Auto merge of #41236 - cengizIO:master, r=nikomatsakis

Move E0101 and E0102 logic into new E0282 mechanism #40013

Hello there!

## What's this?
Previously, me and @nikomatsakis worked on error messages of uninferred locals. (#38812)

This aims to build up on that by moving certain type checks from `writeback`.

With this, `E0101` and `E0102` errors are getting obsoleted and no longer thrown.

They're replaced with customized versions of `E0282`s instead.

## Sample Error Messages

#### `E0101` is getting converted into:
```rust
error[E0282]: type annotations needed
 --> test.rs:2:14
  |
2 |     let x = |_| {};
  |              ^ consider giving this closure parameter a type

error: aborting due to previous error
```

#### `E0102` is getting converted into:
```rust
error[E0282]: type annotations needed
 --> test.rs:2:9
  |
2 |     let x = [];
  |         ^
  |         |
  |         consider giving `x` a type
  |         cannot infer type for `[_; 0]`

error: aborting due to previous error
```

## Annoyances
- I think we need to change our way of type name resolving in relevant places, because that `[_; 0]` looks horrible IMHO.
- I'm not terribly happy with the note ordering of errors. So please do point to code that might help me accomplish this.

## Tests
Tests of `E0101` and `E0102` are getting converted from `compile-fail` to `ui` tests.

## Documentation
Please help me with documentation update. There are some confusing places that needed an update but I'm not sure if I did the right ones.

Please do comment on messages, layouts and other details.

## Appreciation
Huge thanks goes to @nikomatsakis for being a patient and humble mentor along this long journey. 🍻

src/librustc/diagnostics.rs

@@ -1812,6 +1812,8 @@ makes a difference in practice.)
 
 register_diagnostics! {
 //  E0006 // merged with E0005
+//  E0101, // replaced with E0282
+//  E0102, // replaced with E0282
 //  E0134,
 //  E0135,
     E0278, // requirement is not satisfied

src/librustc/traits/error_reporting.rs

@@ -25,16 +25,16 @@ use super::{
 
 use errors::DiagnosticBuilder;
 use fmt_macros::{Parser, Piece, Position};
-use hir::{intravisit, Local, Pat};
+use hir::{self, intravisit, Local, Pat, Body};
 use hir::intravisit::{Visitor, NestedVisitorMap};
 use hir::map::NodeExpr;
 use hir::def_id::DefId;
 use infer::{self, InferCtxt};
 use infer::type_variable::TypeVariableOrigin;
 use rustc::lint::builtin::EXTRA_REQUIREMENT_IN_IMPL;
 use std::fmt;
-use syntax::ast;
-use ty::{self, AdtKind, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable};
+use syntax::ast::{self, NodeId};
+use ty::{self, AdtKind, ToPredicate, ToPolyTraitRef, Ty, TyCtxt, TypeFoldable, TyInfer, TyVar};
 use ty::error::ExpectedFound;
 use ty::fast_reject;
 use ty::fold::TypeFolder;
@@ -66,37 +66,52 @@ 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>,
+    hir_map: &'a hir::map::Map<'gcx>,
+    found_local_pattern: Option<&'gcx Pat>,
+    found_arg_pattern: Option<&'gcx Pat>,
 }
 
 impl<'a, 'gcx, 'tcx> FindLocalByTypeVisitor<'a, 'gcx, 'tcx> {
-    fn is_match(&self, ty: Ty<'tcx>) -> bool {
-        ty == *self.target_ty || match (&ty.sty, &self.target_ty.sty) {
-            (&ty::TyInfer(ty::TyVar(a_vid)), &ty::TyInfer(ty::TyVar(b_vid))) =>
-                self.infcx.type_variables
-                          .borrow_mut()
-                          .sub_unified(a_vid, b_vid),
-
+    fn node_matches_type(&mut self, node_id: &'gcx NodeId) -> bool {
+        match self.infcx.tables.borrow().node_types.get(node_id) {
+            Some(&ty) => {
+                let ty = self.infcx.resolve_type_vars_if_possible(&ty);
+                ty.walk().any(|inner_ty| {
+                    inner_ty == *self.target_ty || match (&inner_ty.sty, &self.target_ty.sty) {
+                        (&TyInfer(TyVar(a_vid)), &TyInfer(TyVar(b_vid))) => {
+                            self.infcx
+                                .type_variables
+                                .borrow_mut()
+                                .sub_unified(a_vid, b_vid)
+                        }
+                        _ => false,
+                    }
+                })
+            }
             _ => false,
         }
     }
 }
 
-impl<'a, 'gcx, 'tcx> Visitor<'a> for FindLocalByTypeVisitor<'a, 'gcx, 'tcx> {
-    fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'a> {
-        NestedVisitorMap::None
+impl<'a, 'gcx, 'tcx> Visitor<'gcx> for FindLocalByTypeVisitor<'a, 'gcx, 'tcx> {
+    fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'gcx> {
+        NestedVisitorMap::OnlyBodies(&self.hir_map)
     }
 
