Auto merge of #33130 - eddyb:mir-const, r=nikomatsakis

Implement constant support in MIR.

All of the intended features in `trans::consts` are now supported by `mir::constant`.
The implementation is considered a temporary measure until `miri` replaces it.

A `-Z orbit` bootstrap build will only translate LLVM IR from AST for `#[rustc_no_mir]` functions.

Furthermore, almost all checks of constant expressions have been moved to MIR.
In non-`const` functions, trees of temporaries are promoted, as per RFC 1414 (rvalue promotion).
Promotion before MIR borrowck would allow reasoning about promoted values' lifetimes.

The improved checking comes at the cost of four `[breaking-change]`s:
* repeat counts must contain a constant expression, e.g.:
`let arr = [0; { println!("foo"); 5 }];` used to be allowed (it behaved like `let arr = [0; 5];`)
* dereference of a reference to a `static` cannot be used in another `static`, e.g.:
`static X: [u8; 1] = [1]; static Y: u8 = (&X)[0];` was unintentionally allowed before
* the type of a `static` *must* be `Sync`, irrespective of the initializer, e.g.
`static FOO: *const T = &BAR;` worked as `&T` is `Sync`, but it shouldn't because `*const T` isn't
* a `static` cannot wrap `UnsafeCell` around a type that *may* need drop, e.g.
`static X: MakeSync<UnsafeCell<Option<String>>> = MakeSync(UnsafeCell::new(None));`
was previously allowed based on the fact `None` alone doesn't need drop, but in `UnsafeCell`
it can be later changed to `Some(String)` which *does* need dropping

The drop restrictions are relaxed by RFC 1440 (#33156), which is implemented, but feature-gated.
However, creating `UnsafeCell` from constants is unstable, so users can just enable the feature gate.

Author: bors

mk/crates.mk

@@ -111,7 +111,7 @@ DEPS_rustc_lint := rustc log syntax rustc_const_eval
 DEPS_rustc_llvm := native:rustllvm libc std rustc_bitflags
 DEPS_rustc_metadata := rustc syntax rbml rustc_const_math
 DEPS_rustc_passes := syntax rustc core rustc_const_eval
-DEPS_rustc_mir := rustc syntax rustc_const_math rustc_const_eval
+DEPS_rustc_mir := rustc syntax rustc_const_math rustc_const_eval rustc_bitflags
 DEPS_rustc_resolve := arena rustc log syntax
 DEPS_rustc_platform_intrinsics := std
 DEPS_rustc_plugin := rustc rustc_metadata syntax rustc_mir

src/librustc/dep_graph/dep_node.rs

@@ -64,6 +64,7 @@ pub enum DepNode<D: Clone + Debug> {
     IntrinsicCheck(D),
     MatchCheck(D),
     MirMapConstruction(D),
+    MirPass(D),
     MirTypeck(D),
     BorrowCheck(D),
     RvalueCheck(D),
@@ -186,6 +187,7 @@ impl<D: Clone + Debug> DepNode<D> {
             IntrinsicCheck(ref d) => op(d).map(IntrinsicCheck),
             MatchCheck(ref d) => op(d).map(MatchCheck),
             MirMapConstruction(ref d) => op(d).map(MirMapConstruction),
+            MirPass(ref d) => op(d).map(MirPass),
             MirTypeck(ref d) => op(d).map(MirTypeck),
             BorrowCheck(ref d) => op(d).map(BorrowCheck),
             RvalueCheck(ref d) => op(d).map(RvalueCheck),