Fix cycle when debug-printing opaque types

Fixes #61577

When printing an opaque type in non-verbose mode, we use the
`tcx.predicates_of` query to retrieve the opaque type's bounds for
pretty-printing. However, the pervasiveness of logging within librustc
means that we may already be executing `tcx.predicates_of` for the
opaque type we're trying to print, leading to a cycle error.

This commit adds a new 'no queries' thread-local flag to the pretty
printer. This flag is enabled during the computation of `predicates_of`
for opaque types, and causes us to print the opaque type in 'verbose'
mode (which does not require computing any additinal queries). This
should only affect debug logging for highly nested log messages, not any
user-visible output.

2 files changed, 95 insertions, 64 deletions

diff --git a/src/librustc/ty/print/pretty.rs b/src/librustc/ty/print/pretty.rs
index 594550dd967..ee05d57d239 100644
--- a/src/librustc/ty/print/pretty.rs
+++ b/src/librustc/ty/print/pretty.rs
@@ -49,6 +49,24 @@ macro_rules! define_scoped_cx {
 thread_local! {
     static FORCE_IMPL_FILENAME_LINE: Cell<bool> = Cell::new(false);
     static SHOULD_PREFIX_WITH_CRATE: Cell<bool> = Cell::new(false);
+    static NO_QUERIES: Cell<bool> = Cell::new(false);
+}
+
+/// Avoids running any queries during any prints that occur
+/// during the closure. This may alter the apperance of some
+/// types (e.g. forcing verbose printing for opaque types).
+/// This method is used during some queries (e.g. `predicates_of`
+/// for opaque types), to ensure that any debug printing that
+/// occurs during the query computation does not end up recursively
+/// calling the same query.
+pub fn with_no_queries<F: FnOnce() -> R, R>(f: F) -> R {
+    NO_QUERIES.with(|no_queries| {
+        let old = no_queries.get();
+        no_queries.set(true);
+        let result = f();
+        no_queries.set(old);
+        result
+    })
 }
 
 /// Force us to name impls with just the filename/line number. We
@@ -556,52 +574,61 @@ pub trait PrettyPrinter<'tcx>:
             }
             ty::Opaque(def_id, substs) => {
                 // FIXME(eddyb) print this with `print_def_path`.
-                if self.tcx().sess.verbose() {
+                // We use verbose printing in 'NO_QUERIES' mode, to
+                // avoid needing to call `predicates_of`. This should
+                // only affect certain debug messages (e.g. messages printed
+                // from `rustc::ty` during the computation of `tcx.predicates_of`),
+                // and should have no effect on any compiler output.
+                if self.tcx().sess.verbose() || NO_QUERIES.with(|q| q.get())  {
                     p!(write("Opaque({:?}, {:?})", def_id, substs));
                     return Ok(self);
                 }
 
-                let def_key = self.tcx().def_key(def_id);
-                if let Some(name) = def_key.disambiguated_data.data.get_opt_name() {
-                    p!(write("{}", name));
-                    let mut substs = substs.iter();
-                    // FIXME(eddyb) print this with `print_def_path`.
-                    if let Some(first) = substs.next() {
-                        p!(write("::<"));
-                        p!(print(first));
-                        for subst in substs {
-                            p!(write(", "), print(subst));
+                return Ok(with_no_queries(|| {
+
+                    let def_key = self.tcx().def_key(def_id);
+                    if let Some(name) = def_key.disambiguated_data.data.get_opt_name() {
+                        p!(write("{}", name));
+                        let mut substs = substs.iter();
+                        // FIXME(eddyb) print this with `print_def_path`.
+                        if let Some(first) = substs.next() {
+                            p!(write("::<"));
+                            p!(print(first));
+                            for subst in substs {
+                                p!(write(", "), print(subst));
+                            }
+                            p!(write(">"));
                         }
-                        p!(write(">"));
+                        return Ok(self);
                     }
-                    return Ok(self);
-                }
-                // Grab the "TraitA + TraitB" from `impl TraitA + TraitB`,
-                // by looking up the projections associated with the def_id.
-                let bounds = self.tcx().predicates_of(def_id).instantiate(self.tcx(), substs);
-
-                let mut first = true;
-                let mut is_sized = false;
-                p!(write("impl"));
-                for predicate in bounds.predicates {
-                    if let Some(trait_ref) = predicate.to_opt_poly_trait_ref() {
-                        // Don't print +Sized, but rather +?Sized if absent.
-                        if Some(trait_ref.def_id()) == self.tcx().lang_items().sized_trait() {
-                            is_sized = true;
-                            continue;
-                        }
+                    // Grab the "TraitA + TraitB" from `impl TraitA + TraitB`,
+                    // by looking up the projections associated with the def_id.
+                    let bounds = self.tcx().predicates_of(def_id).instantiate(self.tcx(), substs);
+
+                    let mut first = true;
+                    let mut is_sized = false;
+                    p!(write("impl"));
+                    for predicate in bounds.predicates {
+                        if let Some(trait_ref) = predicate.to_opt_poly_trait_ref() {
+                            // Don't print +Sized, but rather +?Sized if absent.
+                            if Some(trait_ref.def_id()) == self.tcx().lang_items().sized_trait() {
+                                is_sized = true;
+                                continue;
+                            }
 