-    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| self.is_match(t));
+    fn visit_local(&mut self, local: &'gcx Local) {
+        if self.found_local_pattern.is_none() && self.node_matches_type(&local.id) {
+            self.found_local_pattern = Some(&*local.pat);
+        }
+        intravisit::walk_local(self, local);
+    }
 
-            if is_match && self.found_pattern.is_none() {
-                self.found_pattern = Some(&*local.pat);
+    fn visit_body(&mut self, body: &'gcx Body) {
+        for argument in &body.arguments {
+            if self.found_arg_pattern.is_none() && self.node_matches_type(&argument.id) {
+                self.found_arg_pattern = Some(&*argument.pat);
             }
         }
-        intravisit::walk_local(self, local);
+        intravisit::walk_body(self, body);
     }
 }
 
@@ -721,6 +736,8 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         // coherence violation, so we don't report it here.
 
         let predicate = self.resolve_type_vars_if_possible(&obligation.predicate);
+        let body_id = hir::BodyId { node_id: obligation.cause.body_id };
+        let span = obligation.cause.span;
 
         debug!("maybe_report_ambiguity(predicate={:?}, obligation={:?})",
                predicate,
@@ -768,10 +785,10 @@ 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, self_ty);
+                        self.need_type_info(body_id, span, self_ty);
                     } else {
                         let mut err = struct_span_err!(self.tcx.sess,
-                                                        obligation.cause.span, E0283,
+                                                        span, E0283,
                                                         "type annotations required: \
                                                         cannot resolve `{}`",
                                                         predicate);
@@ -785,7 +802,7 @@ 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, ty);
+                    self.need_type_info(body_id, span, ty);
                 }
             }
 
@@ -796,7 +813,9 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                     let &SubtypePredicate { a_is_expected: _, a, b } = data.skip_binder();
                     // both must be type variables, or the other would've been instantiated
                     assert!(a.is_ty_var() && b.is_ty_var());
-                    self.need_type_info(obligation, a);
+                    self.need_type_info(hir::BodyId { node_id: obligation.cause.body_id },
+                                        obligation.cause.span,
+                                        a);
                 }
             }
 
@@ -874,42 +893,66 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
         }
     }
 
-    fn need_type_info(&self, obligation: &PredicateObligation<'tcx>, ty: Ty<'tcx>) {
+    pub fn need_type_info(&self, body_id: hir::BodyId, span: Span, 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 mut err_span = span;
+        let mut labels = vec![(span, format!("cannot infer type for `{}`", name))];
 
         let mut local_visitor = FindLocalByTypeVisitor {
             infcx: &self,
             target_ty: &ty,
-            found_pattern: None,
+            hir_map: &self.tcx.hir,
+            found_local_pattern: None,
+            found_arg_pattern: None,
         };
 
         // #40294: cause.body_id can also be a fn declaration.
         // Currently, if it's anything other than NodeExpr, we just ignore it
-        match self.tcx.hir.find(cause.body_id) {
+        match self.tcx.hir.find(body_id.node_id) {
             Some(NodeExpr(expr)) => local_visitor.visit_expr(expr),
             _ => ()
         }
 
-        if let Some(pattern) = local_visitor.found_pattern {
-            let pattern_span = pattern.span;
+        if let Some(pattern) = local_visitor.found_arg_pattern {
+            err_span = pattern.span;
+            // We don't want to show the default label for closures.
+            //
+            // So, before clearing, the output would look something like this:
+            // ```
+            // let x = |_| {  };
+            //          -  ^^^^ cannot infer type for `[_; 0]`
+            //          |
+            //          consider giving this closure parameter a type
+            // ```
+            //
+            // After clearing, it looks something like this:
+            // ```
+            // let x = |_| {  };
+            //          ^ consider giving this closure parameter a type
+            // ```
+            labels.clear();
+            labels.push((pattern.span, format!("consider giving this closure parameter a type")));
+        }
+
+        if let Some(pattern) = local_visitor.found_local_pattern {
             if let Some(simple_name) = pattern.simple_name() {
-                err.span_label(pattern_span,
-                               &format!("consider giving `{}` a type",
-                                        simple_name));
+                labels.push((pattern.span, format!("consider giving `{}` a type", simple_name)));
             } else {
-                err.span_label(pattern_span, &format!("consider giving a type to pattern"));
+                labels.push((pattern.span, format!("consider giving the pattern a type")));
             }
         }
 
+        let mut err = struct_span_err!(self.tcx.sess,
+                                       err_span,
+                                       E0282,
+                                       "type annotations needed");
+
+        for (target_span, label_message) in labels {
+            err.span_label(target_span, &label_message);
+        }
+
         err.emit();
     }