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 (Issue #55250) with any variants whose
 115         // sole field is of the found type, suggest such variants. (Issue #42764)
 116         if let ty::Adt(expected_adt, substs) = expected.sty {
 117             if expected_adt.is_enum() {
 118                 let mut 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                     }).peekable();
 131
 132                 if compatible_variants.peek().is_some() {
 133                     let expr_text = print::to_string(print::NO_ANN, |s| s.print_expr(expr));
 134                     let suggestions = compatible_variants
 135                         .map(|v| format!("{}({})", v, expr_text));
 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
 146         self.suggest_ref_or_into(&mut err, expr, expected, expr_ty);
 147
 148         (expected, Some(err))
 149     }
 150
 151     pub fn get_conversion_methods(&self, span: Span, expected: Ty<'tcx>, checked_ty: Ty<'tcx>)
 152                               -> Vec<AssociatedItem> {
 153         let mut methods = self.probe_for_return_type(span,
 154                                                      probe::Mode::MethodCall,
 155                                                      expected,
 156                                                      checked_ty,
 157                                                      ast::DUMMY_NODE_ID);
 158         methods.retain(|m| {
 159             self.has_no_input_arg(m) &&
 160                 self.tcx.get_attrs(m.def_id).iter()
 161                 // This special internal attribute is used to whitelist
 162                 // "identity-like" conversion methods to be suggested here.
 163                 //
 164                 // FIXME (#46459 and #46460): ideally
 165                 // `std::convert::Into::into` and `std::borrow:ToOwned` would
 166                 // also be `#[rustc_conversion_suggestion]`, if not for
 167                 // method-probing false-positives and -negatives (respectively).
 168                 //
 169                 // FIXME? Other potential candidate methods: `as_ref` and
 170                 // `as_mut`?
 171                 .find(|a| a.check_name("rustc_conversion_suggestion")).is_some()
 172         });
 173
 174         methods
 175     }
 176
 177     // This function checks if the method isn't static and takes other arguments than `self`.
 178     fn has_no_input_arg(&self, method: &AssociatedItem) -> bool {
 179         match method.def() {
 180             Def::Method(def_id) => {
 181                 self.tcx.fn_sig(def_id).inputs().skip_binder().len() == 1
 182             }
 183             _ => false,
 184         }
 185     }
 186
 187     /// Identify some cases where `as_ref()` would be appropriate and suggest it.
 188     ///
 189     /// Given the following code:
 190     /// ```
 191     /// struct Foo;
 192     /// fn takes_ref(_: &Foo) {}
 193     /// let ref opt = Some(Foo);
 194     ///
 195     /// opt.map(|arg| takes_ref(arg));
 196     /// ```
 197     /// Suggest using `opt.as_ref().map(|arg| takes_ref(arg));` instead.
 198     ///
 199     /// It only checks for `Option` and `Result` and won't work with
 200     /// ```
 201     /// opt.map(|arg| { takes_ref(arg) });
 202     /// ```
 203     fn can_use_as_ref(&self, expr: &hir::Expr) -> Option<(Span, &'static str, String)> {
 204         if let hir::ExprKind::Path(hir::QPath::Resolved(_, ref path)) = expr.node {
 205             if let hir::def::Def::Local(id) = path.def {
 206                 let parent = self.tcx.hir().get_parent_node(id);
 207                 if let Some(Node::Expr(hir::Expr {
 208                     id,
 209                     node: hir::ExprKind::Closure(_, decl, ..),
 210                     ..
 211                 })) = self.tcx.hir().find(parent) {
 212                     let parent = self.tcx.hir().get_parent_node(*id);
 213                     if let (Some(Node::Expr(hir::Expr {
 214                         node: hir::ExprKind::MethodCall(path, span, expr),
 215                         ..
