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 use check::FnCtxt;
  12 use rustc::infer::InferOk;
  13 use rustc::traits::ObligationCause;
  14
  15 use syntax::ast;
  16 use syntax::util::parser::PREC_POSTFIX;
  17 use syntax_pos::Span;
  18 use rustc::hir;
  19 use rustc::hir::def::Def;
  20 use rustc::hir::Node;
  21 use rustc::hir::{Item, ItemKind, print};
  22 use rustc::ty::{self, Ty, AssociatedItem};
  23 use rustc::ty::adjustment::AllowTwoPhase;
  24 use errors::{Applicability, DiagnosticBuilder, SourceMapper};
  25
  26 use super::method::probe;
  27
  28 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
  29     // Requires that the two types unify, and prints an error message if
  30     // they don't.
  31     pub fn demand_suptype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  32         self.demand_suptype_diag(sp, expected, actual).map(|mut e| e.emit());
  33     }
  34
  35     pub fn demand_suptype_diag(&self,
  36                                sp: Span,
  37                                expected: Ty<'tcx>,
  38                                actual: Ty<'tcx>) -> Option<DiagnosticBuilder<'tcx>> {
  39         let cause = &self.misc(sp);
  40         match self.at(cause, self.param_env).sup(expected, actual) {
  41             Ok(InferOk { obligations, value: () }) => {
  42                 self.register_predicates(obligations);
  43                 None
  44             },
  45             Err(e) => {
  46                 Some(self.report_mismatched_types(&cause, expected, actual, e))
  47             }
  48         }
  49     }
  50
  51     pub fn demand_eqtype(&self, sp: Span, expected: Ty<'tcx>, actual: Ty<'tcx>) {
  52         if let Some(mut err) = self.demand_eqtype_diag(sp, expected, actual) {
  53             err.emit();
  54         }
  55     }
  56
  57     pub fn demand_eqtype_diag(&self,
  58                              sp: Span,
  59                              expected: Ty<'tcx>,
  60                              actual: Ty<'tcx>) -> Option<DiagnosticBuilder<'tcx>> {
  61         self.demand_eqtype_with_origin(&self.misc(sp), expected, actual)
  62     }
  63
  64     pub fn demand_eqtype_with_origin(&self,
  65                                      cause: &ObligationCause<'tcx>,
  66                                      expected: Ty<'tcx>,
  67                                      actual: Ty<'tcx>) -> Option<DiagnosticBuilder<'tcx>> {
  68         match self.at(cause, self.param_env).eq(expected, actual) {
  69             Ok(InferOk { obligations, value: () }) => {
  70                 self.register_predicates(obligations);
  71                 None
  72             }
  73             Err(e) => {
  74                 Some(self.report_mismatched_types(cause, expected, actual, e))
  75             }
  76         }
  77     }
  78
  79     pub fn demand_coerce(&self,
  80                          expr: &hir::Expr,
  81                          checked_ty: Ty<'tcx>,
  82                          expected: Ty<'tcx>,
  83                          allow_two_phase: AllowTwoPhase)
  84                          -> Ty<'tcx> {
  85         let (ty, err) = self.demand_coerce_diag(expr, checked_ty, expected, allow_two_phase);
  86         if let Some(mut err) = err {
  87             err.emit();
  88         }
  89         ty
  90     }
  91
  92     // Checks that the type of `expr` can be coerced to `expected`.
  93     //
  94     // NB: This code relies on `self.diverges` to be accurate. In
  95     // particular, assignments to `!` will be permitted if the
  96     // diverges flag is currently "always".
  97     pub fn demand_coerce_diag(&self,
  98                               expr: &hir::Expr,
  99                               checked_ty: Ty<'tcx>,
 100                               expected: Ty<'tcx>,
 101                               allow_two_phase: AllowTwoPhase)
 102                               -> (Ty<'tcx>, Option<DiagnosticBuilder<'tcx>>) {
 103         let expected = self.resolve_type_vars_with_obligations(expected);
 104
 105         let e = match self.try_coerce(expr, checked_ty, expected, allow_two_phase) {
 106             Ok(ty) => return (ty, None),
 107             Err(e) => e
 108         };
 109
 110         let cause = self.misc(expr.span);
 111         let expr_ty = self.resolve_type_vars_with_obligations(checked_ty);
 112         let mut err = self.report_mismatched_types(&cause, expected, expr_ty, e);
 113
 114         // If the expected type is an enum with any variants whose sole
 115         // field is of the found type, suggest such variants. See Issue
 116         // #42764.