-                        p!(
-                                write("{}", if first { " " } else { "+" }),
-                                print(trait_ref));
-                        first = false;
+                            p!(
+                                    write("{}", if first { " " } else { "+" }),
+                                    print(trait_ref));
+                            first = false;
+                        }
                     }
-                }
-                if !is_sized {
-                    p!(write("{}?Sized", if first { " " } else { "+" }));
-                } else if first {
-                    p!(write(" Sized"));
-                }
+                    if !is_sized {
+                        p!(write("{}?Sized", if first { " " } else { "+" }));
+                    } else if first {
+                        p!(write(" Sized"));
+                    }
+                    Ok(self)
+                })?);
             }
             ty::Str => p!(write("str")),
             ty::Generator(did, substs, movability) => {
diff --git a/src/librustc_typeck/collect.rs b/src/librustc_typeck/collect.rs
index d9f6d79f218..f4c8763f879 100644
--- a/src/librustc_typeck/collect.rs
+++ b/src/librustc_typeck/collect.rs
@@ -2056,20 +2056,22 @@ fn explicit_predicates_of(
 
         Node::ImplItem(item) => match item.kind {
             ImplItemKind::OpaqueTy(ref bounds) => {
-                let substs = InternalSubsts::identity_for_item(tcx, def_id);
-                let opaque_ty = tcx.mk_opaque(def_id, substs);
-
-                // Collect the bounds, i.e., the `A + B + 'c` in `impl A + B + 'c`.
-                let bounds = AstConv::compute_bounds(
-                    &icx,
-                    opaque_ty,
-                    bounds,
-                    SizedByDefault::Yes,
-                    tcx.def_span(def_id),
-                );
+                ty::print::with_no_queries(|| {
+                    let substs = InternalSubsts::identity_for_item(tcx, def_id);
+                    let opaque_ty = tcx.mk_opaque(def_id, substs);
+
+                    // Collect the bounds, i.e., the `A + B + 'c` in `impl A + B + 'c`.
+                    let bounds = AstConv::compute_bounds(
+                        &icx,
+                        opaque_ty,
+                        bounds,
+                        SizedByDefault::Yes,
+                        tcx.def_span(def_id),
+                    );
 
-                predicates.extend(bounds.predicates(tcx, opaque_ty));
-                &item.generics
+                    predicates.extend(bounds.predicates(tcx, opaque_ty));
+                    &item.generics
+                })
             }
             _ => &item.generics,
         },
@@ -2102,19 +2104,21 @@ fn explicit_predicates_of(
                     ref generics,
                     origin: _,
                 }) => {
-                    let substs = InternalSubsts::identity_for_item(tcx, def_id);
-                    let opaque_ty = tcx.mk_opaque(def_id, substs);
-
-                    // Collect the bounds, i.e., the `A + B + 'c` in `impl A + B + 'c`.
-                    let bounds = AstConv::compute_bounds(
-                        &icx,
-                        opaque_ty,
-                        bounds,
-                        SizedByDefault::Yes,
-                        tcx.def_span(def_id),
-                    );
+                    let bounds_predicates = ty::print::with_no_queries(|| {
+                        let substs = InternalSubsts::identity_for_item(tcx, def_id);
+                        let opaque_ty = tcx.mk_opaque(def_id, substs);
+
+                        // Collect the bounds, i.e., the `A + B + 'c` in `impl A + B + 'c`.
+                        let bounds = AstConv::compute_bounds(
+                            &icx,
+                            opaque_ty,
+                            bounds,
+                            SizedByDefault::Yes,
+                            tcx.def_span(def_id),
+                        );
 
-                    let bounds_predicates = bounds.predicates(tcx, opaque_ty);
+                        bounds.predicates(tcx, opaque_ty)
+                    });
                     if impl_trait_fn.is_some() {
                         // opaque types
                         return ty::GenericPredicates {
@@ -2124,7 +2128,7 @@ fn explicit_predicates_of(
                     } else {
                         // named opaque types
                         predicates.extend(bounds_predicates);
-                        generics
+                        generics.clone()
                     }
                 }