src/librustc_typeck/check/demand.rs

   1 // Copyright 2012 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11
  12 use check::FnCtxt;
  13 use rustc::infer::InferOk;
  14 use rustc::traits::ObligationCause;
  15
  16 use syntax::ast;
  17 use syntax_pos::{self, Span};
  18 use rustc::hir;
  19 use rustc::hir::def::Def;
  20 use rustc::ty::{self, Ty, AssociatedItem};
  21 use errors::DiagnosticBuilder;
  22
  23 use super::method::probe;
  24
  25 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
  26     // Requires that the two types unify, and prints an error message if
  27     // they don't.
  28     pub fn demand_suptype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  29         let cause = self.misc(sp);
  30         match self.sub_types(false, &cause, actual, expected) {
  31             Ok(InferOk { obligations, value: () }) => {
  32                 self.register_predicates(obligations);
  33             },
  34             Err(e) => {
  35                 self.report_mismatched_types(&cause, expected, actual, e).emit();
  36             }
  37         }
  38     }
  39
  40     pub fn demand_eqtype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  41         if let Some(mut err) = self.demand_eqtype_diag(sp, expected, actual) {
  42             err.emit();
  43         }
  44     }
  45
  46     pub fn demand_eqtype_diag(&self,
  47                              sp: Span,
  48                              expected: Ty<'tcx>,
  49                              actual: Ty<'tcx>) -> Option<DiagnosticBuilder<'tcx>> {
  50         self.demand_eqtype_with_origin(&self.misc(sp), expected, actual)
  51     }
  52
  53     pub fn demand_eqtype_with_origin(&self,
  54                                      cause: &ObligationCause<'tcx>,
  55                                      expected: Ty<'tcx>,
  56                                      actual: Ty<'tcx>) -> Option<DiagnosticBuilder<'tcx>> {
  57         match self.eq_types(false, cause, actual, expected) {
  58             Ok(InferOk { obligations, value: () }) => {
  59                 self.register_predicates(obligations);
  60                 None
  61             },
  62             Err(e) => {
  63                 Some(self.report_mismatched_types(cause, expected, actual, e))
  64             }
  65         }
  66     }
  67
  68     // Checks that the type of `expr` can be coerced to `expected`.
  69     //
  70     // NB: This code relies on `self.diverges` to be accurate.  In
  71     // particular, assignments to `!` will be permitted if the
  72     // diverges flag is currently "always".
  73     pub fn demand_coerce(&self,
  74                          expr: &hir::Expr,
  75                          checked_ty: Ty<'tcx>,
  76                          expected: Ty<'tcx>) {
  77         let expected = self.resolve_type_vars_with_obligations(expected);
  78
  79         if let Err(e) = self.try_coerce(expr, checked_ty, self.diverges.get(), expected) {
  80             let cause = self.misc(expr.span);
  81             let expr_ty = self.resolve_type_vars_with_obligations(checked_ty);
  82             let mut err = self.report_mismatched_types(&cause, expected, expr_ty, e);
  83             if let Some(suggestion) = self.check_ref(expr,
  84                                                      checked_ty,
  85                                                      expected) {
  86                 err.help(&suggestion);
  87             } else {
  88                 let mode = probe::Mode::MethodCall;
  89                 let suggestions = self.probe_for_return_type(syntax_pos::DUMMY_SP,
  90                                                              mode,
  91                                                              expected,
  92                                                              checked_ty,
  93                                                              ast::DUMMY_NODE_ID);
  94                 if suggestions.len() > 0 {
  95                     err.help(&format!("here are some functions which \
  96                                        might fulfill your needs:\n{}",
  97                                       self.get_best_match(&suggestions).join("\n")));
  98                 }
  99             }
 100             err.emit();
 101         }
 102     }
 103
 104     fn format_method_suggestion(&self, method: &AssociatedItem) -> String {
 105         format!("- .{}({})",
 106                 method.name,
 107                 if self.has_no_input_arg(method) {
 108                     ""
 109                 } else {
 110                     "..."
 111                 })
 112     }
 113
 114     fn display_suggested_methods(&self, methods: &[AssociatedItem]) -> Vec<String> {
 115         methods.iter()
 116                .take(5)
 117                .map(|method| self.format_method_suggestion(&*method))
 118                .collect::<Vec<String>>()
 119     }
 120
 121     fn get_best_match(&self, methods: &[AssociatedItem]) -> Vec<String> {
 122         let no_argument_methods: Vec<_> =
 123             methods.iter()
 124                    .filter(|ref x| self.has_no_input_arg(&*x))
 125                    .map(|x| x.clone())
 126                    .collect();
 127         if no_argument_methods.len() > 0 {
 128             self.display_suggested_methods(&no_argument_methods)
 129         } else {
 130             self.display_suggested_methods(&methods)
 131         }
 132     }
 133
 134     // This function checks if the method isn't static and takes other arguments than `self`.
 135     fn has_no_input_arg(&self, method: &AssociatedItem) -> bool {
 136         match method.def() {
 137             Def::Method(def_id) => {
 138                 match self.tcx.type_of(def_id).sty {
 139                     ty::TypeVariants::TyFnDef(_, _, sig) => {
 140                         sig.inputs().skip_binder().len() == 1
 141                     }
 142                     _ => false,
 143                 }
 144             }
 145             _ => false,
 146         }
 147     }
 148
 149     /// This function is used to determine potential "simple" improvements or users' errors and
 150     /// provide them useful help. For example:
 151     ///
 152     /// ```
 153     /// fn some_fn(s: &str) {}
 154     ///
 155     /// let x = "hey!".to_owned();
 156     /// some_fn(x); // error
 157     /// ```
 158     ///
 159     /// No need to find every potential function which could make a coercion to transform a
 160     /// `String` into a `&str` since a `&` would do the trick!
 161     ///
 162     /// In addition of this check, it also checks between references mutability state. If the
 163     /// expected is mutable but the provided isn't, maybe we could just say "Hey, try with
 164     /// `&mut`!".
 165     fn check_ref(&self,
 166                  expr: &hir::Expr,
 167                  checked_ty: Ty<'tcx>,
 168                  expected: Ty<'tcx>)
 169                  -> Option<String> {
 170         match (&expected.sty, &checked_ty.sty) {
 171             (&ty::TyRef(_, _), &ty::TyRef(_, _)) => None,
 172             (&ty::TyRef(_, mutability), _) => {
 173                 // Check if it can work when put into a ref. For example:
 174                 //
 175                 // ```
 176                 // fn bar(x: &mut i32) {}
 177                 //
 178                 // let x = 0u32;
 179                 // bar(&x); // error, expected &mut
 180                 // ```
 181                 let ref_ty = match mutability.mutbl {
 182                     hir::Mutability::MutMutable => self.tcx.mk_mut_ref(
 183                                                        self.tcx.mk_region(ty::ReStatic),
 184                                                        checked_ty),
 185                     hir::Mutability::MutImmutable => self.tcx.mk_imm_ref(
 186                                                        self.tcx.mk_region(ty::ReStatic),
 187                                                        checked_ty),
 188                 };
 189                 if self.can_coerce(ref_ty, expected) {
 190                     if let Ok(src) = self.tcx.sess.codemap().span_to_snippet(expr.span) {
 191                         return Some(format!("try with `{}{}`",
 192                                             match mutability.mutbl {
 193                                                 hir::Mutability::MutMutable => "&mut ",
 194                                                 hir::Mutability::MutImmutable => "&",
 195                                             },
 196                                             &src));
 197                     }
 198                 }
 199                 None
 200             }
 201             _ => None,
 202         }
 203     }
 204 }