Auto merge of #45225 - eddyb:trans-abi, r=<try>

Refactor type memory layouts and ABIs, to be more general and easier to optimize.

To combat combinatorial explosion, type layouts are now described through 3 orthogonal properties:
* `Variants` describes the plurality of sum types (where applicable)
  * `Single` is for one inhabited/active variant, including all C `struct`s and `union`s
  * `Tagged` has its variants discriminated by an integer tag, including C `enum`s
  * `NicheFilling` uses otherwise-invalid values ("niches") for all but one of its inhabited variants
* `FieldPlacement` describes the number and memory offsets of fields (if any)
  * `Union` has all its fields at offset `0`
  * `Array` has offsets that are a multiple of its `stride`; guarantees all fields have one type
  * `Arbitrary` records all the field offsets, which can be out-of-order
* `Abi` describes how values of the type should be passed around, including for FFI
  * `Uninhabited` corresponds to no values, associated with unreachable control-flow
  * `Scalar` is ABI-identical to its only integer/floating-point/pointer "scalar component"
  * `ScalarPair` has two "scalar components", but only applies to the Rust ABI
  * `Vector` is for SIMD vectors, typically `#[repr(simd)]` `struct`s in Rust
  * `Aggregate` has arbitrary contents, including all non-transparent C `struct`s and `union`s

Size optimizations implemented so far:
* ignoring uninhabited variants (i.e. containing uninhabited fields), e.g.:
  * `Option<!>` is 0 bytes
  * `Result<T, !>` has the same size as `T`
* using arbitrary niches, not just `0`, to represent a data-less variant, e.g.:
  * `Option<bool>`, `Option<Option<bool>>`, `Option<Ordering>` are all 1 byte
  * `Option<char>` is 4 bytes
* using a range of niches to represent *multiple* data-less variants, e.g.:
  * `enum E { A(bool), B, C, D }` is 1 byte

Code generation now takes advantage of `Scalar` and `ScalarPair` to, in more cases, pass around scalar components as immediates instead of indirectly, through pointers into temporary memory, while avoiding LLVM's "first-class aggregates", and there's more untapped potential here.

Closes #44426, fixes #5977, fixes #14540, fixes #43278.

src/liballoc/boxed.rs

@@ -151,7 +151,7 @@ impl<T> Place<T> for IntermediateBox<T> {
 unsafe fn finalize<T>(b: IntermediateBox<T>) -> Box<T> {
     let p = b.ptr as *mut T;
     mem::forget(b);
-    mem::transmute(p)
+    Box::from_raw(p)
 }
 
 fn make_place<T>() -> IntermediateBox<T> {
@@ -300,7 +300,10 @@ impl<T: ?Sized> Box<T> {
                issue = "27730")]
     #[inline]
     pub unsafe fn from_unique(u: Unique<T>) -> Self {
-        mem::transmute(u)
+        #[cfg(stage0)]
+        return mem::transmute(u);
+        #[cfg(not(stage0))]
+        return Box(u);
     }
 
     /// Consumes the `Box`, returning the wrapped raw pointer.
@@ -362,7 +365,14 @@ impl<T: ?Sized> Box<T> {
                issue = "27730")]
     #[inline]
     pub fn into_unique(b: Box<T>) -> Unique<T> {
-        unsafe { mem::transmute(b) }
+        #[cfg(stage0)]
+        return unsafe { mem::transmute(b) };
+        #[cfg(not(stage0))]
+        return {
+            let unique = b.0;
+            mem::forget(b);
+            unique
+        };
     }
 }
 
@@ -627,7 +637,7 @@ impl Box<Any + Send> {
     pub fn downcast<T: Any>(self) -> Result<Box<T>, Box<Any + Send>> {
         <Box<Any>>::downcast(self).map_err(|s| unsafe {
             // reapply the Send marker
-            mem::transmute::<Box<Any>, Box<Any + Send>>(s)
+            Box::from_raw(Box::into_raw(s) as *mut (Any + Send))
         })
     }
 }

src/librustc/lib.rs

@@ -45,7 +45,10 @@
 #![feature(conservative_impl_trait)]
 #![feature(const_fn)]
 #![feature(core_intrinsics)]
+#![feature(i128)]
 #![feature(i128_type)]
+#![feature(inclusive_range)]
+#![feature(inclusive_range_syntax)]
 #![cfg_attr(windows, feature(libc))]
 #![feature(never_type)]
 #![feature(nonzero)]

src/librustc/ty/context.rs

@@ -42,7 +42,7 @@ use ty::{PolyFnSig, InferTy, ParamTy, ProjectionTy, ExistentialPredicate, Predic
 use ty::RegionKind;
 use ty::{TyVar, TyVid, IntVar, IntVid, FloatVar, FloatVid};
 use ty::TypeVariants::*;
-use ty::layout::{Layout, TargetDataLayout};
+use ty::layout::{CachedLayout, TargetDataLayout};
 use ty::inhabitedness::DefIdForest;
 use ty::maps;
 use ty::steal::Steal;
@@ -80,7 +80,7 @@ use hir;
 /// Internal storage
 pub struct GlobalArenas<'tcx> {
     // internings
-    layout: TypedArena<Layout>,
+    layout: TypedArena<CachedLayout>,
 
     // references
     generics: TypedArena<ty::Generics>,
@@ -931,7 +931,7 @@ pub struct GlobalCtxt<'tcx> {
 
     stability_interner: RefCell<FxHashSet<&'tcx attr::Stability>>,
 
-    layout_interner: RefCell<FxHashSet<&'tcx Layout>>,
+    layout_interner: RefCell<FxHashSet<&'tcx CachedLayout>>,
 
     /// A vector of every trait accessible in the whole crate
     /// (i.e. including those from subcrates). This is used only for
@@ -1029,7 +1029,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
         interned
     }
 
-    pub fn intern_layout(self, layout: Layout) -> &'gcx Layout {
+    pub fn intern_layout(self, layout: CachedLayout) -> &'gcx CachedLayout {
         if let Some(layout) = self.layout_interner.borrow().get(&layout) {
             return layout;
         }