fix(material/input): preserve aria-describedby set externally

Currently there are two sources of an `aria-describedby` for a `matInput`: the IDs of the hint/error message in the form field and any custom ones set through `aria-describedby`. This is insufficient, because the ID can also come from a direct DOM manupulation like in the `AriaDescriber`.

These changes tweak the logic to try and preserve them, because currently they get overwritten.

Fixes #30011.

src/material/input/input.spec.ts

@@ -643,6 +643,18 @@ describe('MatMdcInput without forms', () => {
     expect(input.getAttribute('aria-describedby')).toBe('start end');
   }));
 
+  it('should preserve aria-describedby set directly in the DOM', fakeAsync(() => {
+    const fixture = createComponent(MatInputHintLabel2TestController);
+    const input = fixture.nativeElement.querySelector('input');
+    input.setAttribute('aria-describedby', 'custom');
+    fixture.componentInstance.label = 'label';
+    fixture.changeDetectorRef.markForCheck();
+    fixture.detectChanges();
+    const hint = fixture.nativeElement.querySelector('.mat-mdc-form-field-hint');
+
+    expect(input.getAttribute('aria-describedby')).toBe(`${hint.getAttribute('id')} custom`);
+  }));
+
   it('should set a class on the hint element based on its alignment', fakeAsync(() => {
     const fixture = createComponent(MatInputMultipleHintTestController);
 

src/material/input/input.ts

@@ -111,6 +111,9 @@ export class MatInput
   private _webkitBlinkWheelListenerAttached = false;
   private _config = inject(MAT_INPUT_CONFIG, {optional: true});
 
+  /** `aria-describedby` IDs assigned by the form field. */
+  private _formFieldDescribedBy: string[] | undefined;
+
   /** Whether the component is being rendered on the server. */
   readonly _isServer: boolean;
 
@@ -552,9 +555,26 @@ export class MatInput
    */
   setDescribedByIds(ids: string[]) {
     const element = this._elementRef.nativeElement;
+    const existingDescribedBy = element.getAttribute('aria-describedby');
+    let toAssign: string[];
+
+    // In some cases there might be some `aria-describedby` IDs that were assigned directly,
+    // like by the `AriaDescriber` (see #30011). Attempt to preserve them by taking the previous
+    // attribute value and filtering out the IDs that came from the previous `setDescribedByIds`
+    // call. Note the `|| ids` here allows us to avoid duplicating IDs on the first render.
+    if (existingDescribedBy) {
+      const exclude = this._formFieldDescribedBy || ids;
+      toAssign = ids.concat(
+        existingDescribedBy.split(' ').filter(id => id && !exclude.includes(id)),
+      );
+    } else {
+      toAssign = ids;
+    }
+
+    this._formFieldDescribedBy = ids;
 
-    if (ids.length) {
-      element.setAttribute('aria-describedby', ids.join(' '));
+    if (toAssign.length) {
+      element.setAttribute('aria-describedby', toAssign.join(' '));
     } else {
       element.removeAttribute('aria-describedby');
     }