Fix implementation to work for Rust 2015

Prioritizing the added check for the crate's name at the top of
`report_path_resolution_error()` seems to account for differences
in Rust 2015 and 2018's crate/module path systems, allowing
the same suggestion to show up for Rust 2015.

Author: joculatrix

compiler/rustc_resolve/src/diagnostics.rs

@@ -2003,7 +2003,18 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             Some(ModuleOrUniformRoot::Module(module)) => module.res(),
             _ => None,
         };
-        if module_res == self.graph_root.res() {
+        if ident.name == self.tcx().crate_name(0_usize.into()) {
+            (
+                format!("use of unresolved module or unlinked crate `{ident}`"),
+                Some((
+                    vec![(ident.span, String::from("crate"))],
+                    format!(
+                        "the current crate name can not be used in a path, use the keyword `crate` instead"
+                    ),
+                    Applicability::MaybeIncorrect,
+                )),
+            )
+        } else if module_res == self.graph_root.res() {
             let is_mod = |res| matches!(res, Res::Def(DefKind::Mod, _));
             let mut candidates = self.lookup_import_candidates(ident, TypeNS, parent_scope, is_mod);
             candidates
@@ -2202,17 +2213,6 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             });
 
             (format!("use of undeclared type `{ident}`"), suggestion)
-        } else if ident.name == self.tcx().crate_name(0_usize.into()) {
-            (
-                format!("use of unresolved module or unlinked crate `{ident}`"),
-                Some((
-                    vec![(ident.span, String::from("crate"))],
-                    format!(
-                        "the current crate name can not be used in a path, use the keyword `crate` instead"
-                    ),
-                    Applicability::MaybeIncorrect,
-                )),
-            )
         } else {
             let mut suggestion = None;
             if ident.name == sym::alloc {

tests/ui/resolve/current-crate-name-in-path.rs

@@ -1,10 +1,6 @@
 //! Ensure E0433 suggests replacing the crate's name with `crate` when the
 //! current crate's name is used in a path.
 
-//@ edition:2018
-// Rust 2018 is needed for the updated crate/module path system and `crate`
-// keyword.
-
 mod bar {
     pub fn baz() {}
 }

tests/ui/resolve/current-crate-name-in-path.stderr

@@ -1,5 +1,5 @@
 error[E0433]: failed to resolve: use of unresolved module or unlinked crate `current_crate_name_in_path`
-  --> $DIR/current-crate-name-in-path.rs:12:5
+  --> $DIR/current-crate-name-in-path.rs:8:5
    |
 LL | use current_crate_name_in_path::bar::baz;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^ use of unresolved module or unlinked crate `current_crate_name_in_path`