 117         if let ty::Adt(expected_adt, substs) = expected.sty {
 118             let compatible_variants = expected_adt.variants
 119                                                   .iter()
 120                                                   .filter(|variant| variant.fields.len() == 1)
 121                                                   .filter_map(|variant| {
 122                 let sole_field = &variant.fields[0];
 123                 let sole_field_ty = sole_field.ty(self.tcx, substs);
 124                 if self.can_coerce(expr_ty, sole_field_ty) {
 125                     let variant_path = self.tcx.item_path_str(variant.did);
 126                     Some(variant_path.trim_left_matches("std::prelude::v1::").to_string())
 127                 } else {
 128                     None
 129                 }
 130             }).collect::<Vec<_>>();
 131
 132             if !compatible_variants.is_empty() {
 133                 let expr_text = print::to_string(print::NO_ANN, |s| s.print_expr(expr));
 134                 let suggestions = compatible_variants.iter()
 135                     .map(|v| format!("{}({})", v, expr_text)).collect::<Vec<_>>();
 136                 err.span_suggestions_with_applicability(
 137                      expr.span,
 138                      "try using a variant of the expected type",
 139                      suggestions,
 140                      Applicability::MaybeIncorrect,
 141                 );
 142             }
 143         }
 144
 145         self.suggest_ref_or_into(&mut err, expr, expected, expr_ty);
 146
 147         (expected, Some(err))
 148     }
 149
 150     pub fn get_conversion_methods(&self, span: Span, expected: Ty<'tcx>, checked_ty: Ty<'tcx>)
 151                               -> Vec<AssociatedItem> {
 152         let mut methods = self.probe_for_return_type(span,
 153                                                      probe::Mode::MethodCall,
 154                                                      expected,
 155                                                      checked_ty,
 156                                                      ast::DUMMY_NODE_ID);
 157         methods.retain(|m| {
 158             self.has_no_input_arg(m) &&
 159                 self.tcx.get_attrs(m.def_id).iter()
 160                 // This special internal attribute is used to whitelist
 161                 // "identity-like" conversion methods to be suggested here.
 162                 //
 163                 // FIXME (#46459 and #46460): ideally
 164                 // `std::convert::Into::into` and `std::borrow:ToOwned` would
 165                 // also be `#[rustc_conversion_suggestion]`, if not for
 166                 // method-probing false-positives and -negatives (respectively).
 167                 //
 168                 // FIXME? Other potential candidate methods: `as_ref` and
 169                 // `as_mut`?
 170                 .find(|a| a.check_name("rustc_conversion_suggestion")).is_some()
 171         });
 172
 173         methods
 174     }
 175
 176     // This function checks if the method isn't static and takes other arguments than `self`.
 177     fn has_no_input_arg(&self, method: &AssociatedItem) -> bool {
 178         match method.def() {
 179             Def::Method(def_id) => {
 180                 self.tcx.fn_sig(def_id).inputs().skip_binder().len() == 1
 181             }
 182             _ => false,
 183         }
 184     }
 185
 186     /// Identify some cases where `as_ref()` would be appropriate and suggest it.
 187     ///
 188     /// Given the following code:
 189     /// ```
 190     /// struct Foo;
 191     /// fn takes_ref(_: &Foo) {}
 192     /// let ref opt = Some(Foo);
 193     ///
 194     /// opt.map(|arg| takes_ref(arg));
 195     /// ```
 196     /// Suggest using `opt.as_ref().map(|arg| takes_ref(arg));` instead.
