Clarify the role of the PreCodegen dummy pass

Cargo.lock

@@ -3830,7 +3830,6 @@ dependencies = [
  "rustc_metadata",
  "rustc_middle",
  "rustc_mir_build",
- "rustc_mir_transform",
  "rustc_parse",
  "rustc_public",
  "rustc_resolve",

compiler/rustc_driver_impl/Cargo.toml

@@ -27,7 +27,6 @@ rustc_macros = { path = "../rustc_macros" }
 rustc_metadata = { path = "../rustc_metadata" }
 rustc_middle = { path = "../rustc_middle" }
 rustc_mir_build = { path = "../rustc_mir_build" }
-rustc_mir_transform = { path = "../rustc_mir_transform" }
 rustc_parse = { path = "../rustc_parse" }
 rustc_public = { path = "../rustc_public", features = ["rustc_internal"] }
 rustc_resolve = { path = "../rustc_resolve" }

compiler/rustc_driver_impl/src/lib.rs

@@ -6,6 +6,7 @@
 
 // tidy-alphabetical-start
 #![feature(decl_macro)]
+#![feature(file_buffered)]
 #![feature(panic_backtrace_config)]
 #![feature(panic_update_hook)]
 #![feature(trim_prefix_suffix)]
@@ -333,7 +334,7 @@ pub fn run_compiler(at_args: &[String], callbacks: &mut (dyn Callbacks + Send))
             }
 
             if tcx.sess.opts.output_types.contains_key(&OutputType::Mir) {
-                if let Err(error) = rustc_mir_transform::dump_mir::emit_mir(tcx) {
+                if let Err(error) = pretty::emit_mir(tcx) {
                     tcx.dcx().emit_fatal(CantEmitMIR { error });
                 }
             }

compiler/rustc_driver_impl/src/pretty.rs

@@ -2,6 +2,8 @@
 
 use std::cell::Cell;
 use std::fmt::Write;
+use std::fs::File;
+use std::io;
 
 use rustc_ast as ast;
 use rustc_ast_pretty::pprust as pprust_ast;
@@ -12,7 +14,7 @@ use rustc_middle::ty::{self, TyCtxt};
 use rustc_mir_build::thir::print::{thir_flat, thir_tree};
 use rustc_public::rustc_internal::pretty::write_smir_pretty;
 use rustc_session::Session;
-use rustc_session::config::{OutFileName, PpHirMode, PpMode, PpSourceMode};
+use rustc_session::config::{OutFileName, OutputType, PpHirMode, PpMode, PpSourceMode};
 use rustc_span::{FileName, Ident};
 use tracing::debug;
 
@@ -340,3 +342,21 @@ pub fn print<'tcx>(sess: &Session, ppm: PpMode, ex: PrintExtra<'tcx>) {
 
     write_or_print(&out, sess);
 }
+
+/// Implementation of `--emit=mir`.
+pub(crate) fn emit_mir(tcx: TyCtxt<'_>) -> io::Result<()> {
+    match tcx.output_filenames(()).path(OutputType::Mir) {
+        OutFileName::Stdout => {
+            let mut f = io::stdout();
+            write_mir_pretty(tcx, None, &mut f)?;
+        }
+        OutFileName::Real(path) => {
+            let mut f = File::create_buffered(&path)?;
+            write_mir_pretty(tcx, None, &mut f)?;
+            if tcx.sess.opts.json_artifact_notifications {
+                tcx.dcx().emit_artifact_notification(&path, "mir");
+            }
+        }
+    }
+    Ok(())
+}

compiler/rustc_mir_transform/src/lib.rs

@@ -2,7 +2,6 @@
 #![feature(box_patterns)]
 #![feature(const_type_name)]
 #![feature(cow_is_borrowed)]
-#![feature(file_buffered)]
 #![feature(impl_trait_in_assoc_type)]
 #![feature(iterator_try_collect)]
 #![feature(try_blocks)]
@@ -91,8 +90,6 @@ macro_rules! declare_passes {
 
         static PASS_NAMES: LazyLock<FxIndexSet<&str>> = LazyLock::new(|| {
             let mut set = FxIndexSet::default();
-            // Fake marker pass
-            set.insert("PreCodegen");
             $(
                 $(
                     set.extend(pass_names!($mod_name : $pass_name $( { $($ident),* } )? ));
@@ -145,7 +142,6 @@ declare_passes! {
     };
     mod deref_separator : Derefer;
     mod dest_prop : DestinationPropagation;
-    pub mod dump_mir : Marker;
     mod early_otherwise_branch : EarlyOtherwiseBranch;
     mod erase_deref_temps : EraseDerefTemps;
     mod elaborate_box_derefs : ElaborateBoxDerefs;
@@ -162,6 +158,7 @@ declare_passes! {
     mod large_enums : EnumSizeOpt;
     mod lower_intrinsics : LowerIntrinsics;
     mod lower_slice_len : LowerSliceLenCalls;
+    mod marker : PreCodegen;
     mod match_branches : MatchBranchSimplification;
     mod mentioned_items : MentionedItems;
     mod multiple_return_terminators : MultipleReturnTerminators;
@@ -771,8 +768,9 @@ pub(crate) fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'
             // Cleanup for human readability, off by default.
             &prettify::ReorderBasicBlocks,
             &prettify::ReorderLocals,
-            // Dump the end result for testing and debugging purposes.
-            &dump_mir::Marker("PreCodegen"),
+            // Dummy pass to allow dumping the final pre-codegen MIR for testing/debugging.
+            // This must be the final pass!
+            &marker::PreCodegen,
         ],
         Some(MirPhase::Runtime(RuntimePhase::Optimized)),
         optimizations,

compiler/rustc_mir_transform/src/marker.rs

@@ -0,0 +1,20 @@
+use rustc_middle::mir::Body;
+use rustc_middle::ty::TyCtxt;
+
+/// Dummy pass that does nothing, to be used as the final pass.
+///
+/// Its existence allows `-Zdump-mir` and mir-opt tests to easily dump
+/// the final MIR just before codegen, by dumping the input or output
+/// of the `PreCodegen` pass.
+pub(super) struct PreCodegen;
+
+impl<'tcx> crate::MirPass<'tcx> for PreCodegen {
+    fn run_pass(&self, _tcx: TyCtxt<'tcx>, _body: &mut Body<'tcx>) {
+        // This is a dummy pass, so the pass itself doesn't do anything.
+        // Dumping is performed by the normal `-Zdump-mir` machinery.
+    }
+
+    fn is_required(&self) -> bool {
+        false
+    }
+}