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::print;
  20 use rustc::hir::def::Def;
  21 use rustc::ty::{self, Ty, AssociatedItem};
  22 use errors::{DiagnosticBuilder, CodeMapper};
  23
  24 use super::method::probe;
  25
  26 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
  27     // Requires that the two types unify, and prints an error message if
  28     // they don't.
  29     pub fn demand_suptype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  30         self.demand_suptype_diag(sp, expected, actual).map(|mut e| e.emit());
  31     }
  32
  33     pub fn demand_suptype_diag(&self,
  34                                sp: Span,
  35                                expected: Ty<'tcx>,
  36                                actual: Ty<'tcx>) -> Option<DiagnosticBuilder<'tcx>> {
  37         let cause = &self.misc(sp);
  38         match self.at(cause, self.param_env).sup(expected, actual) {
  39             Ok(InferOk { obligations, value: () }) => {
  40                 self.register_predicates(obligations);
  41                 None
  42             },
  43             Err(e) => {
  44                 Some(self.report_mismatched_types(&cause, expected, actual, e))
  45             }
  46         }
  47     }
  48
  49     pub fn demand_eqtype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  50         if let Some(mut err) = self.demand_eqtype_diag(sp, expected, actual) {
  51             err.emit();
  52         }
  53     }
  54
  55     pub fn demand_eqtype_diag(&self,
  56                              sp: Span,
  57                              expected: Ty<'tcx>,
  58                              actual: Ty<'tcx>) -> Option<DiagnosticBuilder<'tcx>> {
  59         self.demand_eqtype_with_origin(&self.misc(sp), expected, actual)
  60     }
  61
  62     pub fn demand_eqtype_with_origin(&self,
  63                                      cause: &ObligationCause<'tcx>,
  64                                      expected: Ty<'tcx>,
  65                                      actual: Ty<'tcx>) -> Option<DiagnosticBuilder<'tcx>> {
  66         match self.at(cause, self.param_env).eq(expected, actual) {
  67             Ok(InferOk { obligations, value: () }) => {
  68                 self.register_predicates(obligations);
  69                 None
  70             },
  71             Err(e) => {
  72                 Some(self.report_mismatched_types(cause, expected, actual, e))
  73             }
  74         }
  75     }
  76
  77     pub fn demand_coerce(&self, expr: &hir::Expr, checked_ty: Ty<'tcx>, expected: Ty<'tcx>) {
  78         if let Some(mut err) = self.demand_coerce_diag(expr, checked_ty, expected) {
  79             err.emit();
  80         }
  81     }
  82
  83     // Checks that the type of `expr` can be coerced to `expected`.
  84     //
  85     // NB: This code relies on `self.diverges` to be accurate.  In
  86     // particular, assignments to `!` will be permitted if the
  87     // diverges flag is currently "always".
  88     pub fn demand_coerce_diag(&self,
  89                               expr: &hir::Expr,
  90                               checked_ty: Ty<'tcx>,
  91                               expected: Ty<'tcx>) -> Option<DiagnosticBuilder<'tcx>> {
  92         let expected = self.resolve_type_vars_with_obligations(expected);
  93
  94         if let Err(e) = self.try_coerce(expr, checked_ty, self.diverges.get(), expected) {
  95             let cause = self.misc(expr.span);
  96             let expr_ty = self.resolve_type_vars_with_obligations(checked_ty);
  97             let mut err = self.report_mismatched_types(&cause, expected, expr_ty, e);
  98
  99             // If the expected type is an enum with any variants whose sole
 100             // field is of the found type, suggest such variants. See Issue
 101             // #42764.
 102             if let ty::TyAdt(expected_adt, substs) = expected.sty {
 103                 let mut compatible_variants = vec![];
 104                 for variant in &expected_adt.variants {
 105                     if variant.fields.len() == 1 {
 106                         let sole_field = &variant.fields[0];
 107                         let sole_field_ty = sole_field.ty(self.tcx, substs);
 108                         if self.can_coerce(expr_ty, sole_field_ty) {
 109                             let mut variant_path = self.tcx.item_path_str(variant.did);
 110                             variant_path = variant_path.trim_left_matches("std::prelude::v1::")
 111                                 .to_string();
 112                             compatible_variants.push(variant_path);
 113                         }
 114                     }
 115                 }
 116                 if !compatible_variants.is_empty() {
 117                     let expr_text = print::to_string(print::NO_ANN, |s| s.print_expr(expr));
 118                     let suggestions = compatible_variants.iter()
 119                         .map(|v| format!("{}({})", v, expr_text)).collect::<Vec<_>>();
 120                     err.span_suggestions(expr.span,
 121                                          "perhaps you meant to use a variant of the expected type",
 122                                          suggestions);
 123                 }
 124             }
 125
 126             if let Some(suggestion) = self.check_ref(expr,
 127                                                      checked_ty,
 128                                                      expected) {
 129                 err.help(&suggestion);
 130             } else {
 131                 let mode = probe::Mode::MethodCall;
 132                 let suggestions = self.probe_for_return_type(syntax_pos::DUMMY_SP,
 133                                                              mode,
 134                                                              expected,
 135                                                              checked_ty,
 136                                                              ast::DUMMY_NODE_ID);
 137                 if suggestions.len() > 0 {
 138                     err.help(&format!("here are some functions which \
 139                                        might fulfill your needs:\n{}",
 140                                       self.get_best_match(&suggestions).join("\n")));
 141                 }
 142             }
 143             return Some(err);
 144         }
 145         None
 146     }
 147
 148     fn format_method_suggestion(&self, method: &AssociatedItem) -> String {
 149         format!("- .{}({})",
 150                 method.name,
 151                 if self.has_no_input_arg(method) {
 152                     ""
 153                 } else {
 154                     "..."