 197     ///
 198     /// It only checks for `Option` and `Result` and won't work with
 199     /// ```
 200     /// opt.map(|arg| { takes_ref(arg) });
 201     /// ```
 202     fn can_use_as_ref(&self, expr: &hir::Expr) -> Option<(Span, &'static str, String)> {
 203         if let hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) = expr.node {
 204             if let hir::def::Def::Local(id) = path.def {
 205                 let parent = self.tcx.hir.get_parent_node(id);
 206                 if let Some(Node::Expr(hir::Expr {
 207                     id,
 208                     node: hir::ExprKind::Closure(_, decl, ..),
 209                     ..
 210                 })) = self.tcx.hir.find(parent) {
 211                     let parent = self.tcx.hir.get_parent_node(*id);
 212                     if let (Some(Node::Expr(hir::Expr {
 213                         node: hir::ExprKind::MethodCall(path, span, expr),
 214                         ..
 215                     })), 1) = (self.tcx.hir.find(parent), decl.inputs.len()) {
 216                         let self_ty = self.tables.borrow().node_id_to_type(expr[0].hir_id);
 217                         let self_ty = format!("{:?}", self_ty);
 218                         let name = path.ident.as_str();
 219                         let is_as_ref_able = (
 220                             self_ty.starts_with("&std::option::Option") ||
 221                             self_ty.starts_with("&std::result::Result") ||
 222                             self_ty.starts_with("std::option::Option") ||
 223                             self_ty.starts_with("std::result::Result")
 224                         ) && (name == "map" || name == "and_then");
 225                         if is_as_ref_able {
 226                             return Some((span.shrink_to_lo(),
 227                                          "consider using `as_ref` instead",
 228                                          "as_ref().".into()));
 229                         }
 230                     }
 231                 }
 232             }
 233         }
 234         None
 235     }
 236
 237     /// This function is used to determine potential "simple" improvements or users' errors and
 238     /// provide them useful help. For example:
 239     ///
 240     /// ```
 241     /// fn some_fn(s: &str) {}
 242     ///
 243     /// let x = "hey!".to_owned();
 244     /// some_fn(x); // error
 245     /// ```
 246     ///
 247     /// No need to find every potential function which could make a coercion to transform a
 248     /// `String` into a `&str` since a `&` would do the trick!
 249     ///
 250     /// In addition of this check, it also checks between references mutability state. If the
 251     /// expected is mutable but the provided isn't, maybe we could just say "Hey, try with
 252     /// `&mut`!".
 253     pub fn check_ref(&self,
 254                  expr: &hir::Expr,
 255                  checked_ty: Ty<'tcx>,
 256                  expected: Ty<'tcx>)
 257                  -> Option<(Span, &'static str, String)> {
 258         let cm = self.sess().source_map();
 259         // Use the callsite's span if this is a macro call. #41858
 260         let sp = cm.call_span_if_macro(expr.span);
 261         if !cm.span_to_filename(sp).is_real() {
 262             return None;
 263         }
 264
 265         match (&expected.sty, &checked_ty.sty) {
 266             (&ty::Ref(_, exp, _), &ty::Ref(_, check, _)) => match (&exp.sty, &check.sty) {
 267                 (&ty::Str, &ty::Array(arr, _)) |
 268                 (&ty::Str, &ty::Slice(arr)) if arr == self.tcx.types.u8 => {
 269                     if let hir::ExprKind::Lit(_) = expr.node {
 270                         if let Ok(src) = cm.span_to_snippet(sp) {
 271                             if src.starts_with("b\"") {
 272                                 return Some((sp,
 273                                              "consider removing the leading `b`",
 274                                              src[1..].to_string()));
 275                             }
 276                         }
 277                     }
 278                 },
 279                 (&ty::Array(arr, _), &ty::Str) |
 280                 (&ty::Slice(arr), &ty::Str) if arr == self.tcx.types.u8 => {
 281                     if let hir::ExprKind::Lit(_) = expr.node {
 282                         if let Ok(src) = cm.span_to_snippet(sp) {
 283                             if src.starts_with("\"") {
 284                                 return Some((sp,
 285                                              "consider adding a leading `b`",
 286                                              format!("b{}", src)));
 287                             }
 288                         }
 289                     }
 290                 }
 291                 _ => {}
 292             },
 293             (&ty::Ref(_, _, mutability), _) => {
