Suggest constant on unused binding in a pattern

```
error: unused variable: `Batery`
  --> $DIR/binding-typo-2.rs:110:9
   |
LL |         Batery => {}
   |         ^^^^^^
   |
help: if this is intentional, prefix it with an underscore
   |
LL |         _Batery => {}
   |         +
help: you might have meant to pattern match on the similarly named constant `Battery`
   |
LL |         Battery => {}
   |            +
```

Author: estebank

compiler/rustc_passes/src/liveness.rs

@@ -1716,8 +1716,25 @@ impl<'tcx> Liveness<'_, 'tcx> {
                     }
                 }
             }
-            // FIXME(estebank): look for consts of the same type with similar names as well, not
-            // just unit structs and variants.
+            if typo.is_none() {
+                for (hir_id, _, span) in &hir_ids_and_spans {
+                    let ty = self.typeck_results.node_type(*hir_id);
+                    // Look for consts of the same type with similar names as well, not just unit
+                    // structs and variants.
+                    for def_id in self.ir.tcx.hir_body_owners() {
+                        if let DefKind::Const = self.ir.tcx.def_kind(def_id)
+                            && self.ir.tcx.type_of(def_id).instantiate_identity() == ty
+                        {
+                            typo = Some(errors::PatternTypo {
+                                span: *span,
+                                code: with_no_trimmed_paths!(self.ir.tcx.def_path_str(def_id)),
+                                kind: "constant".to_string(),
+                                item_name: self.ir.tcx.item_name(def_id).to_string(),
+                            });
+                        }
+                    }
+                }
+            }
             if is_assigned {
                 self.ir.tcx.emit_node_span_lint(
                     lint::builtin::UNUSED_VARIABLES,

tests/ui/or-patterns/binding-typo-2.rs

@@ -5,6 +5,7 @@ enum Lol {
     Bar,
 }
 const Bat: () = ();
+const Battery: () = ();
 struct Bay;
 
 fn foo(x: (Lol, Lol)) {
@@ -62,8 +63,8 @@ fn foo(x: (Lol, Lol)) {
 }
 fn bar(x: (Lol, Lol)) {
     use Lol::*;
-    use Bat;
-    use Bay;
+    use ::Bat;
+    use ::Bay;
     match &x {
         (Foo, _) | (Ban, Foo) => {}
         //~^ ERROR: variable `Ban` is not bound in all patterns
@@ -105,6 +106,12 @@ fn baz(x: (Lol, Lol)) {
         //~| HELP: if this is intentional, prefix it with an underscore
         //~| HELP: you might have meant to pattern match on the similarly named
     }
+    match () {
+        Batery => {}
+        //~^ ERROR: unused variable: `Batery`
+        //~| HELP: if this is intentional, prefix it with an underscore
+        //~| HELP: you might have meant to pattern match on the similarly named constant
+    }
 }
 
 fn main() {

tests/ui/or-patterns/binding-typo-2.stderr

@@ -1,5 +1,5 @@
 error[E0408]: variable `Ban` is not bound in all patterns
-  --> $DIR/binding-typo-2.rs:13:9
+  --> $DIR/binding-typo-2.rs:14:9
    |
 LL |         (Foo, Bar) | (Ban, Foo) => {}
    |         ^^^^^^^^^^    --- variable not in all patterns
@@ -13,7 +13,7 @@ LL +         (Foo, Bar) | (Bar, Foo) => {}
    |
 
 error[E0408]: variable `Ban` is not bound in all patterns
-  --> $DIR/binding-typo-2.rs:24:9
+  --> $DIR/binding-typo-2.rs:25:9
    |
 LL |         (Foo, _) | (Ban, Foo) => {}
    |         ^^^^^^^^    --- variable not in all patterns
@@ -27,7 +27,7 @@ LL +         (Foo, _) | (Bar, Foo) => {}
    |
 
 error[E0408]: variable `Non` is not bound in all patterns
-  --> $DIR/binding-typo-2.rs:43:15
+  --> $DIR/binding-typo-2.rs:44:15
    |
 LL |         Non | None => {}
    |         ---   ^^^^ pattern doesn't bind `Non`
@@ -40,7 +40,7 @@ LL |         None | None => {}
    |            +
 
 error[E0408]: variable `Non` is not bound in all patterns
-  --> $DIR/binding-typo-2.rs:53:15
+  --> $DIR/binding-typo-2.rs:54:15
    |
 LL |         Non | Some(_) => {}
    |         ---   ^^^^^^^ pattern doesn't bind `Non`
@@ -54,7 +54,7 @@ LL +         core::option::Option::None | Some(_) => {}
    |
 
