Rollup merge of rust-lang#39361 - cengizIO:master, r=arielb1

Improve error message for uninferrable types rust-lang#38812

Hello,

I tried to improve the error message for uninferrable types. The error code is `E0282`.

```rust

error[E0282]: type annotations needed
 --> /home/cengizIO/issue38812.rs:2:11
  |
2 |   let x = vec![];
  |       -   ^^^^^^ cannot infer type for `T`
  |       |
  |       consider giving `x` a type
  |
  = note: this error originates in a macro outside of the current crate
```

and

```rust

error[E0282]: type annotations needed
 --> /home/cengizIO/issue38812.rs:2:15
  |
2 |   let (x,) = (vec![],);
  |       ----    ^^^^^^ cannot infer type for `T`
  |       |
  |       consider giving a type to pattern
  |
  = note: this error originates in a macro outside of the current crate
```

Rust compiler now tries to find uninferred `local`s with type `_` and adds them into the error message.

I'm probably wrong on wording that I used. Please feel free to suggest better alternatives.

Thanks @nikomatsakis for mentoring 🍺

Any comments/feedback is more than welcome!

Thank you

Author: frewsxcv

src/librustc/traits/error_reporting.rs

@@ -39,6 +39,8 @@ use util::nodemap::{FxHashMap, FxHashSet};
 use std::cmp;
 use std::fmt;
 use syntax::ast;
+use hir::{intravisit, Local, Pat};
+use hir::intravisit::{Visitor, NestedVisitorMap};
 use syntax_pos::{DUMMY_SP, Span};
 use errors::DiagnosticBuilder;
 
@@ -60,6 +62,30 @@ impl<'a, 'gcx, 'tcx> TraitErrorKey<'tcx> {
     }
 }
 
+struct FindLocalByTypeVisitor<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
+    infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
+    target_ty: &'a Ty<'tcx>,
+    found_pattern: Option<&'a Pat>,
+}
+
+impl<'a, 'gcx, 'tcx> Visitor<'a> for FindLocalByTypeVisitor<'a, 'gcx, 'tcx> {
+    fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'a> {
+        NestedVisitorMap::None
+    }
+
+    fn visit_local(&mut self, local: &'a Local) {
+        if let Some(&ty) = self.infcx.tables.borrow().node_types.get(&local.id) {
+            let ty = self.infcx.resolve_type_vars_if_possible(&ty);
+            let is_match = ty.walk().any(|t| t == *self.target_ty);
+
+            if is_match && self.found_pattern.is_none() {
+                self.found_pattern = Some(&*local.pat);
+            }
+        }
+        intravisit::walk_local(self, local);
+    }
+}
+
 impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
     pub fn report_fulfillment_errors(&self, errors: &Vec<FulfillmentError<'tcx>>) {
         for error in errors {
@@ -775,7 +801,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                             self.tcx.lang_items.sized_trait()
                             .map_or(false, |sized_id| sized_id == trait_ref.def_id())
                         {
-                            self.need_type_info(obligation.cause.span, self_ty);
+                            self.need_type_info(obligation, self_ty);
                         } else {
                             let mut err = struct_span_err!(self.tcx.sess,
                                                            obligation.cause.span, E0283,
@@ -793,7 +819,7 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                 // Same hacky approach as above to avoid deluging user
                 // with error messages.
                 if !ty.references_error() && !self.tcx.sess.has_errors() {
-                    self.need_type_info(obligation.cause.span, ty);
+                    self.need_type_info(obligation, ty);
                 }
             }
 
@@ -857,27 +883,53 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         })
     }
 
-
-    fn need_type_info(&self, span: Span, ty: Ty<'tcx>) {
-        let ty = self.resolve_type_vars_if_possible(&ty);
-        let name = if let ty::TyInfer(ty::TyVar(ty_vid)) = ty.sty {
+    fn extract_type_name(&self, ty: &'a Ty<'tcx>) -> String {
+        if let ty::TyInfer(ty::TyVar(ty_vid)) = (*ty).sty {
             let ty_vars = self.type_variables.borrow();
             if let TypeVariableOrigin::TypeParameterDefinition(_, name) =
-                    *ty_vars.var_origin(ty_vid)
-            {
+                *ty_vars.var_origin(ty_vid) {
                 name.to_string()
             } else {
                 ty.to_string()
             }
         } else {
             ty.to_string()
+        }
+    }
+
+    fn need_type_info(&self, obligation: &PredicateObligation<'tcx>, ty: Ty<'tcx>) {
+        let ty = self.resolve_type_vars_if_possible(&ty);
+        let name = self.extract_type_name(&ty);
+        let ref cause = obligation.cause;
+
+        let mut err = struct_span_err!(self.tcx.sess,
+                                       cause.span,
+                                       E0282,
+                                       "type annotations needed");
+
+        err.span_label(cause.span, &format!("cannot infer type for `{}`", name));
+
+        let expr = self.tcx.hir.expect_expr(cause.body_id);
+
+        let mut local_visitor = FindLocalByTypeVisitor {
+            infcx: &self,
+            target_ty: &ty,
+            found_pattern: None,
         };
 