 294                 // Check if it can work when put into a ref. For example:
 295                 //
 296                 // ```
 297                 // fn bar(x: &mut i32) {}
 298                 //
 299                 // let x = 0u32;
 300                 // bar(&x); // error, expected &mut
 301                 // ```
 302                 let ref_ty = match mutability {
 303                     hir::Mutability::MutMutable => self.tcx.mk_mut_ref(
 304                                                        self.tcx.mk_region(ty::ReStatic),
 305                                                        checked_ty),
 306                     hir::Mutability::MutImmutable => self.tcx.mk_imm_ref(
 307                                                        self.tcx.mk_region(ty::ReStatic),
 308                                                        checked_ty),
 309                 };
 310                 if self.can_coerce(ref_ty, expected) {
 311                     if let Ok(src) = cm.span_to_snippet(sp) {
 312                         let sugg_expr = match expr.node { // parenthesize if needed (Issue #46756)
 313                             hir::ExprKind::Cast(_, _) |
 314                             hir::ExprKind::Binary(_, _, _) => format!("({})", src),
 315                             _ => src,
 316                         };
 317                         if let Some(sugg) = self.can_use_as_ref(expr) {
 318                             return Some(sugg);
 319                         }
 320                         return Some(match mutability {
 321                             hir::Mutability::MutMutable => {
 322                                 (sp, "consider mutably borrowing here", format!("&mut {}",
 323                                                                                 sugg_expr))
 324                             }
 325                             hir::Mutability::MutImmutable => {
 326                                 (sp, "consider borrowing here", format!("&{}", sugg_expr))
 327                             }
 328                         });
 329                     }
 330                 }
 331             }
 332             (_, &ty::Ref(_, checked, _)) => {
 333                 // We have `&T`, check if what was expected was `T`. If so,
 334                 // we may want to suggest adding a `*`, or removing
 335                 // a `&`.
 336                 //
 337                 // (But, also check check the `expn_info()` to see if this is
 338                 // a macro; if so, it's hard to extract the text and make a good
 339                 // suggestion, so don't bother.)
 340                 if self.infcx.can_sub(self.param_env, checked, &expected).is_ok() &&
 341                    sp.ctxt().outer().expn_info().is_none() {
 342                     match expr.node {
 343                         // Maybe remove `&`?
 344                         hir::ExprKind::AddrOf(_, ref expr) => {
 345                             if !cm.span_to_filename(expr.span).is_real() {
 346                                 return None;
 347                             }
 348                             if let Ok(code) = cm.span_to_snippet(expr.span) {
 349                                 return Some((sp, "consider removing the borrow", code));
 350                             }
 351                         }
 352
 353                         // Maybe add `*`? Only if `T: Copy`.
 354                         _ => {
 355                             if !self.infcx.type_moves_by_default(self.param_env,
 356                                                                 checked,
 357                                                                 sp) {
 358                                 // do not suggest if the span comes from a macro (#52783)
 359                                 if let (Ok(code),
 360                                         true) = (cm.span_to_snippet(sp), sp == expr.span) {
 361                                     return Some((
 362                                         sp,
 363                                         "consider dereferencing the borrow",
 364                                         format!("*{}", code),
 365                                     ));
 366                                 }
 367                             }
 368                         }
 369                     }
 370                 }
 371             }
 372             _ => {}
 373         }
 374         None
 375     }
 376
 377     pub fn check_for_cast(&self,
 378                       err: &mut DiagnosticBuilder<'tcx>,
 379                       expr: &hir::Expr,
 380                       checked_ty: Ty<'tcx>,
 381                       expected_ty: Ty<'tcx>)
 382                       -> bool {
 383         let parent_id = self.tcx.hir.get_parent_node(expr.id);
 384         if let Some(parent) = self.tcx.hir.find(parent_id) {
 385             // Shouldn't suggest `.into()` on `const`s.
