suppress trait errors that are implied by other errors

Instead of suppressing only trait errors that are "exact duplicates",
display only the "most high-level" error when there are multiple trait
errors with the same span that imply each-other.

e.g. when there are both `[closure]: Fn` and `[closure]: FnOnce`, omit
displaying the `[closure]: FnOnce` bound.

Author: arielb1

src/librustc/infer/error_reporting/mod.rs

@@ -74,6 +74,7 @@ use syntax_pos::{Pos, Span};
 use errors::{DiagnosticBuilder, DiagnosticStyledString};
 
 mod note;
+mod need_type_info;
 
 impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     pub fn note_and_explain_region(self,

src/librustc/infer/error_reporting/need_type_info.rs

@@ -0,0 +1,153 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use hir::{self, map, Local, Pat, Body};
+use hir::intravisit::{self, Visitor, NestedVisitorMap};
+use infer::InferCtxt;
+use infer::type_variable::TypeVariableOrigin;
+use ty::{self, Ty, TyInfer, TyVar};
+
+use syntax::ast::NodeId;
+use syntax_pos::Span;
+
+struct FindLocalByTypeVisitor<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
+    infcx: &'a InferCtxt<'a, 'gcx, 'tcx>,
+    target_ty: &'a Ty<'tcx>,
+    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 node_matches_type(&mut self, node_id: NodeId) -> bool {
+        let ty_opt = self.infcx.in_progress_tables.and_then(|tables| {
+            tables.borrow().node_id_to_type_opt(node_id)
+        });
+        match ty_opt {
+            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,
+                    }
+                })
+            }
+            None => false,
+        }
+    }
+}
+
+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: &'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);
+    }
+
+    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_body(self, body);
+    }
+}
+
+
+impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
+    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) {
+                name.to_string()
+            } else {
+                ty.to_string()
+            }
+        } else {
+            ty.to_string()
+        }
+    }
+
+    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 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,
+            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(body_id.node_id) {
+            Some(map::NodeExpr(expr)) => local_visitor.visit_expr(expr),
+            _ => ()
+        }
+
+        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() {
+                labels.push((pattern.span, format!("consider giving `{}` a type", simple_name)));
+            } else {
+                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();
+    }
+}

src/librustc/infer/mod.rs

@@ -36,7 +36,7 @@ use std::fmt;
 use syntax::ast;
 use errors::DiagnosticBuilder;
 use syntax_pos::{self, Span, DUMMY_SP};
-use util::nodemap::{FxHashMap, FxHashSet};
+use util::nodemap::FxHashMap;
 use arena::DroplessArena;
 
 use self::combine::CombineFields;
@@ -110,7 +110,7 @@ pub struct InferCtxt<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 
     // the set of predicates on which errors have been reported, to
     // avoid reporting the same error twice.
-    pub reported_trait_errors: RefCell<FxHashSet<traits::TraitErrorKey<'tcx>>>,
+    pub reported_trait_errors: RefCell<FxHashMap<Span, Vec<ty::Predicate<'tcx>>>>,
 
     // When an error occurs, we want to avoid reporting "derived"
     // errors that are due to this original failure. Normally, we
@@ -350,6 +350,7 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'gcx> {
             global_tcx: self,
             arena: DroplessArena::new(),
             fresh_tables: None,
+
         }
     }
 }
@@ -381,7 +382,7 @@ impl<'a, 'gcx, 'tcx> InferCtxtBuilder<'a, 'gcx, 'tcx> {
             region_vars: RegionVarBindings::new(tcx),
             selection_cache: traits::SelectionCache::new(),
             evaluation_cache: traits::EvaluationCache::new(),
-            reported_trait_errors: RefCell::new(FxHashSet()),
+            reported_trait_errors: RefCell::new(FxHashMap()),
             tainted_by_errors_flag: Cell::new(false),
             err_count_on_creation: tcx.sess.err_count(),
             in_snapshot: Cell::new(false),