Fix try_print_visible_def_path for Rust 2018

The recursive check of `try_print_visible_def_path` did not properly handle the Rust 2018 case of crate-paths without 'extern crate'. Instead, it returned a "not found" via (false, self). This fixes issue #56175.

Author: da-x

src/librustc_middle/ty/print/pretty.rs

@@ -282,26 +282,27 @@ pub trait PrettyPrinter<'tcx>:
  // where there is no explicit `extern crate`, we just prepend
  // the crate name.
  match self.tcx().extern_crate(def_id) {
- Some(&ExternCrate {
- src: ExternCrateSource::Extern(def_id),
- dependency_of: LOCAL_CRATE,
- span,
- ..
- }) => {
- debug!("try_print_visible_def_path: def_id={:?}", def_id);
- return Ok((
- if !span.is_dummy() {
- self.print_def_path(def_id, &[])?
- } else {
- self.path_crate(cnum)?
- },
- true,
- ));
- }
+ Some(&ExternCrate { src, dependency_of, span, .. }) => match (src, dependency_of) {
+ (ExternCrateSource::Extern(def_id), LOCAL_CRATE) => {
+ debug!("try_print_visible_def_path: def_id={:?}", def_id);
+ return Ok((
+ if !span.is_dummy() {
+ self.print_def_path(def_id, &[])?
+ } else {
+ self.path_crate(cnum)?
+ },
+ true,
+ ));
+ }
+ (ExternCrateSource::Path, LOCAL_CRATE) if self.tcx().sess.rust_2018() => {
+ debug!("try_print_visible_def_path: def_id={:?}", def_id);
+ return Ok((self.path_crate(cnum)?, true));
+ }
+ _ => {}
+ },
  None => {
  return Ok((self.path_crate(cnum)?, true));
  }
- _ => {}
  }
  }
 

src/test/ui/issues/issue-56175.stderr

@@ -7,7 +7,7 @@ LL | reexported_trait::FooStruct.trait_method();
  = help: items from traits can only be used if the trait is in scope
 help: the following trait is implemented but not in scope; perhaps add a `use` for it:
  |
-LL | use reexported_trait::private::Trait;
+LL | use reexported_trait::Trait;
  |
 
 error[E0599]: no method named `trait_method_b` found for struct `reexported_trait::FooStruct` in the current scope
@@ -19,7 +19,7 @@ LL | reexported_trait::FooStruct.trait_method_b();
  = help: items from traits can only be used if the trait is in scope
 help: the following trait is implemented but not in scope; perhaps add a `use` for it:
  |
-LL | use reexported_trait::private::TraitB;
+LL | use reexported_trait::TraitBRename;
  |
 
 error: aborting due to 2 previous errors