 386             if let Node::Item(Item { node: ItemKind::Const(_, _), .. }) = parent {
 387                 // FIXME(estebank): modify once we decide to suggest `as` casts
 388                 return false;
 389             }
 390         };
 391
 392         let will_truncate = "will truncate the source value";
 393         let depending_on_isize = "will truncate or zero-extend depending on the bit width of \
 394                                   `isize`";
 395         let depending_on_usize = "will truncate or zero-extend depending on the bit width of \
 396                                   `usize`";
 397         let will_sign_extend = "will sign-extend the source value";
 398         let will_zero_extend = "will zero-extend the source value";
 399
 400         // If casting this expression to a given numeric type would be appropriate in case of a type
 401         // mismatch.
 402         //
 403         // We want to minimize the amount of casting operations that are suggested, as it can be a
 404         // lossy operation with potentially bad side effects, so we only suggest when encountering
 405         // an expression that indicates that the original type couldn't be directly changed.
 406         //
 407         // For now, don't suggest casting with `as`.
 408         let can_cast = false;
 409
 410         let needs_paren = expr.precedence().order() < (PREC_POSTFIX as i8);
 411
 412         if let Ok(src) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
 413             let msg = format!("you can cast an `{}` to `{}`", checked_ty, expected_ty);
 414             let cast_suggestion = format!("{}{}{} as {}",
 415                                           if needs_paren { "(" } else { "" },
 416                                           src,
 417                                           if needs_paren { ")" } else { "" },
 418                                           expected_ty);
 419             let into_suggestion = format!(
 420                 "{}{}{}.into()",
 421                 if needs_paren { "(" } else { "" },
 422                 src,
 423                 if needs_paren { ")" } else { "" },
 424             );
 425             let literal_is_ty_suffixed = |expr: &hir::Expr| {
 426                 if let hir::ExprKind::Lit(lit) = &expr.node {
 427                     lit.node.is_suffixed()
 428                 } else {
 429                     false
 430                 }
 431             };
 432
 433             let into_sugg = into_suggestion.clone();
 434             let suggest_to_change_suffix_or_into = |err: &mut DiagnosticBuilder,
 435                                                     note: Option<&str>| {
 436                 let suggest_msg = if literal_is_ty_suffixed(expr) {
 437                     format!(
 438                         "change the type of the numeric literal from `{}` to `{}`",
 439                         checked_ty,
 440                         expected_ty,
 441                     )
 442                 } else {
 443                     match note {
 444                         Some(note) => format!("{}, which {}", msg, note),
 445                         _ => format!("{} in a lossless way", msg),
 446                     }
 447                 };
 448
 449                 let suffix_suggestion = format!(
 450                     "{}{}{}{}",
 451                     if needs_paren { "(" } else { "" },
 452                     src.trim_right_matches(&checked_ty.to_string()),
 453                     expected_ty,
 454                     if needs_paren { ")" } else { "" },
 455                 );
 456
 457                 err.span_suggestion_with_applicability(
 458                     expr.span,
 459                     &suggest_msg,
 460                     if literal_is_ty_suffixed(expr) {
 461                         suffix_suggestion
 462                     } else {
 463                         into_sugg
 464                     },
 465                     Applicability::MachineApplicable,
 466                 );
 467             };
 468
 469             match (&expected_ty.sty, &checked_ty.sty) {
 470                 (&ty::Int(ref exp), &ty::Int(ref found)) => {
 471                     match (found.bit_width(), exp.bit_width()) {
 472                         (Some(found), Some(exp)) if found > exp => {
 473                             if can_cast {
 474                                 err.span_suggestion_with_applicability(
 475                                     expr.span,
 476                                     &format!("{}, which {}", msg, will_truncate),
 477                                     cast_suggestion,
 478                                     Applicability::MaybeIncorrect // lossy conversion
 479                                 );
 480                             }
 481                         }
 482                         (None, _) | (_, None) => {
 483                             if can_cast {
 484                                 err.span_suggestion_with_applicability(
 485                                     expr.span,
 486                                     &format!("{}, which {}", msg, depending_on_isize),
 487                                     cast_suggestion,
 488                                     Applicability::MaybeIncorrect // lossy conversion
 489                                 );
 490                             }
 491                         }
 492                         _ => {
 493                             suggest_to_change_suffix_or_into(
 494                                 err,
 495                                 Some(will_sign_extend),
 496                             );
 497                         }
 498                     }
 499                     true
 500                 }
