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 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.map(|v|
 135                     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 needs_parens = match expr.node {
 313                             // parenthesize if needed (Issue #46756)
 314                             hir::ExprKind::Cast(_, _) |
 315                             hir::ExprKind::Binary(_, _, _) => true,
 316                             // parenthesize borrows of range literals (Issue #54505)
 317                             _ if self.is_range_literal(expr) => true,
 318                             _ => false,
 319                         };
 320                         let sugg_expr = if needs_parens {
 321                             format!("({})", src)
 322                         } else {
 323                             src
 324                         };
 325
 326                         if let Some(sugg) = self.can_use_as_ref(expr) {
 327                             return Some(sugg);
 328                         }
 329                         return Some(match mutability {
 330                             hir::Mutability::MutMutable => {
 331                                 (sp, "consider mutably borrowing here", format!("&mut {}",
 332                                                                                 sugg_expr))
 333                             }
 334                             hir::Mutability::MutImmutable => {
 335                                 (sp, "consider borrowing here", format!("&{}", sugg_expr))
 336                             }
 337                         });
 338                     }
 339                 }
 340             }
 341             (_, &ty::Ref(_, checked, _)) => {
 342                 // We have `&T`, check if what was expected was `T`. If so,
 343                 // we may want to suggest adding a `*`, or removing
 344                 // a `&`.
 345                 //
 346                 // (But, also check check the `expn_info()` to see if this is
 347                 // a macro; if so, it's hard to extract the text and make a good
 348                 // suggestion, so don't bother.)
 349                 if self.infcx.can_sub(self.param_env, checked, &expected).is_ok() &&
 350                    sp.ctxt().outer().expn_info().is_none() {
 351                     match expr.node {
 352                         // Maybe remove `&`?
 353                         hir::ExprKind::AddrOf(_, ref expr) => {
 354                             if !cm.span_to_filename(expr.span).is_real() {
 355                                 return None;
 356                             }
 357                             if let Ok(code) = cm.span_to_snippet(expr.span) {
 358                                 return Some((sp, "consider removing the borrow", code));
 359                             }
 360                         }
 361
 362                         // Maybe add `*`? Only if `T: Copy`.
 363                         _ => {
 364                             if !self.infcx.type_moves_by_default(self.param_env,
 365                                                                 checked,
 366                                                                 sp) {
 367                                 // do not suggest if the span comes from a macro (#52783)
 368                                 if let (Ok(code),
 369                                         true) = (cm.span_to_snippet(sp), sp == expr.span) {
 370                                     return Some((
 371                                         sp,
 372                                         "consider dereferencing the borrow",
 373                                         format!("*{}", code),
 374                                     ));
 375                                 }
 376                             }
 377                         }
 378                     }
 379                 }
 380             }
 381             _ => {}
 382         }
 383         None
 384     }
 385
 386     /// This function checks if the specified expression is a built-in range literal.
 387     /// (See: `LoweringContext::lower_expr()` in `src/librustc/hir/lowering.rs`).
 388     fn is_range_literal(&self, expr: &hir::Expr) -> bool {
 389         use hir::{Path, QPath, ExprKind, TyKind};
 390
 391         // We support `::std::ops::Range` and `::core::ops::Range` prefixes
 392         let is_range_path = |path: &Path| {
 393             let mut segs = path.segments.iter()
 394                 .map(|seg| seg.ident.as_str());
 395
 396             if let (Some(root), Some(std_core), Some(ops), Some(range), None) =
 397                 (segs.next(), segs.next(), segs.next(), segs.next(), segs.next())
 398             {
 399                 // "{{root}}" is the equivalent of `::` prefix in Path
 400                 root == "{{root}}" && (std_core == "std" || std_core == "core")
 401                     && ops == "ops" && range.starts_with("Range")
 402             } else {
 403                 false
 404             }
 405         };
 406
 407         let span_is_range_literal = |span: &Span| {
 408             // Check whether a span corresponding to a range expression
 409             // is a range literal, rather than an explicit struct or `new()` call.
