From 5e126c944b83d8402e0414914ef7d6c05004d483 Mon Sep 17 00:00:00 2001
From: Gus Wynn <guswynn@gmail.com>
Date: Tue, 1 Sep 2020 15:53:04 -0700
Subject: [PATCH] better diag when const ranges are used in patterns

---
 compiler/rustc_typeck/src/check/pat.rs | 39 ++++++++++++++++++++++----
 src/test/ui/issues/issue-76191.rs      | 14 +++++++++
 src/test/ui/issues/issue-76191.stderr  | 21 ++++++++++++++
 3 files changed, 69 insertions(+), 5 deletions(-)
 create mode 100644 src/test/ui/issues/issue-76191.rs
 create mode 100644 src/test/ui/issues/issue-76191.stderr

diff --git a/compiler/rustc_typeck/src/check/pat.rs b/compiler/rustc_typeck/src/check/pat.rs
index 1896155e327..171d5ee4ff2 100644
--- a/compiler/rustc_typeck/src/check/pat.rs
+++ b/compiler/rustc_typeck/src/check/pat.rs
@@ -10,7 +10,7 @@ use rustc_hir::{HirId, Pat, PatKind};
 use rustc_infer::infer;
 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
 use rustc_middle::ty::subst::GenericArg;
-use rustc_middle::ty::{self, BindingMode, Ty, TypeFoldable};
+use rustc_middle::ty::{self, Adt, BindingMode, Ty, TypeFoldable};
 use rustc_span::hygiene::DesugaringKind;
 use rustc_span::source_map::{Span, Spanned};
 use rustc_span::symbol::Ident;
@@ -735,7 +735,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         if let Some(err) =
             self.demand_suptype_with_origin(&self.pattern_cause(ti, pat.span), expected, pat_ty)
         {
-            self.emit_bad_pat_path(err, pat.span, res, pat_res, segments, ti.parent_pat);
+            self.emit_bad_pat_path(err, pat.span, res, pat_res, pat_ty, segments, ti.parent_pat);
         }
         pat_ty
     }
@@ -746,6 +746,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         pat_span: Span,
         res: Res,
         pat_res: Res,
+        pat_ty: Ty<'tcx>,
         segments: &'b [hir::PathSegment<'b>],
         parent_pat: Option<&Pat<'_>>,
     ) {
@@ -771,9 +772,37 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         );
                     }
                     _ => {
-                        let msg = "introduce a new binding instead";
-                        let sugg = format!("other_{}", ident.as_str().to_lowercase());
-                        e.span_suggestion(ident.span, msg, sugg, Applicability::HasPlaceholders);
+                        let const_def_id = match pat_ty.kind() {
+                            Adt(def, _) => match res {
+                                Res::Def(DefKind::Const, _) => Some(def.did),
+                                _ => None,
+                            },
+                            _ => None,
+                        };
+
+                        let ranges = &[
+                            self.tcx.lang_items().range_struct(),
+                            self.tcx.lang_items().range_from_struct(),
+                            self.tcx.lang_items().range_to_struct(),
+                            self.tcx.lang_items().range_full_struct(),
+                            self.tcx.lang_items().range_inclusive_struct(),
+                            self.tcx.lang_items().range_to_inclusive_struct(),
+                        ];
+                        if const_def_id != None && ranges.contains(&const_def_id) {
+                            let msg = "constants only support matching by type, \
+                                if you meant to match against a range of values, \
+                                consider using a range pattern like `min ..= max` in the match block";
+                            e.note(msg);
+                        } else {
+                            let msg = "introduce a new binding instead";
+                            let sugg = format!("other_{}", ident.as_str().to_lowercase());
+                            e.span_suggestion(
+                                ident.span,
+                                msg,
+                                sugg,
+                                Applicability::HasPlaceholders,
+                            );
+                        }
                     }
                 };
             }
diff --git a/src/test/ui/issues/issue-76191.rs b/src/test/ui/issues/issue-76191.rs
new file mode 100644
index 00000000000..bc327123c6f
--- /dev/null
+++ b/src/test/ui/issues/issue-76191.rs
@@ -0,0 +1,14 @@
+// Regression test for diagnostic issue #76191
+#![allow(non_snake_case)]
+
+use std::ops::RangeInclusive;
+const RANGE: RangeInclusive<i32> = 0..=255;
+
+fn main() {
+    let n: i32 = 1;
+    match n {
+        RANGE => {}
+        //~^ ERROR mismatched types
+        _ => {}
+    }
+}
diff --git a/src/test/ui/issues/issue-76191.stderr b/src/test/ui/issues/issue-76191.stderr
new file mode 100644
index 00000000000..a5544d9e9da
--- /dev/null
+++ b/src/test/ui/issues/issue-76191.stderr
@@ -0,0 +1,21 @@
+error[E0308]: mismatched types
+  --> $DIR/issue-76191.rs:10:9
+   |
+LL | const RANGE: RangeInclusive<i32> = 0..=255;
+   | ------------------------------------------- constant defined here
+...
+LL |     match n {
+   |           - this expression has type `i32`
+LL |         RANGE => {}
+   |         ^^^^^
+   |         |
+   |         expected `i32`, found struct `RangeInclusive`
+   |         `RANGE` is interpreted as a constant, not a new binding
+   |
+   = note: expected type `i32`
+            found struct `RangeInclusive<i32>`
+   = note: constants only support matching by type, if you meant to match against a range of values, consider using a range pattern like `min ..= max` in the match block
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.