Use a hook to decouple rustc_mir_transform from rustc_mir_build

Cargo.lock

@@ -4360,7 +4360,6 @@ dependencies = [
  "rustc_infer",
  "rustc_macros",
  "rustc_middle",
- "rustc_mir_build",
  "rustc_mir_dataflow",
  "rustc_session",
  "rustc_span",

compiler/rustc_middle/src/hooks/mod.rs

@@ -102,6 +102,11 @@ declare_hooks! {
     /// Ensure the given scalar is valid for the given type.
     /// This checks non-recursive runtime validity.
     hook validate_scalar_in_layout(scalar: crate::ty::ScalarInt, ty: Ty<'tcx>) -> bool;
+
+    /// **Do not call this directly; call the `mir_built` query instead.**
+    ///
+    /// Creates the MIR for a given `DefId`, including unreachable code.
+    hook build_mir_inner_impl(def: LocalDefId) -> mir::Body<'tcx>;
 }
 
 #[cold]

compiler/rustc_mir_build/src/builder/mod.rs

@@ -64,9 +64,11 @@ pub(crate) fn closure_saved_names_of_captured_variables<'tcx>(
         .collect()
 }
 
-/// Create the MIR for a given `DefId`, including unreachable code. Do not call
-/// this directly; instead use the cached version via `mir_built`.
-pub fn build_mir<'tcx>(tcx: TyCtxt<'tcx>, def: LocalDefId) -> Body<'tcx> {
+/// Create the MIR for a given `DefId`, including unreachable code.
+///
+/// This is the implementation of hook `build_mir_inner_impl`, which should only
+/// be called by the query `mir_built`.
+pub(crate) fn build_mir_inner_impl<'tcx>(tcx: TyCtxt<'tcx>, def: LocalDefId) -> Body<'tcx> {
     tcx.ensure_done().thir_abstract_const(def);
     if let Err(e) = tcx.ensure_ok().check_match(def) {
         return construct_error(tcx, def, e);

compiler/rustc_mir_build/src/lib.rs

@@ -12,7 +12,7 @@
 // The `builder` module used to be named `build`, but that was causing GitHub's
 // "Go to file" feature to silently ignore all files in the module, probably
 // because it assumes that "build" is a build-output directory. See #134365.
-pub mod builder;
+mod builder;
 mod check_tail_calls;
 mod check_unsafety;
 mod errors;
@@ -30,4 +30,5 @@ pub fn provide(providers: &mut Providers) {
     providers.check_unsafety = check_unsafety::check_unsafety;
     providers.check_tail_calls = check_tail_calls::check_tail_calls;
     providers.thir_body = thir::cx::thir_body;
+    providers.hooks.build_mir_inner_impl = builder::build_mir_inner_impl;
 }

compiler/rustc_mir_transform/Cargo.toml

@@ -20,7 +20,6 @@ rustc_index = { path = "../rustc_index" }
 rustc_infer = { path = "../rustc_infer" }
 rustc_macros = { path = "../rustc_macros" }
 rustc_middle = { path = "../rustc_middle" }
-rustc_mir_build = { path = "../rustc_mir_build" }
 rustc_mir_dataflow = { path = "../rustc_mir_dataflow" }
 rustc_session = { path = "../rustc_session" }
 rustc_span = { path = "../rustc_span" }

compiler/rustc_mir_transform/src/lib.rs

@@ -30,7 +30,6 @@ use rustc_middle::mir::{
 use rustc_middle::ty::{self, TyCtxt, TypeVisitableExt};
 use rustc_middle::util::Providers;
 use rustc_middle::{bug, query, span_bug};
-use rustc_mir_build::builder::build_mir;
 use rustc_span::source_map::Spanned;
 use rustc_span::{DUMMY_SP, sym};
 use tracing::debug;
@@ -378,8 +377,11 @@ fn mir_const_qualif(tcx: TyCtxt<'_>, def: LocalDefId) -> ConstQualifs {
     validator.qualifs_in_return_place()
 }
 
+/// Implementation of the `mir_built` query.
 fn mir_built(tcx: TyCtxt<'_>, def: LocalDefId) -> &Steal<Body<'_>> {
-    let mut body = build_mir(tcx, def);
+    // Delegate to the main MIR building code in the `rustc_mir_build` crate.
+    // This is the one place that is allowed to call `build_mir_inner_impl`.
+    let mut body = tcx.build_mir_inner_impl(def);
 
     // Identifying trivial consts based on their mir_built is easy, but a little wasteful.
     // Trying to push this logic earlier in the compiler and never even produce the Body would