 410             let source_map = self.tcx.sess.source_map();
 411             let end_point = source_map.end_point(*span);
 412
 413             if let Ok(end_string) = source_map.span_to_snippet(end_point) {
 414                 !(end_string.ends_with("}") || end_string.ends_with(")"))
 415             } else {
 416                 false
 417             }
 418         };
 419
 420         match expr.node {
 421             // All built-in range literals but `..=` and `..` desugar to Structs
 422             ExprKind::Struct(QPath::Resolved(None, ref path), _, _) |
 423             // `..` desugars to its struct path
 424             ExprKind::Path(QPath::Resolved(None, ref path)) => {
 425                 return is_range_path(&path) && span_is_range_literal(&expr.span);
 426             }
 427
 428             // `..=` desugars into `::std::ops::RangeInclusive::new(...)`
 429             ExprKind::Call(ref func, _) => {
 430                 if let ExprKind::Path(QPath::TypeRelative(ref ty, ref segment)) = func.node {
 431                     if let TyKind::Path(QPath::Resolved(None, ref path)) = ty.node {
 432                         let call_to_new = segment.ident.as_str() == "new";
 433
 434                         return is_range_path(&path) && span_is_range_literal(&expr.span)
 435                             && call_to_new;
 436                     }
 437                 }
 438             }
 439
 440             _ => {}
 441         }
 442
 443         false
 444     }
 445
 446     pub fn check_for_cast(&self,
 447                       err: &mut DiagnosticBuilder<'tcx>,
 448                       expr: &hir::Expr,
 449                       checked_ty: Ty<'tcx>,
 450                       expected_ty: Ty<'tcx>)
 451                       -> bool {
 452         let parent_id = self.tcx.hir.get_parent_node(expr.id);
 453         if let Some(parent) = self.tcx.hir.find(parent_id) {
 454             // Shouldn't suggest `.into()` on `const`s.
 455             if let Node::Item(Item { node: ItemKind::Const(_, _), .. }) = parent {
 456                 // FIXME(estebank): modify once we decide to suggest `as` casts
 457                 return false;
 458             }
 459         };
 460
 461         let will_truncate = "will truncate the source value";
 462         let depending_on_isize = "will truncate or zero-extend depending on the bit width of \
 463                                   `isize`";
 464         let depending_on_usize = "will truncate or zero-extend depending on the bit width of \
 465                                   `usize`";
 466         let will_sign_extend = "will sign-extend the source value";
 467         let will_zero_extend = "will zero-extend the source value";
 468
 469         // If casting this expression to a given numeric type would be appropriate in case of a type
 470         // mismatch.
 471         //
 472         // We want to minimize the amount of casting operations that are suggested, as it can be a
 473         // lossy operation with potentially bad side effects, so we only suggest when encountering
 474         // an expression that indicates that the original type couldn't be directly changed.
 475         //
 476         // For now, don't suggest casting with `as`.
 477         let can_cast = false;
 478
 479         let needs_paren = expr.precedence().order() < (PREC_POSTFIX as i8);
 480
 481         if let Ok(src) = self.tcx.sess.source_map().span_to_snippet(expr.span) {
 482             let msg = format!("you can cast an `{}` to `{}`", checked_ty, expected_ty);
 483             let cast_suggestion = format!("{}{}{} as {}",
 484                                           if needs_paren { "(" } else { "" },
 485                                           src,
 486                                           if needs_paren { ")" } else { "" },
 487                                           expected_ty);
 488             let into_suggestion = format!(
 489                 "{}{}{}.into()",
 490                 if needs_paren { "(" } else { "" },
 491                 src,
 492                 if needs_paren { ")" } else { "" },
 493             );
 494             let literal_is_ty_suffixed = |expr: &hir::Expr| {
 495                 if let hir::ExprKind::Lit(lit) = &expr.node {
 496                     lit.node.is_suffixed()
 497                 } else {
 498                     false
 499                 }
 500             };
 501
 502             let into_sugg = into_suggestion.clone();
 503             let suggest_to_change_suffix_or_into = |err: &mut DiagnosticBuilder,
 504                                                     note: Option<&str>| {
 505                 let suggest_msg = if literal_is_ty_suffixed(expr) {
 506                     format!(
 507                         "change the type of the numeric literal from `{}` to `{}`",
 508                         checked_ty,
 509                         expected_ty,
 510                     )
 511                 } else {
 512                     match note {
 513                         Some(note) => format!("{}, which {}", msg, note),
 514                         _ => format!("{} in a lossless way", msg),
 515                     }
 516                 };
 517
 518                 let suffix_suggestion = format!(
 519                     "{}{}{}{}",
 520                     if needs_paren { "(" } else { "" },
 521                     src.trim_right_matches(&checked_ty.to_string()),
 522                     expected_ty,
 523                     if needs_paren { ")" } else { "" },
 524                 );
 525
 526                 err.span_suggestion_with_applicability(
 527                     expr.span,
 528                     &suggest_msg,
 529                     if literal_is_ty_suffixed(expr) {
 530                         suffix_suggestion
 531                     } else {
 532                         into_sugg
 533                     },
 534                     Applicability::MachineApplicable,
 535                 );
 536             };
 537
 538             match (&expected_ty.sty, &checked_ty.sty) {
 539                 (&ty::Int(ref exp), &ty::Int(ref found)) => {
 540                     match (found.bit_width(), exp.bit_width()) {
 541                         (Some(found), Some(exp)) if found > exp => {
 542                             if can_cast {
 543                                 err.span_suggestion_with_applicability(
