
     /// executes in "reveal all" mode, and will normalize the input type.
     query layout_of(