suggest declaring modules when file found but module not defined

compiler/rustc_resolve/src/diagnostics.rs

@@ -2423,6 +2423,8 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             } else {
                 let suggestion = if suggestion.is_some() {
                     suggestion
+                } else if let Some(m) = self.undeclared_module_exists(ident) {
+                    self.undeclared_module_suggest_declare(ident, m)
                 } else if was_invoked_from_cargo() {
                     Some((
                         vec![],
@@ -2444,6 +2446,55 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         }
     }
 
+    fn undeclared_module_suggest_declare(
+        &mut self,
+        ident: Ident,
+        path: std::path::PathBuf,
+    ) -> Option<(Vec<(Span, String)>, String, Applicability)> {
+        Some((
+            vec![(self.current_crate_outer_attr_insert_span, format!("mod {ident};\n"))],
+            format!(
+                "to make use of source file {}, use `mod {ident}` \
+                 in this file to declare the module",
+                path.display()
+            ),
+            Applicability::MaybeIncorrect,
+        ))
+    }
+
+    fn undeclared_module_exists(&mut self, ident: Ident) -> Option<std::path::PathBuf> {
+        let map = self.tcx.sess.source_map();
+
+        let src = map.span_to_filename(ident.span).into_local_path()?;
+        let i = ident.as_str();
+        // FIXME: add case where non parent using undeclared module (hard?)
+        let dir = src.parent()?;
+        let src = src.file_stem()?.to_str()?;
+        for file in [
+            // …/x.rs
+            dir.join(i).with_extension("rs"),
+            // …/x/mod.rs
+            dir.join(i).join("mod.rs"),
+        ] {
+            if file.exists() {
+                return Some(file);
+            }
+        }
+        if !matches!(src, "main" | "lib" | "mod") {
+            for file in [
+                // …/x/y.rs
+                dir.join(src).join(i).with_extension("rs"),
+                // …/x/y/mod.rs
+                dir.join(src).join(i).join("mod.rs"),
+            ] {
+                if file.exists() {
+                    return Some(file);
+                }
+            }
+        }
+        None
+    }
+
     /// Adds suggestions for a path that cannot be resolved.
     #[instrument(level = "debug", skip(self, parent_scope))]
     pub(crate) fn make_path_suggestion(

tests/ui/modules_and_files_visibility/mod_file_disambig.stderr

@@ -12,7 +12,10 @@ error[E0433]: failed to resolve: use of unresolved module or unlinked crate `mod
 LL |     assert_eq!(mod_file_aux::bar(), 10);
    |                ^^^^^^^^^^^^ use of unresolved module or unlinked crate `mod_file_aux`
    |
-   = help: you might be missing a crate named `mod_file_aux`
+help: to make use of source file $DIR/mod_file_aux.rs, use `mod mod_file_aux` in this file to declare the module
+   |
+LL + mod mod_file_aux;
+   |
 
 error: aborting due to 2 previous errors