 216                     })), 1) = (self.tcx.hir().find(parent), decl.inputs.len()) {
 217                         let self_ty = self.tables.borrow().node_id_to_type(expr[0].hir_id);
 218                         let self_ty = format!("{:?}", self_ty);
 219                         let name = path.ident.as_str();
 220                         let is_as_ref_able = (
 221                             self_ty.starts_with("&std::option::Option") ||
 222                             self_ty.starts_with("&std::result::Result") ||
 223                             self_ty.starts_with("std::option::Option") ||
 224                             self_ty.starts_with("std::result::Result")
 225                         ) && (name == "map" || name == "and_then");
 226                         if is_as_ref_able {
 227                             return Some((span.shrink_to_lo(),
 228                                          "consider using `as_ref` instead",
 229                                          "as_ref().".into()));
 230                         }
 231                     }
 232                 }
 233             }
 234         }
 235         None
 236     }
 237
 238     /// This function is used to determine potential "simple" improvements or users' errors and
 239     /// provide them useful help. For example:
 240     ///
 241     /// ```
 242     /// fn some_fn(s: &str) {}
 243     ///
 244     /// let x = "hey!".to_owned();
 245     /// some_fn(x); // error
 246     /// ```
 247     ///
 248     /// No need to find every potential function which could make a coercion to transform a
 249     /// `String` into a `&str` since a `&` would do the trick!
 250     ///
 251     /// In addition of this check, it also checks between references mutability state. If the
 252     /// expected is mutable but the provided isn't, maybe we could just say "Hey, try with
 253     /// `&mut`!".
 254     pub fn check_ref(&self,
 255                  expr: &hir::Expr,
 256                  checked_ty: Ty<'tcx>,
 257                  expected: Ty<'tcx>)
 258                  -> Option<(Span, &'static str, String)> {
 259         let cm = self.sess().source_map();
 260         // Use the callsite's span if this is a macro call. #41858
 261         let sp = cm.call_span_if_macro(expr.span);
 262         if !cm.span_to_filename(sp).is_real() {
 263             return None;
 264         }
 265
 266         match (&expected.sty, &checked_ty.sty) {
 267             (&ty::Ref(_, exp, _), &ty::Ref(_, check, _)) => match (&exp.sty, &check.sty) {
 268                 (&ty::Str, &ty::Array(arr, _)) |
 269                 (&ty::Str, &ty::Slice(arr)) if arr == self.tcx.types.u8 => {
 270                     if let hir::ExprKind::Lit(_) = expr.node {
 271                         if let Ok(src) = cm.span_to_snippet(sp) {
 272                             if src.starts_with("b\"") {
 273                                 return Some((sp,
 274                                              "consider removing the leading `b`",
 275                                              src[1..].to_string()));
 276                             }
 277                         }
 278                     }
 279                 },
 280                 (&ty::Array(arr, _), &ty::Str) |
 281                 (&ty::Slice(arr), &ty::Str) if arr == self.tcx.types.u8 => {
 282                     if let hir::ExprKind::Lit(_) = expr.node {
 283                         if let Ok(src) = cm.span_to_snippet(sp) {
 284                             if src.starts_with("\"") {
 285                                 return Some((sp,
 286                                              "consider adding a leading `b`",
 287                                              format!("b{}", src)));
 288                             }
 289                         }
 290                     }
 291                 }
 292                 _ => {}
 293             },
 294             (&ty::Ref(_, _, mutability), _) => {
 295                 // Check if it can work when put into a ref. For example:
 296                 //
 297                 // ```
 298                 // fn bar(x: &mut i32) {}
 299                 //
 300                 // let x = 0u32;
 301                 // bar(&x); // error, expected &mut
 302                 // ```
 303                 let ref_ty = match mutability {
 304                     hir::Mutability::MutMutable => self.tcx.mk_mut_ref(
 305                                                        self.tcx.mk_region(ty::ReStatic),
 306                                                        checked_ty),
 307                     hir::Mutability::MutImmutable => self.tcx.mk_imm_ref(
 308                                                        self.tcx.mk_region(ty::ReStatic),
 309                                                        checked_ty),
 310                 };
 311                 if self.can_coerce(ref_ty, expected) {
 312                     if let Ok(src) = cm.span_to_snippet(sp) {
 313                         let needs_parens = match expr.node {
 314                             // parenthesize if needed (Issue #46756)
 315                             hir::ExprKind::Cast(_, _) |
 316                             hir::ExprKind::Binary(_, _, _) => true,
 317                             // parenthesize borrows of range literals (Issue #54505)
 318                             _ if self.is_range_literal(expr) => true,
 319                             _ => false,
 320                         };
 321                         let sugg_expr = if needs_parens {
 322                             format!("({})", src)
 323                         } else {
 324                             src
 325                         };
 326
 327                         if let Some(sugg) = self.can_use_as_ref(expr) {
 328                             return Some(sugg);
 329                         }
 330                         return Some(match mutability {
 331                             hir::Mutability::MutMutable => {
 332                                 (sp, "consider mutably borrowing here", format!("&mut {}",
 333                                                                                 sugg_expr))
 334                             }
 335                             hir::Mutability::MutImmutable => {
 336                                 (sp, "consider borrowing here", format!("&{}", sugg_expr))
 337                             }
 338                         });
 339                     }
 340                 }
 341             }
 342             (_, &ty::Ref(_, checked, _)) => {
 343                 // We have `&T`, check if what was expected was `T`. If so,
 344                 // we may want to suggest adding a `*`, or removing
 345                 // a `&`.
