Auto merge of #38099 - GuillaumeGomez:cast_suggestions, r=nikomatsakis

Cast suggestions

r? @nikomatsakis

src/librustc/infer/mod.rs

@@ -1367,9 +1367,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                                    cause: &ObligationCause<'tcx>,
                                    expected: Ty<'tcx>,
                                    actual: Ty<'tcx>,
-                                   err: TypeError<'tcx>) {
+                                   err: TypeError<'tcx>)
+                                   -> DiagnosticBuilder<'tcx> {
         let trace = TypeTrace::types(cause, true, expected, actual);
-        self.report_and_explain_type_error(trace, &err).emit();
+        self.report_and_explain_type_error(trace, &err)
     }
 
     pub fn report_conflicting_default_types(&self,

src/librustc/ty/sty.rs

@@ -543,6 +543,7 @@ pub struct ProjectionTy<'tcx> {
 pub struct BareFnTy<'tcx> {
     pub unsafety: hir::Unsafety,
     pub abi: abi::Abi,
+    /// Signature (inputs and output) of this function type.
     pub sig: PolyFnSig<'tcx>,
 }
 

src/librustc_typeck/check/_match.rs

@@ -481,7 +481,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                     } else {
                         (result_ty, arm_ty)
                     };
-                    self.report_mismatched_types(&cause, expected, found, e);
+                    self.report_mismatched_types(&cause, expected, found, e).emit();
                     self.tcx.types.err
                 }
             };

src/librustc_typeck/check/autoderef.rs

@@ -131,10 +131,18 @@ impl<'a, 'gcx, 'tcx> Autoderef<'a, 'gcx, 'tcx> {
         Some(self.fcx.resolve_type_vars_if_possible(&normalized.value))
     }
 
+    /// Returns the final type, generating an error if it is an
+    /// unresolved inference variable.
     pub fn unambiguous_final_ty(&self) -> Ty<'tcx> {
         self.fcx.structurally_resolved_type(self.span, self.cur_ty)
     }
 
+    /// Returns the final type we ended up with, which may well be an
+    /// inference variable (we will resolve it first, if possible).
+    pub fn maybe_ambiguous_final_ty(&self) -> Ty<'tcx> {
+        self.fcx.resolve_type_vars_if_possible(&self.cur_ty)
+    }
+
     pub fn finalize<'b, I>(self, pref: LvaluePreference, exprs: I)
         where I: IntoIterator<Item = &'b hir::Expr>
     {

src/librustc_typeck/check/demand.rs

@@ -14,8 +14,13 @@ use rustc::ty::Ty;
 use rustc::infer::{InferOk};
 use rustc::traits::ObligationCause;
 
-use syntax_pos::Span;
+use syntax::ast;
+use syntax_pos::{self, Span};
 use rustc::hir;
+use rustc::hir::def::Def;
+use rustc::ty::{self, AssociatedItem};
+
+use super::method::probe;
 
