Be consistent about linting singletons

compiler/rustc_mir_build/src/thir/pattern/deconstruct_pat.rs

@@ -161,7 +161,7 @@ fn suspicious_intersection(&self, other: &Self) -> bool {
         // 2       --------   // 2 -------
         let (lo, hi) = self.boundaries();
         let (other_lo, other_hi) = other.boundaries();
-        lo == other_hi || hi == other_lo
+        (lo == other_hi || hi == other_lo) && !self.is_singleton() && !other.is_singleton()
     }
 
     fn to_pat<'tcx>(&self, tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Pat<'tcx> {

src/test/ui/pattern/usefulness/integer-ranges/overlapping_range_endpoints.rs

@@ -29,9 +29,9 @@ fn main() {
     m!(0u8, 20..=30, 25);
     m!(0u8, 20..=30, 30);
     m!(0u8, 20.. 30, 29);
-    m!(0u8, 20, 20..=30); //~ ERROR multiple patterns overlap on their endpoints
+    m!(0u8, 20, 20..=30);
     m!(0u8, 25, 20..=30);
-    m!(0u8, 30, 20..=30); //~ ERROR multiple patterns overlap on their endpoints
+    m!(0u8, 30, 20..=30);
 
     match 0u8 {
         0..=10 => {}

src/test/ui/pattern/usefulness/integer-ranges/overlapping_range_endpoints.stderr

@@ -44,22 +44,6 @@ LL |     m!(0u8, 20..=30, 19..=20);
    |             |
    |             this range overlaps on `20_u8`
 
-error: multiple patterns overlap on their endpoints
-  --> $DIR/overlapping_range_endpoints.rs:32:17
-   |
-LL |     m!(0u8, 20, 20..=30);
-   |             --  ^^^^^^^ overlapping range endpoints
-   |             |
-   |             this range overlaps on `20_u8`
-
-error: multiple patterns overlap on their endpoints
-  --> $DIR/overlapping_range_endpoints.rs:34:17
-   |
-LL |     m!(0u8, 30, 20..=30);
-   |             --  ^^^^^^^ overlapping range endpoints
-   |             |
-   |             this range overlaps on `30_u8`
-
 error: multiple patterns overlap on their endpoints
   --> $DIR/overlapping_range_endpoints.rs:39:9
    |
@@ -86,5 +70,5 @@ LL |         Some(0..=10) => {}
 LL |         Some(10..20) => {}
    |              ^^^^^^ overlapping range endpoints
 
-error: aborting due to 10 previous errors
+error: aborting due to 8 previous errors