 346                 //
 347                 // (But, also check check the `expn_info()` to see if this is
 348                 // a macro; if so, it's hard to extract the text and make a good
 349                 // suggestion, so don't bother.)
 350                 if self.infcx.can_sub(self.param_env, checked, &expected).is_ok() &&
 351                    sp.ctxt().outer().expn_info().is_none() {
 352                     match expr.node {
 353                         // Maybe remove `&`?
 354                         hir::ExprKind::AddrOf(_, ref expr) => {
 355                             if !cm.span_to_filename(expr.span).is_real() {
 356                                 return None;
 357                             }
 358                             if let Ok(code) = cm.span_to_snippet(expr.span) {
 359                                 return Some((sp, "consider removing the borrow", code));
 360                             }
 361                         }
 362
 363                         // Maybe add `*`? Only if `T: Copy`.
 364                         _ => {
 365                             if !self.infcx.type_moves_by_default(self.param_env,
 366                                                                 checked,
 367                                                                 sp) {
 368                                 // do not suggest if the span comes from a macro (#52783)
 369                                 if let (Ok(code),
 370                                         true) = (cm.span_to_snippet(sp), sp == expr.span) {
 371                                     return Some((
 372                                         sp,
 373                                         "consider dereferencing the borrow",
 374                                         format!("*{}", code),
 375                                     ));
 376                                 }
 377                             }
 378                         }
 379                     }
 380                 }
 381             }
 382             _ => {}
 383         }
 384         None
 385     }
 386
 387     /// This function checks if the specified expression is a built-in range literal.
 388     /// (See: `LoweringContext::lower_expr()` in `src/librustc/hir/lowering.rs`).
 389     fn is_range_literal(&self, expr: &hir::Expr) -> bool {
 390         use hir::{Path, QPath, ExprKind, TyKind};
 391
 392         // We support `::std::ops::Range` and `::core::ops::Range` prefixes
 393         let is_range_path = |path: &Path| {
 394             let mut segs = path.segments.iter()
 395                 .map(|seg| seg.ident.as_str());
 396
 397             if let (Some(root), Some(std_core), Some(ops), Some(range), None) =
 398                 (segs.next(), segs.next(), segs.next(), segs.next(), segs.next())
 399             {
 400                 // "{{root}}" is the equivalent of `::` prefix in Path
 401                 root == "{{root}}" && (std_core == "std" || std_core == "core")
 402                     && ops == "ops" && range.starts_with("Range")
 403             } else {
 404                 false
 405             }
 406         };
 407
 408         let span_is_range_literal = |span: &Span| {
 409             // Check whether a span corresponding to a range expression
 410             // is a range literal, rather than an explicit struct or `new()` call.
 411             let source_map = self.tcx.sess.source_map();
 412             let end_point = source_map.end_point(*span);
 413
 414             if let Ok(end_string) = source_map.span_to_snippet(end_point) {
 415                 !(end_string.ends_with("}") || end_string.ends_with(")"))
 416             } else {
 417                 false
 418             }
 419         };
 420
 421         match expr.node {
 422             // All built-in range literals but `..=` and `..` desugar to Structs
 423             ExprKind::Struct(QPath::Resolved(None, ref path), _, _) |
 424             // `..` desugars to its struct path
 425             ExprKind::Path(QPath::Resolved(None, ref path)) => {
 426                 return is_range_path(&path) && span_is_range_literal(&expr.span);
 427             }
 428
 429             // `..=` desugars into `::std::ops::RangeInclusive::new(...)`
 430             ExprKind::Call(ref func, _) => {
 431                 if let ExprKind::Path(QPath::TypeRelative(ref ty, ref segment)) = func.node {
 432                     if let TyKind::Path(QPath::Resolved(None, ref path)) = ty.node {
 433                         let call_to_new = segment.ident.as_str() == "new";
 434
 435                         return is_range_path(&path) && span_is_range_literal(&expr.span)
 436                             && call_to_new;
 437                     }
 438                 }
 439             }
 440
 441             _ => {}
 442         }
 443
 444         false
 445     }
 446
 447     pub fn check_for_cast(&self,
 448                       err: &mut DiagnosticBuilder<'tcx>,
 449                       expr: &hir::Expr,
 450                       checked_ty: Ty<'tcx>,
 451                       expected_ty: Ty<'tcx>)
 452                       -> bool {
 453         let parent_id = self.tcx.hir().get_parent_node(expr.id);
 454         if let Some(parent) = self.tcx.hir().find(parent_id) {
 455             // Shouldn't suggest `.into()` on `const`s.