 544                                     expr.span,
 545                                     &format!("{}, which {}", msg, will_truncate),
 546                                     cast_suggestion,
 547                                     Applicability::MaybeIncorrect // lossy conversion
 548                                 );
 549                             }
 550                         }
 551                         (None, _) | (_, None) => {
 552                             if can_cast {
 553                                 err.span_suggestion_with_applicability(
 554                                     expr.span,
 555                                     &format!("{}, which {}", msg, depending_on_isize),
 556                                     cast_suggestion,
 557                                     Applicability::MaybeIncorrect // lossy conversion
 558                                 );
 559                             }
 560                         }
 561                         _ => {
 562                             suggest_to_change_suffix_or_into(
 563                                 err,
 564                                 Some(will_sign_extend),
 565                             );
 566                         }
 567                     }
 568                     true
 569                 }
 570                 (&ty::Uint(ref exp), &ty::Uint(ref found)) => {
 571                     match (found.bit_width(), exp.bit_width()) {
 572                         (Some(found), Some(exp)) if found > exp => {
 573                             if can_cast {
 574                                 err.span_suggestion_with_applicability(
 575                                     expr.span,
 576                                     &format!("{}, which {}", msg, will_truncate),
 577                                     cast_suggestion,
 578                                     Applicability::MaybeIncorrect  // lossy conversion
 579                                 );
 580                             }
 581                         }
 582                         (None, _) | (_, None) => {
 583                             if can_cast {
 584                                 err.span_suggestion_with_applicability(
 585                                     expr.span,
 586                                     &format!("{}, which {}", msg, depending_on_usize),
 587                                     cast_suggestion,
 588                                     Applicability::MaybeIncorrect  // lossy conversion
 589                                 );
 590                             }
 591                         }
 592                         _ => {
 593                            suggest_to_change_suffix_or_into(
 594                                err,
 595                                Some(will_zero_extend),
 596                            );
 597                         }
 598                     }
 599                     true
 600                 }
 601                 (&ty::Int(ref exp), &ty::Uint(ref found)) => {
 602                     if can_cast {
 603                         match (found.bit_width(), exp.bit_width()) {
 604                             (Some(found), Some(exp)) if found > exp - 1 => {
 605                                 err.span_suggestion_with_applicability(
 606                                     expr.span,
 607                                     &format!("{}, which {}", msg, will_truncate),
 608                                     cast_suggestion,
 609                                     Applicability::MaybeIncorrect  // lossy conversion
 610                                 );
 611                             }
 612                             (None, None) => {
 613                                 err.span_suggestion_with_applicability(
 614                                     expr.span,
 615                                     &format!("{}, which {}", msg, will_truncate),
 616                                     cast_suggestion,
 617                                     Applicability::MaybeIncorrect  // lossy conversion
 618                                 );
 619                             }
 620                             (None, _) => {
 621                                 err.span_suggestion_with_applicability(
 622                                     expr.span,
 623                                     &format!("{}, which {}", msg, depending_on_isize),
 624                                     cast_suggestion,
 625                                     Applicability::MaybeIncorrect  // lossy conversion
 626                                 );
 627                             }
 628                             (_, None) => {
 629                                 err.span_suggestion_with_applicability(
 630                                     expr.span,
 631                                     &format!("{}, which {}", msg, depending_on_usize),
 632                                     cast_suggestion,
 633                                     Applicability::MaybeIncorrect  // lossy conversion
 634                                 );
 635                             }
 636                             _ => {
 637                                 err.span_suggestion_with_applicability(
 638                                     expr.span,
 639                                     &format!("{}, which {}", msg, will_zero_extend),
 640                                     cast_suggestion,
 641                                     Applicability::MachineApplicable
 642                                 );
 643                             }
 644                         }
 645                     }
 646                     true
 647                 }
 648                 (&ty::Uint(ref exp), &ty::Int(ref found)) => {
 649                     if can_cast {
 650                         match (found.bit_width(), exp.bit_width()) {
 651                             (Some(found), Some(exp)) if found - 1 > exp => {
 652                                 err.span_suggestion_with_applicability(
 653                                     expr.span,
 654                                     &format!("{}, which {}", msg, will_truncate),
 655                                     cast_suggestion,
 656                                     Applicability::MaybeIncorrect  // lossy conversion
 657                                 );
 658                             }
 659                             (None, None) => {
