Add check for overlapping ranges to unreachable patterns lint

src/librustc/hir/mod.rs

@@ -989,6 +989,15 @@ pub enum RangeEnd {
     Excluded,
 }
 
+impl fmt::Display for RangeEnd {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.write_str(match self {
+            RangeEnd::Included => "..=",
+            RangeEnd::Excluded => "..",
+        })
+    }
+}
+
 #[derive(RustcEncodable, RustcDecodable, Debug, HashStable)]
 pub enum PatKind {
     /// Represents a wildcard pattern (i.e., `_`).

src/librustc/lint/builtin.rs

@@ -80,6 +80,12 @@ declare_lint! {
     "detects unreachable patterns"
 }
 
+declare_lint! {
+    pub OVERLAPPING_PATTERNS,
+    Warn,
+    "detects overlapping patterns"
+}
+
 declare_lint! {
     pub UNUSED_MACROS,
     Warn,
@@ -423,6 +429,7 @@ declare_lint_pass! {
         DEAD_CODE,
         UNREACHABLE_CODE,
         UNREACHABLE_PATTERNS,
+        OVERLAPPING_PATTERNS,
         UNUSED_MACROS,
         WARNINGS,
         UNUSED_FEATURES,

src/librustc_lint/lib.rs

@@ -255,6 +255,7 @@ pub fn register_builtins(store: &mut lint::LintStore, sess: Option<&Session>) {
                     UNUSED_MUT,
                     UNREACHABLE_CODE,
                     UNREACHABLE_PATTERNS,
+                    OVERLAPPING_PATTERNS,
                     UNUSED_MUST_USE,
                     UNUSED_UNSAFE,
                     PATH_STATEMENTS,