 456             if let Node::Item(Item { node: ItemKind::Const(_, _), .. }) = parent {
 457                 // FIXME(estebank): modify once we decide to suggest `as` casts
 458                 return false;
 459             }
 460         };
 461
 462         let will_truncate = "will truncate the source value";
 463         let depending_on_isize = "will truncate or zero-extend depending on the bit width of \
 464                                   `isize`";
 465         let depending_on_usize = "will truncate or zero-extend depending on the bit width of \
 466                                   `usize`";
 467         let will_sign_extend = "will sign-extend the source value";
 468         let will_zero_extend = "will zero-extend the source value";
 469
 470         // If casting this expression to a given numeric type would be appropriate in case of a type
 471         // mismatch.
 472         //
 473         // We want to minimize the amount of casting operations that are suggested, as it can be a
 474         // lossy operation with potentially bad side effects, so we only suggest when encountering
 475         // an expression that indicates that the original type couldn't be directly changed.
 476         //
 477         // For now, don't suggest casting with `as`.
 478         let can_cast = false;
 479
 480         let needs_paren = expr.precedence().order() < (PREC_POSTFIX as i8);
 481
 482         if let Ok(src) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
 483             let msg = format!("you can cast an `{}` to `{}`", checked_ty, expected_ty);
 484             let cast_suggestion = format!("{}{}{} as {}",
 485                                           if needs_paren { "(" } else { "" },
 486                                           src,
 487                                           if needs_paren { ")" } else { "" },
 488                                           expected_ty);
 489             let into_suggestion = format!(
 490                 "{}{}{}.into()",
 491                 if needs_paren { "(" } else { "" },
 492                 src,
 493                 if needs_paren { ")" } else { "" },
 494             );
 495             let literal_is_ty_suffixed = |expr: &hir::Expr| {
 496                 if let hir::ExprKind::Lit(lit) = &expr.node {
 497                     lit.node.is_suffixed()
 498                 } else {
 499                     false
 500                 }
 501             };
 502
 503             let into_sugg = into_suggestion.clone();
 504             let suggest_to_change_suffix_or_into = |err: &mut DiagnosticBuilder,
 505                                                     note: Option<&str>| {
 506                 let suggest_msg = if literal_is_ty_suffixed(expr) {
 507                     format!(
 508                         "change the type of the numeric literal from `{}` to `{}`",
 509                         checked_ty,
 510                         expected_ty,
 511                     )
 512                 } else {
 513                     match note {
 514                         Some(note) => format!("{}, which {}", msg, note),
 515                         _ => format!("{} in a lossless way", msg),
 516                     }
 517                 };
 518
 519                 let suffix_suggestion = format!(
 520                     "{}{}{}{}",
 521                     if needs_paren { "(" } else { "" },
 522                     src.trim_right_matches(&checked_ty.to_string()),
 523                     expected_ty,
 524                     if needs_paren { ")" } else { "" },
 525                 );
 526
 527                 err.span_suggestion_with_applicability(
 528                     expr.span,
 529                     &suggest_msg,
 530                     if literal_is_ty_suffixed(expr) {
 531                         suffix_suggestion
 532                     } else {
 533                         into_sugg
 534                     },
 535                     Applicability::MachineApplicable,
 536                 );
 537             };
 538
 539             match (&expected_ty.sty, &checked_ty.sty) {
 540                 (&ty::Int(ref exp), &ty::Int(ref found)) => {
 541                     match (found.bit_width(), exp.bit_width()) {
 542                         (Some(found), Some(exp)) if found > exp => {
 543                             if can_cast {
 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                         }
 552                         (None, _) | (_, None) => {
 553                             if can_cast {
 554                                 err.span_suggestion_with_applicability(
 555                                     expr.span,
 556                                     &format!("{}, which {}", msg, depending_on_isize),
 557                                     cast_suggestion,
 558                                     Applicability::MaybeIncorrect // lossy conversion
 559                                 );
 560                             }
 561                         }
 562                         _ => {
 563                             suggest_to_change_suffix_or_into(
 564                                 err,
 565                                 Some(will_sign_extend),
 566                             );
 567                         }
 568                     }
 569                     true
 570                 }
 571                 (&ty::Uint(ref exp), &ty::Uint(ref found)) => {
 572                     match (found.bit_width(), exp.bit_width()) {
 573                         (Some(found), Some(exp)) if found > exp => {
 574                             if can_cast {