 error[E0408]: variable `Ban` is not bound in all patterns
-  --> $DIR/binding-typo-2.rs:68:9
+  --> $DIR/binding-typo-2.rs:69:9
    |
 LL |         (Foo, _) | (Ban, Foo) => {}
    |         ^^^^^^^^    --- variable not in all patterns
@@ -78,7 +78,7 @@ LL +         (Foo, _) | (Bat, Foo) => {}
    |
 
 error[E0408]: variable `Ban` is not bound in all patterns
-  --> $DIR/binding-typo-2.rs:85:9
+  --> $DIR/binding-typo-2.rs:86:9
    |
 LL |         (Foo, _) | (Ban, Foo) => {}
    |         ^^^^^^^^    --- variable not in all patterns
@@ -97,7 +97,7 @@ LL +         (Foo, _) | (Bat, Foo) => {}
    |
 
 error: variable `Ban` is assigned to, but never used
-  --> $DIR/binding-typo-2.rs:13:23
+  --> $DIR/binding-typo-2.rs:14:23
    |
 LL |         (Foo, Bar) | (Ban, Foo) => {}
    |                       ^^^
@@ -115,7 +115,7 @@ LL +         (Foo, Bar) | (Lol::Bar, Foo) => {}
    |
 
 error: variable `Ban` is assigned to, but never used
-  --> $DIR/binding-typo-2.rs:24:21
+  --> $DIR/binding-typo-2.rs:25:21
    |
 LL |         (Foo, _) | (Ban, Foo) => {}
    |                     ^^^
@@ -128,7 +128,7 @@ LL +         (Foo, _) | (Lol::Bar, Foo) => {}
    |
 
 error: unused variable: `Non`
-  --> $DIR/binding-typo-2.rs:36:9
+  --> $DIR/binding-typo-2.rs:37:9
    |
 LL |         Non => {}
    |         ^^^
@@ -144,7 +144,7 @@ LL +         std::prelude::v1::None => {}
    |
 
 error: unused variable: `Non`
-  --> $DIR/binding-typo-2.rs:43:9
+  --> $DIR/binding-typo-2.rs:44:9
    |
 LL |         Non | None => {}
    |         ^^^
@@ -160,7 +160,7 @@ LL +         std::prelude::v1::None | None => {}
    |
 
 error: unused variable: `Non`
-  --> $DIR/binding-typo-2.rs:53:9
+  --> $DIR/binding-typo-2.rs:54:9
    |
 LL |         Non | Some(_) => {}
    |         ^^^
@@ -176,7 +176,7 @@ LL +         std::prelude::v1::None | Some(_) => {}
    |
 
 error: variable `Ban` is assigned to, but never used
-  --> $DIR/binding-typo-2.rs:68:21
+  --> $DIR/binding-typo-2.rs:69:21
    |
 LL |         (Foo, _) | (Ban, Foo) => {}
    |                     ^^^
@@ -189,7 +189,7 @@ LL +         (Foo, _) | (Lol::Bar, Foo) => {}
    |
 
 error: variable `Ban` is assigned to, but never used
-  --> $DIR/binding-typo-2.rs:85:21
+  --> $DIR/binding-typo-2.rs:86:21
    |
 LL |         (Foo, _) | (Ban, Foo) => {}
    |                     ^^^
@@ -202,7 +202,7 @@ LL +         (Foo, _) | (Lol::Bar, Foo) => {}
    |
 
 error: unused variable: `Ban`
-  --> $DIR/binding-typo-2.rs:97:10
+  --> $DIR/binding-typo-2.rs:98:10
    |
 LL |         (Ban, _) => {}
    |          ^^^
@@ -218,7 +218,7 @@ LL +         (Lol::Bar, _) => {}
    |
 
 error: unused variable: `Ban`
-  --> $DIR/binding-typo-2.rs:103:9
+  --> $DIR/binding-typo-2.rs:104:9
    |
 LL |         Ban => {}
    |         ^^^
@@ -233,6 +233,21 @@ LL -         Ban => {}
 LL +         Bay => {}
    |
 
-error: aborting due to 15 previous errors
+error: unused variable: `Batery`
+  --> $DIR/binding-typo-2.rs:110:9
+   |
+LL |         Batery => {}
+   |         ^^^^^^
+   |
+help: if this is intentional, prefix it with an underscore
+   |
+LL |         _Batery => {}
+   |         +
+help: you might have meant to pattern match on the similarly named constant `Battery`
+   |
+LL |         Battery => {}
+   |            +
+
+error: aborting due to 16 previous errors
 
 For more information about this error, try `rustc --explain E0408`.