Auto merge of #118607 - Mark-Simulacrum:stable-next, r=Mark-Simulacrum 1.74.1

[stable] 1.74.1 release

This includes backports of:

*  Dispose llvm::TargetMachines prior to llvm::Context being disposed #118464
*  clarify fn discriminant guarantees: only free lifetimes may get erased #118006
*  Move subtyper below reveal_all and change reveal_all #116415
   *  Make subtyping explicit in MIR #115025 (needed for above)

As well as infrastructure fix:

*  Don't ask for a specific branch in cargotest #118597

r? `@Mark-Simulacrum`

1 files changed, 12 insertions, 0 deletions

diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index e30172727f2..0b95fdfa1fb 100644
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -1075,6 +1075,18 @@ pub enum ProjectionElem<V, T> {
     /// Like an explicit cast from an opaque type to a concrete type, but without
     /// requiring an intermediate variable.
     OpaqueCast(T),
+
+    /// A `Subtype(T)` projection is applied to any `StatementKind::Assign` where
+    /// type of lvalue doesn't match the type of rvalue, the primary goal is making subtyping
+    /// explicit during optimizations and codegen.
+    ///
+    /// This projection doesn't impact the runtime behavior of the program except for potentially changing
+    /// some type metadata of the interpreter or codegen backend.
+    ///
+    /// This goal is achieved with mir_transform pass `Subtyper`, which runs right after
+    /// borrowchecker, as we only care about subtyping that can affect trait selection and
+    /// `TypeId`.
+    Subtype(T),
 }
 
 /// Alias for projections as they appear in places, where the base is a place