Merge from rustc

[rust.git] / compiler / rustc_middle / src / mir / syntax.rs

diff --git a/compiler/rustc_middle/src/mir/syntax.rs b/compiler/rustc_middle/src/mir/syntax.rs
index a6ca04f5e627ade49119c343de8f3fd405470dc6..6b4489026d3d379dee958125b73ced3eb3d40fb1 100644 (file)
--- a/compiler/rustc_middle/src/mir/syntax.rs
+++ b/compiler/rustc_middle/src/mir/syntax.rs
@@ -320,8 +320,10 @@ pub enum StatementKind<'tcx> {
     /// <https://internals.rust-lang.org/t/stacked-borrows-an-aliasing-model-for-rust/8153/> for
     /// more details.
     ///
-    /// For code that is not specific to stacked borrows, you should consider retags to read
-    /// and modify the place in an opaque way.
+    /// For code that is not specific to stacked borrows, you should consider retags to read and
+    /// modify the place in an opaque way.
+    ///
+    /// Only `RetagKind::Default` and `RetagKind::FnEntry` are permitted.
     Retag(RetagKind, Box<Place<'tcx>>),
 
     /// Encodes a user's type ascription. These need to be preserved
@@ -562,14 +564,13 @@ pub enum TerminatorKind<'tcx> {
     Unreachable,
 
     /// The behavior of this statement differs significantly before and after drop elaboration.
-    /// After drop elaboration, `Drop` executes the drop glue for the specified place, after which
-    /// it continues execution/unwinds at the given basic blocks. It is possible that executing drop
-    /// glue is special - this would be part of Rust's memory model. (**FIXME**: due we have an
-    /// issue tracking if drop glue has any interesting semantics in addition to those of a function
-    /// call?)
     ///
-    /// `Drop` before drop elaboration is a *conditional* execution of the drop glue. Specifically, the
-    /// `Drop` will be executed if...
+    /// After drop elaboration: `Drop` terminators are a complete nop for types that have no drop
+    /// glue. For other types, `Drop` terminators behave exactly like a call to
+    /// `core::mem::drop_in_place` with a pointer to the given place.
+    ///
+    /// `Drop` before drop elaboration is a *conditional* execution of the drop glue. Specifically,
+    /// the `Drop` will be executed if...
     ///
     /// **Needs clarification**: End of that sentence. This in effect should document the exact
     /// behavior of drop elaboration. The following sounds vaguely right, but I'm not quite sure:
@@ -890,18 +891,11 @@ pub struct Place<'tcx> {
     pub projection: &'tcx List<PlaceElem<'tcx>>,
 }
 
-/// The different kinds of projections that can be used in the projection of a `Place`.
-///
-/// `T1` is the generic type for a field projection. For an actual projection on a `Place`
-/// this parameter will always be `Ty`, but the field type can be unavailable when
-/// building (by using `PlaceBuilder`) places that correspond to upvars.
-/// `T2` is the generic type for an `OpaqueCast` (is generic since it's abstracted over
-/// in dataflow analysis, see `AbstractElem`).
 #[derive(Copy, Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
 #[derive(TyEncodable, TyDecodable, HashStable, TypeFoldable, TypeVisitable)]
-pub enum ProjectionElem<V, T1, T2> {
+pub enum ProjectionElem<V, T> {
     Deref,
-    Field(Field, T1),
+    Field(Field, T),
     /// Index into a slice/array.
     ///
     /// Note that this does not also dereference, and so it does not exactly correspond to slice
@@ -957,36 +951,12 @@ pub enum ProjectionElem<V, T1, T2> {
 
     /// Like an explicit cast from an opaque type to a concrete type, but without
     /// requiring an intermediate variable.
-    OpaqueCast(T2),
+    OpaqueCast(T),
 }
 
 /// Alias for projections as they appear in places, where the base is a place
 /// and the index is a local.
-pub type PlaceElem<'tcx> = ProjectionElem<Local, Ty<'tcx>, Ty<'tcx>>;
-
-/// Alias for projections that appear in `PlaceBuilder::Upvar`, for which
-/// we cannot provide any field types.
-pub type UpvarProjectionElem<'tcx> = ProjectionElem<Local, (), Ty<'tcx>>;
-
-impl<'tcx> From<PlaceElem<'tcx>> for UpvarProjectionElem<'tcx> {
-    fn from(elem: PlaceElem<'tcx>) -> Self {
-        match elem {
-            ProjectionElem::Deref => ProjectionElem::Deref,
-            ProjectionElem::Field(field, _) => ProjectionElem::Field(field, ()),
-            ProjectionElem::Index(v) => ProjectionElem::Index(v),
-            ProjectionElem::ConstantIndex { offset, min_length, from_end } => {
-                ProjectionElem::ConstantIndex { offset, min_length, from_end }
-            }
-            ProjectionElem::Subslice { from, to, from_end } => {
-                ProjectionElem::Subslice { from, to, from_end }
-            }
-            ProjectionElem::Downcast(opt_sym, variant_idx) => {
-                ProjectionElem::Downcast(opt_sym, variant_idx)
-            }
-            ProjectionElem::OpaqueCast(ty) => ProjectionElem::OpaqueCast(ty),
-        }
-    }
-}
+pub type PlaceElem<'tcx> = ProjectionElem<Local, Ty<'tcx>>;
 
 ///////////////////////////////////////////////////////////////////////////
 // Operands