Fix standalone build

Add extern declarations and optional dependencies to fix build done
directly via `cargo build`.

Cargo.lock

@@ -4235,6 +4235,7 @@ dependencies = [
  "rustc_middle",
  "rustc_span",
  "scoped-tls",
+ "rustc_target",
  "tracing",
 ]
 

compiler/rustc_smir/Cargo.toml

@@ -4,14 +4,18 @@ version = "0.0.0"
 edition = "2021"
 
 [dependencies]
-rustc_hir = { path = "../rustc_hir" }
+# Use optional dependencies for rustc_* in order to support building this crate separately.
+rustc_hir = { path = "../rustc_hir", optional = true }
 rustc_middle = { path = "../rustc_middle", optional = true }
 rustc_span = { path = "../rustc_span", optional = true }
+rustc_target = { path = "../rustc_target", optional = true }
 tracing = "0.1"
 scoped-tls = "1.0"
 
 [features]
 default = [
+    "rustc_hir",
     "rustc_middle",
     "rustc_span",
+    "rustc_target",
 ]

compiler/rustc_smir/rust-toolchain.toml

@@ -1,3 +1,3 @@
 [toolchain]
-channel = "nightly-2023-02-28"
+channel = "nightly-2023-06-14"
 components = [ "rustfmt", "rustc-dev" ]

compiler/rustc_smir/src/lib.rs

@@ -14,6 +14,16 @@
 #![feature(local_key_cell_methods)]
 #![feature(ptr_metadata)]
 
+// Declare extern rustc_* crates to enable building this crate separately from the compiler.
+#[cfg(not(feature = "default"))]
+extern crate rustc_hir;
+#[cfg(not(feature = "default"))]
+extern crate rustc_middle;
+#[cfg(not(feature = "default"))]
+extern crate rustc_span;
+#[cfg(not(feature = "default"))]
+extern crate rustc_target;
+
 pub mod rustc_internal;
 pub mod stable_mir;