Revert "Remove missing_fragment_specifier lint"

This reverts commit 5ba9610.

Author: Mark-Simulacrum

src/doc/rustc/src/lints/listing/deny-by-default.md

@@ -45,6 +45,15 @@ error: defaults for type parameters are only allowed in `struct`, `enum`, `type`
   = note: for more information, see issue #36887 <https://github.com/rust-lang/rust/issues/36887>
 ```
 
+## missing-fragment-specifier
+
+The missing_fragment_specifier warning is issued when an unused pattern in a
+`macro_rules!` macro definition has a meta-variable (e.g. `$e`) that is not
+followed by a fragment specifier (e.g. `:expr`).
+
+This warning can always be fixed by removing the unused pattern in the
+`macro_rules!` macro definition.
+
 ## mutable-transmutes
 
 This lint catches transmuting from `&T` to `&mut T` because it is undefined

src/librustc_session/lint/builtin.rs

@@ -252,6 +252,16 @@ declare_lint! {
     };
 }
 
+declare_lint! {
+    pub MISSING_FRAGMENT_SPECIFIER,
+    Deny,
+    "detects missing fragment specifiers in unused `macro_rules!` patterns",
+    @future_incompatible = FutureIncompatibleInfo {
+        reference: "issue #40107 <https://github.com/rust-lang/rust/issues/40107>",
+        edition: None,
+    };
+}
+
 declare_lint! {
     pub LATE_BOUND_LIFETIME_ARGUMENTS,
     Warn,
@@ -574,6 +584,7 @@ declare_lint_pass! {
         UNALIGNED_REFERENCES,
         SAFE_PACKED_BORROWS,
         PATTERNS_IN_FNS_WITHOUT_BODY,
+        MISSING_FRAGMENT_SPECIFIER,
         LATE_BOUND_LIFETIME_ARGUMENTS,
         ORDER_DEPENDENT_TRAIT_OBJECTS,
         COHERENCE_LEAK_CHECK,