 660                                 err.span_suggestion_with_applicability(
 661                                     expr.span,
 662                                     &format!("{}, which {}", msg, will_sign_extend),
 663                                     cast_suggestion,
 664                                     Applicability::MachineApplicable  // lossy conversion
 665                                 );
 666                             }
 667                             (None, _) => {
 668                                 err.span_suggestion_with_applicability(
 669                                     expr.span,
 670                                     &format!("{}, which {}", msg, depending_on_usize),
 671                                     cast_suggestion,
 672                                     Applicability::MaybeIncorrect  // lossy conversion
 673                                 );
 674                             }
 675                             (_, None) => {
 676                                 err.span_suggestion_with_applicability(
 677                                     expr.span,
 678                                     &format!("{}, which {}", msg, depending_on_isize),
 679                                     cast_suggestion,
 680                                     Applicability::MaybeIncorrect  // lossy conversion
 681                                 );
 682                             }
 683                             _ => {
 684                                 err.span_suggestion_with_applicability(
 685                                     expr.span,
 686                                     &format!("{}, which {}", msg, will_sign_extend),
 687                                     cast_suggestion,
 688                                     Applicability::MachineApplicable
 689                                 );
 690                             }
 691                         }
 692                     }
 693                     true
 694                 }
 695                 (&ty::Float(ref exp), &ty::Float(ref found)) => {
 696                     if found.bit_width() < exp.bit_width() {
 697                        suggest_to_change_suffix_or_into(
 698                            err,
 699                            None,
 700                        );
 701                     } else if can_cast {
 702                         err.span_suggestion_with_applicability(
 703                             expr.span,
 704                             &format!("{}, producing the closest possible value", msg),
 705                             cast_suggestion,
 706                             Applicability::MaybeIncorrect  // lossy conversion
 707                         );
 708                     }
 709                     true
 710                 }
 711                 (&ty::Uint(_), &ty::Float(_)) | (&ty::Int(_), &ty::Float(_)) => {
 712                     if can_cast {
 713                         err.span_suggestion_with_applicability(
 714                             expr.span,
 715                             &format!("{}, rounding the float towards zero", msg),
 716                             cast_suggestion,
 717                             Applicability::MaybeIncorrect  // lossy conversion
 718                         );
 719                         err.warn("casting here will cause undefined behavior if the rounded value \
 720                                   cannot be represented by the target integer type, including \
 721                                   `Inf` and `NaN` (this is a bug and will be fixed)");
 722                     }
 723                     true
 724                 }
 725                 (&ty::Float(ref exp), &ty::Uint(ref found)) => {
 726                     // if `found` is `None` (meaning found is `usize`), don't suggest `.into()`
 727                     if exp.bit_width() > found.bit_width().unwrap_or(256) {
 728                         err.span_suggestion_with_applicability(
 729                             expr.span,
 730                             &format!("{}, producing the floating point representation of the \
 731                                       integer",
 732                                      msg),
 733                             into_suggestion,
 734                             Applicability::MachineApplicable
 735                         );
 736                     } else if can_cast {
 737                         err.span_suggestion_with_applicability(expr.span,
 738                             &format!("{}, producing the floating point representation of the \
 739                                       integer, rounded if necessary",
 740                                      msg),
 741                             cast_suggestion,
 742                             Applicability::MaybeIncorrect  // lossy conversion
 743                         );
 744                     }
 745                     true
 746                 }
 747                 (&ty::Float(ref exp), &ty::Int(ref found)) => {
 748                     // if `found` is `None` (meaning found is `isize`), don't suggest `.into()`
 749                     if exp.bit_width() > found.bit_width().unwrap_or(256) {
 750                         err.span_suggestion_with_applicability(
 751                             expr.span,
 752                             &format!("{}, producing the floating point representation of the \
 753                                       integer",
 754                                      msg),
 755                             into_suggestion,
 756                             Applicability::MachineApplicable
 757                         );
 758                     } else if can_cast {
 759                         err.span_suggestion_with_applicability(
 760                             expr.span,
 761                             &format!("{}, producing the floating point representation of the \
 762                                       integer, rounded if necessary",
 763                                      msg),
 764                             cast_suggestion,
 765                             Applicability::MaybeIncorrect  // lossy conversion
 766                         );
 767                     }
 768                     true
 769                 }
 770                 _ => false,
 771             }
 772         } else {
 773             false
 774         }
 775     }
 776 }