 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
     // Requires that the two types unify, and prints an error message if
@@ -27,7 +32,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 self.register_predicates(obligations);
             },
             Err(e) => {
-                self.report_mismatched_types(&cause, expected, actual, e);
+                self.report_mismatched_types(&cause, expected, actual, e).emit();
             }
         }
     }
@@ -46,7 +51,7 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                 self.register_predicates(obligations);
             },
             Err(e) => {
-                self.report_mismatched_types(cause, expected, actual, e);
+                self.report_mismatched_types(cause, expected, actual, e).emit();
             }
         }
     }
@@ -57,7 +62,65 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
         if let Err(e) = self.try_coerce(expr, checked_ty, expected) {
             let cause = self.misc(expr.span);
             let expr_ty = self.resolve_type_vars_with_obligations(checked_ty);
-            self.report_mismatched_types(&cause, expected, expr_ty, e);
+            let mode = probe::Mode::MethodCall;
+            let suggestions = self.probe_for_return_type(syntax_pos::DUMMY_SP,
+                                                         mode,
+                                                         expected,
+                                                         checked_ty,
+                                                         ast::DUMMY_NODE_ID);
+            let mut err = self.report_mismatched_types(&cause, expected, expr_ty, e);
+            if suggestions.len() > 0 {
+                err.help(&format!("here are some functions which \
+                                   might fulfill your needs:\n - {}",
+                                  self.get_best_match(&suggestions)));
+            };
+            err.emit();
+        }
+    }
+
+    fn format_method_suggestion(&self, method: &AssociatedItem) -> String {
+        format!(".{}({})",
+                method.name,
+                if self.has_no_input_arg(method) {
+                    ""
+                } else {
+                    "..."
+                })
+    }
+
+    fn display_suggested_methods(&self, methods: &[AssociatedItem]) -> String {
+        methods.iter()
+               .take(5)
+               .map(|method| self.format_method_suggestion(&*method))
+               .collect::<Vec<String>>()
+               .join("\n - ")
+    }
+
+    fn get_best_match(&self, methods: &[AssociatedItem]) -> String {
+        let no_argument_methods: Vec<_> =
+            methods.iter()
+                   .filter(|ref x| self.has_no_input_arg(&*x))
+                   .map(|x| x.clone())
+                   .collect();
+        if no_argument_methods.len() > 0 {
+            self.display_suggested_methods(&no_argument_methods)
+        } else {
+            self.display_suggested_methods(&methods)
+        }
+    }
+
+    // This function checks if the method isn't static and takes other arguments than `self`.
+    fn has_no_input_arg(&self, method: &AssociatedItem) -> bool {
+        match method.def() {
+            Def::Method(def_id) => {
+                match self.tcx.item_type(def_id).sty {
+                    ty::TypeVariants::TyFnDef(_, _, fty) => {
+                        fty.sig.skip_binder().inputs().len() == 1
+                    }
+                    _ => false,
+                }
+            }
+            _ => false,
         }
     }
 }

src/librustc_typeck/check/method/mod.rs

@@ -30,9 +30,11 @@ pub use self::CandidateSource::*;
 pub use self::suggest::AllTraitsVec;
 
 mod confirm;
-mod probe;
+pub mod probe;
 mod suggest;
 
+use self::probe::IsSuggestion;
+
 pub enum MethodError<'tcx> {
     // Did not find an applicable method, but we did find various near-misses that may work.
     NoMatch(NoMatchData<'tcx>),
@@ -91,7 +93,8 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                          allow_private: bool)
                          -> bool {
         let mode = probe::Mode::MethodCall;
-        match self.probe_method(span, mode, method_name, self_ty, call_expr_id) {
+        match self.probe_for_name(span, mode, method_name, IsSuggestion(false),
+                                  self_ty, call_expr_id) {
             Ok(..) => true,
             Err(NoMatch(..)) => false,
             Err(Ambiguity(..)) => true,
@@ -130,7 +133,8 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
 
         let mode = probe::Mode::MethodCall;
         let self_ty = self.resolve_type_vars_if_possible(&self_ty);
-        let pick = self.probe_method(span, mode, method_name, self_ty, call_expr.id)?;
+        let pick = self.probe_for_name(span, mode, method_name, IsSuggestion(false),
+                                       self_ty, call_expr.id)?;
 
         if let Some(import_id) = pick.import_id {
             self.tcx.used_trait_imports.borrow_mut().insert(import_id);
@@ -328,7 +332,8 @@ impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
                         expr_id: ast::NodeId)
                         -> Result<Def, MethodError<'tcx>> {
         let mode = probe::Mode::Path;
-        let pick = self.probe_method(span, mode, method_name, self_ty, expr_id)?;
+        let pick = self.probe_for_name(span, mode, method_name, IsSuggestion(false),
+                                       self_ty, expr_id)?;
 
         if let Some(import_id) = pick.import_id {
             self.tcx.used_trait_imports.borrow_mut().insert(import_id);