 575                                 err.span_suggestion_with_applicability(
 576                                     expr.span,
 577                                     &format!("{}, which {}", msg, will_truncate),
 578                                     cast_suggestion,
 579                                     Applicability::MaybeIncorrect  // lossy conversion
 580                                 );
 581                             }
 582                         }
 583                         (None, _) | (_, None) => {
 584                             if can_cast {
 585                                 err.span_suggestion_with_applicability(
 586                                     expr.span,
 587                                     &format!("{}, which {}", msg, depending_on_usize),
 588                                     cast_suggestion,
 589                                     Applicability::MaybeIncorrect  // lossy conversion
 590                                 );
 591                             }
 592                         }
 593                         _ => {
 594                            suggest_to_change_suffix_or_into(
 595                                err,
 596                                Some(will_zero_extend),
 597                            );
 598                         }
 599                     }
 600                     true
 601                 }
 602                 (&ty::Int(ref exp), &ty::Uint(ref found)) => {
 603                     if can_cast {
 604                         match (found.bit_width(), exp.bit_width()) {
 605                             (Some(found), Some(exp)) if found > exp - 1 => {
 606                                 err.span_suggestion_with_applicability(
 607                                     expr.span,
 608                                     &format!("{}, which {}", msg, will_truncate),
 609                                     cast_suggestion,
 610                                     Applicability::MaybeIncorrect  // lossy conversion
 611                                 );
 612                             }
 613                             (None, None) => {
 614                                 err.span_suggestion_with_applicability(
 615                                     expr.span,
 616                                     &format!("{}, which {}", msg, will_truncate),
 617                                     cast_suggestion,
 618                                     Applicability::MaybeIncorrect  // lossy conversion
 619                                 );
 620                             }
 621                             (None, _) => {
 622                                 err.span_suggestion_with_applicability(
 623                                     expr.span,
 624                                     &format!("{}, which {}", msg, depending_on_isize),
 625                                     cast_suggestion,
 626                                     Applicability::MaybeIncorrect  // lossy conversion
 627                                 );
 628                             }
 629                             (_, None) => {
 630                                 err.span_suggestion_with_applicability(
 631                                     expr.span,
 632                                     &format!("{}, which {}", msg, depending_on_usize),
 633                                     cast_suggestion,
 634                                     Applicability::MaybeIncorrect  // lossy conversion
 635                                 );
 636                             }
 637                             _ => {
 638                                 err.span_suggestion_with_applicability(
 639                                     expr.span,
 640                                     &format!("{}, which {}", msg, will_zero_extend),
 641                                     cast_suggestion,
 642                                     Applicability::MachineApplicable
 643                                 );
 644                             }
 645                         }
 646                     }
 647                     true
 648                 }
 649                 (&ty::Uint(ref exp), &ty::Int(ref found)) => {
 650                     if can_cast {
 651                         match (found.bit_width(), exp.bit_width()) {
 652                             (Some(found), Some(exp)) if found - 1 > exp => {
 653                                 err.span_suggestion_with_applicability(
 654                                     expr.span,
 655                                     &format!("{}, which {}", msg, will_truncate),
 656                                     cast_suggestion,
 657                                     Applicability::MaybeIncorrect  // lossy conversion
 658                                 );
 659                             }
 660                             (None, None) => {
 661                                 err.span_suggestion_with_applicability(
 662                                     expr.span,
 663                                     &format!("{}, which {}", msg, will_sign_extend),
 664                                     cast_suggestion,
 665                                     Applicability::MachineApplicable  // lossy conversion
 666                                 );
 667                             }
 668                             (None, _) => {
 669                                 err.span_suggestion_with_applicability(
 670                                     expr.span,
 671                                     &format!("{}, which {}", msg, depending_on_usize),
 672                                     cast_suggestion,
