From 760338526f8a7d7238682fca6b776b28412fd528 Mon Sep 17 00:00:00 2001
From: bjorn3 <17426603+bjorn3@users.noreply.github.com>
Date: Mon, 7 Oct 2024 10:25:58 +0200
Subject: [PATCH] Show actual MIR when MIR building forgot to terminate block

This makes it significantly easier to debug bugs of this kind.
---
 compiler/rustc_middle/src/mir/pretty.rs   | 46 ++++++++++++-----------
 compiler/rustc_mir_build/src/build/mod.rs | 23 +++++++++---
 2 files changed, 41 insertions(+), 28 deletions(-)

diff --git a/compiler/rustc_middle/src/mir/pretty.rs b/compiler/rustc_middle/src/mir/pretty.rs
index e690bf74b6b4b..80ae5a7146dad 100644
--- a/compiler/rustc_middle/src/mir/pretty.rs
+++ b/compiler/rustc_middle/src/mir/pretty.rs
@@ -762,33 +762,35 @@ where
 
     // Terminator at the bottom.
     extra_data(PassWhere::BeforeLocation(current_location), w)?;
-    let indented_terminator = format!("{0}{0}{1:?};", INDENT, data.terminator().kind);
-    if options.include_extra_comments {
-        writeln!(
+    if data.terminator.is_some() {
+        let indented_terminator = format!("{0}{0}{1:?};", INDENT, data.terminator().kind);
+        if options.include_extra_comments {
+            writeln!(
+                w,
+                "{:A$} // {}{}",
+                indented_terminator,
+                if tcx.sess.verbose_internals() {
+                    format!("{current_location:?}: ")
+                } else {
+                    String::new()
+                },
+                comment(tcx, data.terminator().source_info),
+                A = ALIGN,
+            )?;
+        } else {
+            writeln!(w, "{indented_terminator}")?;
+        }
+
+        write_extra(
+            tcx,
             w,
-            "{:A$} // {}{}",
-            indented_terminator,
-            if tcx.sess.verbose_internals() {
-                format!("{current_location:?}: ")
-            } else {
-                String::new()
+            |visitor| {
+                visitor.visit_terminator(data.terminator(), current_location);
             },
-            comment(tcx, data.terminator().source_info),
-            A = ALIGN,
+            options,
         )?;
-    } else {
-        writeln!(w, "{indented_terminator}")?;
     }
 
-    write_extra(
-        tcx,
-        w,
-        |visitor| {
-            visitor.visit_terminator(data.terminator(), current_location);
-        },
-        options,
-    )?;
-
     extra_data(PassWhere::AfterLocation(current_location), w)?;
     extra_data(PassWhere::AfterTerminator(block), w)?;
 
diff --git a/compiler/rustc_mir_build/src/build/mod.rs b/compiler/rustc_mir_build/src/build/mod.rs
index 1f853f6e1c302..46be2aee63742 100644
--- a/compiler/rustc_mir_build/src/build/mod.rs
+++ b/compiler/rustc_mir_build/src/build/mod.rs
@@ -791,12 +791,6 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
     }
 
     fn finish(self) -> Body<'tcx> {
-        for (index, block) in self.cfg.basic_blocks.iter().enumerate() {
-            if block.terminator.is_none() {
-                span_bug!(self.fn_span, "no terminator on block {:?}", index);
-            }
-        }
-
         let mut body = Body::new(
             MirSource::item(self.def_id.to_def_id()),
             self.cfg.basic_blocks,
@@ -810,6 +804,23 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             None,
         );
         body.coverage_info_hi = self.coverage_info.map(|b| b.into_done());
+
+        for (index, block) in body.basic_blocks.iter().enumerate() {
+            if block.terminator.is_none() {
+                use rustc_middle::mir::pretty;
+                let options = pretty::PrettyPrintMirOptions::from_cli(self.tcx);
+                pretty::write_mir_fn(
+                    self.tcx,
+                    &body,
+                    &mut |_, _| Ok(()),
+                    &mut std::io::stdout(),
+                    options,
+                )
+                .unwrap();
+                span_bug!(self.fn_span, "no terminator on block {:?}", index);
+            }
+        }
+
         body
     }