-        let mut err = struct_span_err!(self.tcx.sess, span, E0282,
-                                       "unable to infer enough type information about `{}`",
-                                       name);
-        err.note("type annotations or generic parameter binding required");
-        err.span_label(span, &format!("cannot infer type for `{}`", name));
+        local_visitor.visit_expr(expr);
+
+        if let Some(pattern) = local_visitor.found_pattern {
+            let pattern_span = pattern.span;
+            if let Some(simple_name) = pattern.simple_name() {
+                err.span_label(pattern_span,
+                               &format!("consider giving `{}` a type",
+                                        simple_name));
+            } else {
+                err.span_label(pattern_span, &format!("consider giving a type to pattern"));
+            }
+        }
+
         err.emit();
     }
 

src/test/compile-fail/issue-12187-1.rs

@@ -14,7 +14,6 @@ fn new<T>() -> &'static T {
 
 fn main() {
     let &v = new();
-    //~^ ERROR unable to infer enough type information about `_` [E0282]
+    //~^ ERROR type annotations needed [E0282]
     //~| NOTE cannot infer type for `_`
-    //~| NOTE type annotations or generic parameter binding
 }

src/test/compile-fail/issue-12187-2.rs

@@ -14,7 +14,6 @@ fn new<'r, T>() -> &'r T {
 
 fn main() {
     let &v = new();
-    //~^ ERROR unable to infer enough type information about `_` [E0282]
+    //~^ ERROR type annotations needed [E0282]
     //~| NOTE cannot infer type for `_`
-    //~| NOTE type annotations or generic parameter binding
 }

src/test/compile-fail/issue-16966.rs

@@ -8,7 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-// error-pattern:type annotations or generic parameter binding required
+// error-pattern:type annotations needed
 fn main() {
     panic!(
         std::default::Default::default()

src/test/compile-fail/issue-17551.rs

@@ -13,6 +13,6 @@ use std::marker;
 struct B<T>(marker::PhantomData<T>);
 
 fn main() {
-    let foo = B(marker::PhantomData); //~ ERROR unable to infer enough type information
+    let foo = B(marker::PhantomData); //~ ERROR type annotations needed
     let closure = || foo;
 }

src/test/compile-fail/issue-18159.rs

@@ -9,5 +9,5 @@
 // except according to those terms.
 
 fn main() {
-    let x; //~ ERROR unable to infer enough type information
+    let x; //~ ERROR type annotations needed
 }

src/test/compile-fail/issue-23041.rs

@@ -15,5 +15,4 @@ fn main()
     let b:Box<Any> = Box::new(bar as fn(_)->_);
     b.downcast_ref::<fn(_)->_>(); //~ ERROR E0282
                                   //~| NOTE cannot infer type for `_`
-                                  //~| NOTE type annotations or generic parameter binding required
 }

src/test/compile-fail/issue-23046.rs

@@ -25,6 +25,6 @@ pub fn let_<'var, VAR, F: for<'v: 'var> Fn(Expr<'v, VAR>) -> Expr<'v, VAR>>
 
 fn main() {
     let ex = |x| {
-        let_(add(x,x), |y| { //~ ERROR unable to infer enough type information about `VAR`
+        let_(add(x,x), |y| { //~ ERROR type annotations needed
             let_(add(x, x), |x|x)})};
 }

src/test/compile-fail/issue-24013.rs

@@ -13,5 +13,5 @@ fn main() {
     let a = 1;
     let b = 2;
     unsafe {swap::<&mut _>(transmute(&a), transmute(&b))};
-    //~^ ERROR unable to infer enough type information about `_`
+    //~^ ERROR type annotations needed
 }

src/test/compile-fail/issue-5062.rs

@@ -9,4 +9,4 @@
 // except according to those terms.
 
 fn main() { format!("{:?}", None); }
-    //~^ ERROR unable to infer enough type information about `T` [E0282]
+    //~^ ERROR type annotations needed [E0282]