 501                 (&ty::Uint(ref exp), &ty::Uint(ref found)) => {
 502                     match (found.bit_width(), exp.bit_width()) {
 503                         (Some(found), Some(exp)) if found > exp => {
 504                             if can_cast {
 505                                 err.span_suggestion_with_applicability(
 506                                     expr.span,
 507                                     &format!("{}, which {}", msg, will_truncate),
 508                                     cast_suggestion,
 509                                     Applicability::MaybeIncorrect  // lossy conversion
 510                                 );
 511                             }
 512                         }
 513                         (None, _) | (_, None) => {
 514                             if can_cast {
 515                                 err.span_suggestion_with_applicability(
 516                                     expr.span,
 517                                     &format!("{}, which {}", msg, depending_on_usize),
 518                                     cast_suggestion,
 519                                     Applicability::MaybeIncorrect  // lossy conversion
 520                                 );
 521                             }
 522                         }
 523                         _ => {
 524                            suggest_to_change_suffix_or_into(
 525                                err,
 526                                Some(will_zero_extend),
 527                            );
 528                         }
 529                     }
 530                     true
 531                 }
 532                 (&ty::Int(ref exp), &ty::Uint(ref found)) => {
 533                     if can_cast {
 534                         match (found.bit_width(), exp.bit_width()) {
 535                             (Some(found), Some(exp)) if found > exp - 1 => {
 536                                 err.span_suggestion_with_applicability(
 537                                     expr.span,
 538                                     &format!("{}, which {}", msg, will_truncate),
 539                                     cast_suggestion,
 540                                     Applicability::MaybeIncorrect  // lossy conversion
 541                                 );
 542                             }
 543                             (None, None) => {
 544                                 err.span_suggestion_with_applicability(
 545                                     expr.span,
 546                                     &format!("{}, which {}", msg, will_truncate),
 547                                     cast_suggestion,
 548                                     Applicability::MaybeIncorrect  // lossy conversion
 549                                 );
 550                             }
 551                             (None, _) => {
 552                                 err.span_suggestion_with_applicability(
 553                                     expr.span,
 554                                     &format!("{}, which {}", msg, depending_on_isize),
 555                                     cast_suggestion,
 556                                     Applicability::MaybeIncorrect  // lossy conversion
 557                                 );
 558                             }
 559                             (_, None) => {
 560                                 err.span_suggestion_with_applicability(
 561                                     expr.span,
 562                                     &format!("{}, which {}", msg, depending_on_usize),
 563                                     cast_suggestion,
 564                                     Applicability::MaybeIncorrect  // lossy conversion
 565                                 );
 566                             }
 567                             _ => {
 568                                 err.span_suggestion_with_applicability(
 569                                     expr.span,
 570                                     &format!("{}, which {}", msg, will_zero_extend),
 571                                     cast_suggestion,
 572                                     Applicability::MachineApplicable
 573                                 );
 574                             }
 575                         }
 576                     }
 577                     true
 578                 }
 579                 (&ty::Uint(ref exp), &ty::Int(ref found)) => {
 580                     if can_cast {
 581                         match (found.bit_width(), exp.bit_width()) {
 582                             (Some(found), Some(exp)) if found - 1 > exp => {
 583                                 err.span_suggestion_with_applicability(
 584                                     expr.span,
 585                                     &format!("{}, which {}", msg, will_truncate),
 586                                     cast_suggestion,
 587                                     Applicability::MaybeIncorrect  // lossy conversion
 588                                 );
 589                             }
 590                             (None, None) => {
 591                                 err.span_suggestion_with_applicability(
 592                                     expr.span,
 593                                     &format!("{}, which {}", msg, will_sign_extend),
 594                                     cast_suggestion,
 595                                     Applicability::MachineApplicable  // lossy conversion
 596                                 );
 597                             }
 598                             (None, _) => {
 599                                 err.span_suggestion_with_applicability(
 600                                     expr.span,
