Avoid stack overflowing when printing unevaluated const

Fixes rust-lang#68104

When printing the DefPath for an unevaluated const, we may end up trying
to print the same const again. To avoid infinite recursion, we use the
'verbose' format when we try to re-entrantly print a const. This ensures
that the pretty-printing process will always terminate.

src/librustc_middle/ty/print/pretty.rs

@@ -54,6 +54,7 @@ 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);
+    static NO_CONST_PATH: Cell<bool> = Cell::new(false);
 }
 
 /// Avoids running any queries during any prints that occur
@@ -72,6 +73,15 @@ pub fn with_no_queries<F: FnOnce() -> R, R>(f: F) -> R {
     })
 }
 
+pub fn with_no_const_path<F: FnOnce() -> R, R>(f: F) -> R {
+    NO_CONST_PATH.with(|no_const_path| {
+        let old = no_const_path.replace(true);
+        let result = f();
+        no_const_path.set(old);
+        result
+    })
+}
+
 /// Force us to name impls with just the filename/line number. We
 /// normally try to use types. But at some points, notably while printing
 /// cycle errors, this can result in extra or suboptimal error output,
@@ -858,7 +868,9 @@ pub trait PrettyPrinter<'tcx>:
     ) -> Result<Self::Const, Self::Error> {
         define_scoped_cx!(self);
 
-        if self.tcx().sess.verbose() {
+        // See the call to `with_no_const_path` inside the
+        // `ty::ConstKind::Unevaluated` for why we check `NO_CONST_PATH`
+        if self.tcx().sess.verbose() || NO_CONST_PATH.with(|q| q.get()) {
             p!(write("Const({:?}: {:?})", ct.val, ct.ty));
             return Ok(self);
         }
@@ -882,29 +894,39 @@ pub trait PrettyPrinter<'tcx>:
 
         match ct.val {
             ty::ConstKind::Unevaluated(did, substs, promoted) => {
-                if let Some(promoted) = promoted {
-                    p!(print_value_path(did, substs));
-                    p!(write("::{:?}", promoted));
-                } else {
-                    match self.tcx().def_kind(did) {
-                        DefKind::Static | DefKind::Const | DefKind::AssocConst => {
-                            p!(print_value_path(did, substs))
-                        }
-                        _ => {
-                            if did.is_local() {
-                                let span = self.tcx().def_span(did);
-                                if let Ok(snip) = self.tcx().sess.source_map().span_to_snippet(span)
-                                {
-                                    p!(write("{}", snip))
+                // Don't re-entrantly enter this code path: that is,
+                // don't try to print the DefPath of an unevaluated const
+                // while we're already printing the DefPath of an unevaluated
+                // const. This ensures that we never end up trying to recursively
+                // print a const that we're already printing.
+                // See issue #68104 for more details
+                self = with_no_const_path(|| {
+                    if let Some(promoted) = promoted {
+                        p!(print_value_path(did, substs));
+                        p!(write("::{:?}", promoted));
+                    } else {
+                        match self.tcx().def_kind(did) {
+                            DefKind::Static | DefKind::Const | DefKind::AssocConst => {
+                                p!(print_value_path(did, substs))
+                            }
+                            _ => {
+                                if did.is_local() {
+                                    let span = self.tcx().def_span(did);
+                                    if let Ok(snip) =
+                                        self.tcx().sess.source_map().span_to_snippet(span)
+                                    {
+                                        p!(write("{}", snip))
+                                    } else {
+                                        print_underscore!()
+                                    }
                                 } else {
                                     print_underscore!()
                                 }
-                            } else {
-                                print_underscore!()
                             }
                         }
                     }
-                }
+                    Ok(self)
+                })?;
             }
             ty::ConstKind::Infer(..) => print_underscore!(),
             ty::ConstKind::Param(ParamConst { name, .. }) => p!(write("{}", name)),

src/test/ui/const-generics/auxiliary/impl-const.rs

@@ -0,0 +1,9 @@
+#![feature(const_generics)]
+
+pub struct Num<const N: usize>;
+
+// Braces around const expression causes crash
+impl Num<{5}> {
+    pub fn five(&self) {
+    }
+}

src/test/ui/const-generics/issue-68104-print-stack-overflow.rs

@@ -0,0 +1,14 @@
+// aux-build:impl-const.rs
+// run-pass
+
+#![feature(const_generics)]
+#![allow(incomplete_features)]
+
+extern crate impl_const;
+
+use impl_const::*;
+
+pub fn main() {
+    let n = Num::<5>;
+    n.five();
+}