 155                 })
 156     }
 157
 158     fn display_suggested_methods(&self, methods: &[AssociatedItem]) -> Vec<String> {
 159         methods.iter()
 160                .take(5)
 161                .map(|method| self.format_method_suggestion(&*method))
 162                .collect::<Vec<String>>()
 163     }
 164
 165     fn get_best_match(&self, methods: &[AssociatedItem]) -> Vec<String> {
 166         let no_argument_methods: Vec<_> =
 167             methods.iter()
 168                    .filter(|ref x| self.has_no_input_arg(&*x))
 169                    .map(|x| x.clone())
 170                    .collect();
 171         if no_argument_methods.len() > 0 {
 172             self.display_suggested_methods(&no_argument_methods)
 173         } else {
 174             self.display_suggested_methods(&methods)
 175         }
 176     }
 177
 178     // This function checks if the method isn't static and takes other arguments than `self`.
 179     fn has_no_input_arg(&self, method: &AssociatedItem) -> bool {
 180         match method.def() {
 181             Def::Method(def_id) => {
 182                 self.tcx.fn_sig(def_id).inputs().skip_binder().len() == 1
 183             }
 184             _ => false,
 185         }
 186     }
 187
 188     /// This function is used to determine potential "simple" improvements or users' errors and
 189     /// provide them useful help. For example:
 190     ///
 191     /// ```
 192     /// fn some_fn(s: &str) {}
 193     ///
 194     /// let x = "hey!".to_owned();
 195     /// some_fn(x); // error
 196     /// ```
 197     ///
 198     /// No need to find every potential function which could make a coercion to transform a
 199     /// `String` into a `&str` since a `&` would do the trick!
 200     ///
 201     /// In addition of this check, it also checks between references mutability state. If the
 202     /// expected is mutable but the provided isn't, maybe we could just say "Hey, try with
 203     /// `&mut`!".
 204     fn check_ref(&self,
 205                  expr: &hir::Expr,
 206                  checked_ty: Ty<'tcx>,
 207                  expected: Ty<'tcx>)
 208                  -> Option<String> {
 209         match (&expected.sty, &checked_ty.sty) {
 210             (&ty::TyRef(_, _), &ty::TyRef(_, _)) => None,
 211             (&ty::TyRef(_, mutability), _) => {
 212                 // Check if it can work when put into a ref. For example:
 213                 //
 214                 // ```
 215                 // fn bar(x: &mut i32) {}
 216                 //
 217                 // let x = 0u32;
 218                 // bar(&x); // error, expected &mut
 219                 // ```
 220                 let ref_ty = match mutability.mutbl {
 221                     hir::Mutability::MutMutable => self.tcx.mk_mut_ref(
 222                                                        self.tcx.mk_region(ty::ReStatic),
 223                                                        checked_ty),
 224                     hir::Mutability::MutImmutable => self.tcx.mk_imm_ref(
 225                                                        self.tcx.mk_region(ty::ReStatic),
 226                                                        checked_ty),
 227                 };
 228                 if self.can_coerce(ref_ty, expected) {
 229                     // Use the callsite's span if this is a macro call. #41858
 230                     let sp = self.sess().codemap().call_span_if_macro(expr.span);
 231                     if let Ok(src) = self.tcx.sess.codemap().span_to_snippet(sp) {
 232                         return Some(format!("try with `{}{}`",
 233                                             match mutability.mutbl {
 234                                                 hir::Mutability::MutMutable => "&mut ",
 235                                                 hir::Mutability::MutImmutable => "&",
 236                                             },
 237                                             &src));
 238                     }
 239                 }
 240                 None
 241             }
 242             _ => None,
 243         }
 244     }
 245 }