 601                                     &format!("{}, which {}", msg, depending_on_usize),
 602                                     cast_suggestion,
 603                                     Applicability::MaybeIncorrect  // lossy conversion
 604                                 );
 605                             }
 606                             (_, None) => {
 607                                 err.span_suggestion_with_applicability(
 608                                     expr.span,
 609                                     &format!("{}, which {}", msg, depending_on_isize),
 610                                     cast_suggestion,
 611                                     Applicability::MaybeIncorrect  // lossy conversion
 612                                 );
 613                             }
 614                             _ => {
 615                                 err.span_suggestion_with_applicability(
 616                                     expr.span,
 617                                     &format!("{}, which {}", msg, will_sign_extend),
 618                                     cast_suggestion,
 619                                     Applicability::MachineApplicable
 620                                 );
 621                             }
 622                         }
 623                     }
 624                     true
 625                 }
 626                 (&ty::Float(ref exp), &ty::Float(ref found)) => {
 627                     if found.bit_width() < exp.bit_width() {
 628                        suggest_to_change_suffix_or_into(
 629                            err,
 630                            None,
 631                        );
 632                     } else if can_cast {
 633                         err.span_suggestion_with_applicability(
 634                             expr.span,
 635                             &format!("{}, producing the closest possible value", msg),
 636                             cast_suggestion,
 637                             Applicability::MaybeIncorrect  // lossy conversion
 638                         );
 639                     }
 640                     true
 641                 }
 642                 (&ty::Uint(_), &ty::Float(_)) | (&ty::Int(_), &ty::Float(_)) => {
 643                     if can_cast {
 644                         err.span_suggestion_with_applicability(
 645                             expr.span,
 646                             &format!("{}, rounding the float towards zero", msg),
 647                             cast_suggestion,
 648                             Applicability::MaybeIncorrect  // lossy conversion
 649                         );
 650                         err.warn("casting here will cause undefined behavior if the rounded value \
 651                                   cannot be represented by the target integer type, including \
 652                                   `Inf` and `NaN` (this is a bug and will be fixed)");
 653                     }
 654                     true
 655                 }
 656                 (&ty::Float(ref exp), &ty::Uint(ref found)) => {
 657                     // if `found` is `None` (meaning found is `usize`), don't suggest `.into()`
 658                     if exp.bit_width() > found.bit_width().unwrap_or(256) {
 659                         err.span_suggestion_with_applicability(
 660                             expr.span,
 661                             &format!("{}, producing the floating point representation of the \
 662                                       integer",
 663                                      msg),
 664                             into_suggestion,
 665                             Applicability::MachineApplicable
 666                         );
 667                     } else if can_cast {
 668                         err.span_suggestion_with_applicability(expr.span,
 669                             &format!("{}, producing the floating point representation of the \
 670                                       integer, rounded if necessary",
 671                                      msg),
 672                             cast_suggestion,
 673                             Applicability::MaybeIncorrect  // lossy conversion
 674                         );
 675                     }
 676                     true
 677                 }
 678                 (&ty::Float(ref exp), &ty::Int(ref found)) => {
 679                     // if `found` is `None` (meaning found is `isize`), don't suggest `.into()`
 680                     if exp.bit_width() > found.bit_width().unwrap_or(256) {
 681                         err.span_suggestion_with_applicability(
 682                             expr.span,
 683                             &format!("{}, producing the floating point representation of the \
 684                                       integer",
 685                                      msg),
 686                             into_suggestion,
 687                             Applicability::MachineApplicable
 688                         );
 689                     } else if can_cast {
 690                         err.span_suggestion_with_applicability(
 691                             expr.span,
 692                             &format!("{}, producing the floating point representation of the \
 693                                       integer, rounded if necessary",
 694                                      msg),
 695                             cast_suggestion,
 696                             Applicability::MaybeIncorrect  // lossy conversion
 697                         );
 698                     }
 699                     true
 700                 }
 701                 _ => false,
 702             }
 703         } else {
 704             false
 705         }
 706     }
 707 }