fix: add cfg diagnostic for unresolved import error

compiler/rustc_resolve/src/imports.rs

@@ -609,7 +609,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         }
     }
 
-    fn throw_unresolved_import_error(&self, errors: Vec<(&Import<'_>, UnresolvedImportError)>) {
+    fn throw_unresolved_import_error(&mut self, errors: Vec<(&Import<'_>, UnresolvedImportError)>) {
         if errors.is_empty() {
             return;
         }
@@ -679,6 +679,17 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                     _ => {}
                 }
             }
+
+            match &import.kind {
+                ImportKind::Single { source, .. } => {
+                    if let Some(ModuleOrUniformRoot::Module(module)) = import.imported_module.get()
+                     && let Some(module) = module.opt_def_id()
+                    {
+                        self.find_cfg_stripped(&mut diag, &source.name, module)
+                    }
+                },
+                _ => {}
+            }
         }
 
         diag.emit();

tests/ui/cfg/diagnostics-reexport.rs

@@ -9,6 +9,30 @@ pub mod inner {
     //~^ NOTE found an item that was configured out
 }
 
+pub use a::x;
+//~^ ERROR unresolved import `a::x`
+//~| NOTE no `x` in `a`
+
+mod a {
+    #[cfg(no)]
+    pub fn x() {}
+    //~^ NOTE found an item that was configured out
+}
+
+pub use b::{x, y};
+//~^ ERROR unresolved imports `b::x`, `b::y`
+//~| NOTE no `x` in `b`
+//~| NOTE no `y` in `b`
+
+mod b {
+    #[cfg(no)]
+    pub fn x() {}
+    //~^ NOTE found an item that was configured out
+    #[cfg(no)]
+    pub fn y() {}
+    //~^ NOTE found an item that was configured out
+}
+
 fn main() {
     // There is no uwu at this path - no diagnostic.
     inner::uwu(); //~ ERROR cannot find function

tests/ui/cfg/diagnostics-reexport.stderr

@@ -1,5 +1,36 @@
+error[E0432]: unresolved import `a::x`
+  --> $DIR/diagnostics-reexport.rs:12:9
+   |
+LL | pub use a::x;
+   |         ^^^^ no `x` in `a`
+   |
+note: found an item that was configured out
+  --> $DIR/diagnostics-reexport.rs:18:12
+   |
+LL |     pub fn x() {}
+   |            ^
+
+error[E0432]: unresolved imports `b::x`, `b::y`
+  --> $DIR/diagnostics-reexport.rs:22:13
+   |
+LL | pub use b::{x, y};
+   |             ^  ^ no `y` in `b`
+   |             |
+   |             no `x` in `b`
+   |
+note: found an item that was configured out
+  --> $DIR/diagnostics-reexport.rs:29:12
+   |
+LL |     pub fn x() {}
+   |            ^
+note: found an item that was configured out
+  --> $DIR/diagnostics-reexport.rs:32:12
+   |
+LL |     pub fn y() {}
+   |            ^
+
 error[E0425]: cannot find function `uwu` in module `inner`
-  --> $DIR/diagnostics-reexport.rs:14:12
+  --> $DIR/diagnostics-reexport.rs:38:12
    |
 LL |     inner::uwu();
    |            ^^^ not found in `inner`
@@ -10,6 +41,7 @@ note: found an item that was configured out
 LL |     pub use super::uwu;
    |                    ^^^
 
-error: aborting due to previous error
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0425`.
+Some errors have detailed explanations: E0425, E0432.
+For more information about an error, try `rustc --explain E0425`.