Rollup merge of #42017 - nikomatsakis:issue-41677, r=arielb1

avoid cycles in mir-dump, take 2

This fixes #41697, for real this time, but I'm not sure how best to add a regression test. I was considering maybe adding some flag so that the MIR dumping doesn't actually get written to files (e.g., overloading the directory flag so you can specify nil or something).

cc @dwrensha @oli-obk

src/librustc/ty/maps.rs

@@ -592,7 +592,7 @@ macro_rules! define_map_struct {
      output: $output:tt) => {
         define_map_struct! {
             tcx: $tcx,
-            ready: ([pub] $attrs $name),
+            ready: ([] $attrs $name),
             input: ($($input)*),
             output: $output
         }

src/librustc_mir/util/pretty.rs

@@ -324,7 +324,9 @@ fn write_mir_sig(tcx: TyCtxt, src: MirSource, mir: &Mir, w: &mut Write)
         MirSource::Promoted(_, i) => write!(w, "{:?} in", i)?
     }
 
-    write!(w, " {}", tcx.node_path_str(src.item_id()))?;
+    item_path::with_forced_impl_filename_line(|| { // see notes on #41697 elsewhere
+        write!(w, " {}", tcx.node_path_str(src.item_id()))
+    })?;
 
     if let MirSource::Fn(_) = src {
         write!(w, "(")?;

src/test/mir-opt/issue-41697.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// compile-flags:-Zdump-mir=NEVER_MATCHED
-
 // Regression test for #41697. Using dump-mir was triggering
 // artificial cycles: during type-checking, we had to get the MIR for
 // the constant expressions in `[u8; 2]`, which in turn would trigger