add test for Result<&T, _> where T; Deref

Author: tesuji

tests/ui/mismatched_types/transforming-option-ref-issue-127545.rs

@@ -12,3 +12,7 @@ pub fn bar(arg: Option<&Vec<i32>>) -> &[i32] {
 pub fn barzz<'a>(arg: Option<&'a Vec<i32>>, v: &'a [i32]) -> &'a [i32] {
     arg.unwrap_or(v) //~ ERROR 13:19: 13:20: mismatched types [E0308]
 }
+
+pub fn convert_result(arg: Result<&Vec<i32>, ()>) -> &[i32] {
+    arg.unwrap_or(&[]) //~ ERROR 17:19: 17:22: mismatched types [E0308]
+}

tests/ui/mismatched_types/transforming-option-ref-issue-127545.stderr

@@ -61,6 +61,26 @@ help: use `Option::map_or` to deref inner value of `Option`
 LL |     arg.map_or(v, |v| v)
    |         ~~~~~~~~~~~~~~~~
 
-error: aborting due to 3 previous errors
+error[E0308]: mismatched types
+  --> $DIR/transforming-option-ref-issue-127545.rs:17:19
+   |
+LL |     arg.unwrap_or(&[])
+   |         --------- ^^^ expected `&Vec<i32>`, found `&[_; 0]`
+   |         |
+   |         arguments to this method are incorrect
+   |
+   = note: expected reference `&Vec<i32>`
+              found reference `&[_; 0]`
+help: the return type of this call is `&[_; 0]` due to the type of the argument passed
+  --> $DIR/transforming-option-ref-issue-127545.rs:17:5
+   |
+LL |     arg.unwrap_or(&[])
+   |     ^^^^^^^^^^^^^^---^
+   |                   |
+   |                   this argument influences the return type of `unwrap_or`
+note: method defined here
+  --> $SRC_DIR/core/src/result.rs:LL:COL
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0308`.