Auto merge of #45409 - tamird:suggest-match-default-bindings, r=nikomatsakis

typeck: suggest use of match_default_bindings feature

Fixes #45383.
Updates #42640.

r? @nikomatsakis
cc @tschottdorf

This needs a UI test, but thought I'd get some early feedback.

Author: bors

src/librustc_typeck/check/_match.rs

@@ -23,6 +23,7 @@ use std::collections::hash_map::Entry::{Occupied, Vacant};
 use std::cmp;
 use syntax::ast;
 use syntax::codemap::Spanned;
+use syntax::feature_gate;
 use syntax::ptr::P;
 use syntax_pos::Span;
 
@@ -68,7 +69,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
             PatKind::Binding(..) |
             PatKind::Ref(..) => false,
         };
-        if is_non_ref_pat && tcx.sess.features.borrow().match_default_bindings {
+        if is_non_ref_pat {
             debug!("pattern is non reference pattern");
             let mut exp_ty = self.resolve_type_vars_with_obligations(&expected);
 
@@ -113,10 +114,24 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 }
             };
             if pat_adjustments.len() > 0 {
-                debug!("default binding mode is now {:?}", def_bm);
-                self.inh.tables.borrow_mut()
-                    .pat_adjustments_mut()
-                    .insert(pat.hir_id, pat_adjustments);
+                if tcx.sess.features.borrow().match_default_bindings {
+                    debug!("default binding mode is now {:?}", def_bm);
+                    self.inh.tables.borrow_mut()
+                        .pat_adjustments_mut()
+                        .insert(pat.hir_id, pat_adjustments);
+                } else {
+                    let mut err = feature_gate::feature_err(
+                        &tcx.sess.parse_sess,
+                        "match_default_bindings",
+                        pat.span,
+                        feature_gate::GateIssue::Language,
+                        "non-reference pattern used to match a reference",
+                    );
+                    if let Ok(snippet) = tcx.sess.codemap().span_to_snippet(pat.span) {
+                        err.span_suggestion(pat.span, "consider using", format!("&{}", &snippet));
+                    }
+                    err.emit();
+                }
             }
         }
 
@@ -325,8 +340,8 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                             if let Some(mut err) = err {
                                 if is_arg {
                                     if let PatKind::Binding(..) = inner.node {
-                                        if let Ok(snippet) = self.sess().codemap()
-                                                                        .span_to_snippet(pat.span)
+                                        if let Ok(snippet) = tcx.sess.codemap()
+                                                                     .span_to_snippet(pat.span)
                                         {
                                             err.help(&format!("did you mean `{}: &{}`?",
                                                               &snippet[1..],

src/test/compile-fail/E0029.rs

@@ -17,6 +17,7 @@ fn main() {
         //~| NOTE ranges require char or numeric types
         //~| NOTE start type: &'static str
         //~| NOTE end type: &'static str
+        //~| ERROR non-reference pattern used to match a reference
         _ => {}
     }
 }

src/test/compile-fail/feature-gate-match_default_bindings.rs

@@ -10,7 +10,8 @@
 
 pub fn main() {
     match &Some(3) {
-        Some(n) => {}, //~ ERROR mismatched types [E0308]
+        Some(n) => {},
+        //~^ ERROR non-reference pattern used to match a reference
         _ => panic!(),
     }
 }

src/test/compile-fail/issue-16338.rs

@@ -16,7 +16,6 @@ struct Slice<T> {
 fn main() {
     let Slice { data: data, len: len } = "foo";
     //~^ ERROR mismatched types
-    //~| expected type `&str`
     //~| found type `Slice<_>`
-    //~| expected &str, found struct `Slice`
+    //~| ERROR non-reference pattern used to match a reference
 }

src/test/compile-fail/issue-20261.rs

@@ -10,7 +10,8 @@
 
 fn main() {
     // NB: this (almost) typechecks when default binding modes are enabled.
-    for (ref i,) in [].iter() { //~ ERROR mismatched types [E0308]
+    for (ref i,) in [].iter() {
+        //~^ ERROR non-reference pattern used to match a reference
         i.clone();
     }
 }

src/test/compile-fail/keyword-false-as-identifier.rs

@@ -9,5 +9,5 @@
 // except according to those terms.
 
 fn main() {
-    let false = "foo"; //~ error: mismatched types
+    let false = 22; //~ error: mismatched types
 }

src/test/compile-fail/keyword-self-as-identifier.rs

@@ -9,5 +9,5 @@
 // except according to those terms.
 
 fn main() {
-    let Self = "foo"; //~ ERROR cannot find unit struct/variant or constant `Self` in this scope
+    let Self = 22; //~ ERROR cannot find unit struct/variant or constant `Self` in this scope
 }

src/test/compile-fail/keyword-super-as-identifier.rs

@@ -9,5 +9,5 @@
 // except according to those terms.
 
 fn main() {
-    let super = "foo"; //~ ERROR failed to resolve. There are too many initial `super`s
+    let super = 22; //~ ERROR failed to resolve. There are too many initial `super`s
 }

src/test/compile-fail/keyword-true-as-identifier.rs

@@ -9,5 +9,5 @@
 // except according to those terms.
 
 fn main() {
-    let true = "foo"; //~ error: mismatched types
+    let true = 22; //~ error: mismatched types
 }

src/test/compile-fail/match-range-fail.rs

@@ -15,13 +15,15 @@ fn main() {
     //~^^ ERROR only char and numeric types are allowed in range
     //~| start type: &'static str
     //~| end type: &'static str
+    //~| ERROR non-reference pattern used to match a reference
 
     match "wow" {
         10 ... "what" => ()
     };
     //~^^ ERROR only char and numeric types are allowed in range
     //~| start type: {integer}
     //~| end type: &'static str
+    //~| ERROR non-reference pattern used to match a reference
 
     match 5 {
         'c' ... 100 => { }