Add WIP stable MIR crate

Cargo.toml

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+[package]
+name = "rustc_smir"
+version = "0.0.0"
+edition = "2021"
+
+[dependencies]
+rustc_middle = { path = "../rustc_middle" }
+rustc_driver = { path = "../rustc_driver" }
+rustc_borrowck = { path = "../rustc_borrowck" }
+rustc_interface = { path = "../rustc_interface" }

src/lib.rs

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+//! The WIP stable interface to rustc internals.
+
+#![doc(
+    html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/",
+    test(attr(allow(unused_variables), deny(warnings)))
+)]
+
+pub mod mir;
+
+pub mod very_unstable;

src/mir.rs

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+pub use rustc_middle::mir::{
+    visit::MutVisitor, AggregateKind, AssertKind, BasicBlock, BasicBlockData, BinOp, BindingForm,
+    BlockTailInfo, Body, BorrowKind, CastKind, ClearCrossCrate, Constant, ConstantKind,
+    CopyNonOverlapping, Coverage, FakeReadCause, Field, GeneratorInfo, ImplicitSelfKind,
+    InlineAsmOperand, Local, LocalDecl, LocalInfo, LocalKind, Location, MirPhase, MirSource,
+    NullOp, Operand, Place, PlaceRef, ProjectionElem, ProjectionKind, Promoted, RetagKind, Rvalue,
+    Safety, SourceInfo, SourceScope, SourceScopeData, SourceScopeLocalData, Statement,
+    StatementKind, UnOp, UserTypeProjection, UserTypeProjections, VarBindingForm, VarDebugInfo,
+    VarDebugInfoContents,
+};

src/very_unstable.rs

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+//! This module reexports various crates and modules from unstable rustc APIs.
+//! Add anything you need here and it will get slowly transferred to a stable API.
+//! Only use rustc_smir in your dependencies and use the reexports here instead of
+//! directly referring to the unstable crates.
+
+pub use rustc_borrowck as borrowck;
+pub use rustc_driver as driver;
+pub use rustc_interface as interface;
+pub use rustc_middle as middle;