 673                                     Applicability::MaybeIncorrect  // lossy conversion
 674                                 );
 675                             }
 676                             (_, None) => {
 677                                 err.span_suggestion_with_applicability(
 678                                     expr.span,
 679                                     &format!("{}, which {}", msg, depending_on_isize),
 680                                     cast_suggestion,
 681                                     Applicability::MaybeIncorrect  // lossy conversion
 682                                 );
 683                             }
 684                             _ => {
 685                                 err.span_suggestion_with_applicability(
 686                                     expr.span,
 687                                     &format!("{}, which {}", msg, will_sign_extend),
 688                                     cast_suggestion,
 689                                     Applicability::MachineApplicable
 690                                 );
 691                             }
 692                         }
 693                     }
 694                     true
 695                 }
 696                 (&ty::Float(ref exp), &ty::Float(ref found)) => {
 697                     if found.bit_width() < exp.bit_width() {
 698                        suggest_to_change_suffix_or_into(
 699                            err,
 700                            None,
 701                        );
 702                     } else if can_cast {
 703                         err.span_suggestion_with_applicability(
 704                             expr.span,
 705                             &format!("{}, producing the closest possible value", msg),
 706                             cast_suggestion,
 707                             Applicability::MaybeIncorrect  // lossy conversion
 708                         );
 709                     }
 710                     true
 711                 }
 712                 (&ty::Uint(_), &ty::Float(_)) | (&ty::Int(_), &ty::Float(_)) => {
 713                     if can_cast {
 714                         err.span_suggestion_with_applicability(
 715                             expr.span,
 716                             &format!("{}, rounding the float towards zero", msg),
 717                             cast_suggestion,
 718                             Applicability::MaybeIncorrect  // lossy conversion
 719                         );
 720                         err.warn("casting here will cause undefined behavior if the rounded value \
 721                                   cannot be represented by the target integer type, including \
 722                                   `Inf` and `NaN` (this is a bug and will be fixed)");
 723                     }
 724                     true
 725                 }
 726                 (&ty::Float(ref exp), &ty::Uint(ref found)) => {
 727                     // if `found` is `None` (meaning found is `usize`), don't suggest `.into()`
 728                     if exp.bit_width() > found.bit_width().unwrap_or(256) {
 729                         err.span_suggestion_with_applicability(
 730                             expr.span,
 731                             &format!("{}, producing the floating point representation of the \
 732                                       integer",
 733                                      msg),
 734                             into_suggestion,
 735                             Applicability::MachineApplicable
 736                         );
 737                     } else if can_cast {
 738                         err.span_suggestion_with_applicability(expr.span,
 739                             &format!("{}, producing the floating point representation of the \
 740                                       integer, rounded if necessary",
 741                                      msg),
 742                             cast_suggestion,
 743                             Applicability::MaybeIncorrect  // lossy conversion
 744                         );
 745                     }
 746                     true
 747                 }
 748                 (&ty::Float(ref exp), &ty::Int(ref found)) => {
 749                     // if `found` is `None` (meaning found is `isize`), don't suggest `.into()`
 750                     if exp.bit_width() > found.bit_width().unwrap_or(256) {
 751                         err.span_suggestion_with_applicability(
 752                             expr.span,
 753                             &format!("{}, producing the floating point representation of the \
 754                                       integer",
 755                                      msg),
 756                             into_suggestion,
 757                             Applicability::MachineApplicable
 758                         );
 759                     } else if can_cast {
 760                         err.span_suggestion_with_applicability(
 761                             expr.span,
 762                             &format!("{}, producing the floating point representation of the \
 763                                       integer, rounded if necessary",
 764                                      msg),
 765                             cast_suggestion,
 766                             Applicability::MaybeIncorrect  // lossy conversion
 767                         );
 768                     }
 769                     true
 770                 }
 771                 _ => false,
 772             }
 773         } else {
 774             false
 775         }
 776     }
 777 }