1 // Copyright 2014 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.
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.
11 #![allow(non_snake_case)]
13 register_long_diagnostics! {
16 A pattern used to match against an enum variant must provide a sub-pattern for
17 each field of the enum variant. This error indicates that a pattern attempted to
18 extract an incorrect number of fields from a variant.
27 Here the `Apple` variant has two fields, and should be matched against like so:
36 Matching with the wrong number of fields has no sensible interpretation:
46 Check how many fields the enum was declared with and ensure that your pattern
51 This error indicates that a pattern attempted to extract the fields of an enum
52 variant with no fields. Here's a tiny example of this error:
55 // This enum has two variants.
57 // This variant has no fields.
59 // This variant has one field.
63 // Assuming x is a Number we can pattern match on its contents.
70 The pattern match `Zero(inside)` is incorrect because the `Zero` variant
71 contains no fields, yet the `inside` name attempts to bind the first field of
76 Each field of a struct can only be bound once in a pattern. Each occurrence of a
77 field name binds the value of that field, so to fix this error you will have to
78 remove or alter the duplicate uses of the field name. Perhaps you misspelt
83 This error indicates that a struct pattern attempted to extract a non-existant
84 field from a struct. Struct fields are identified by the name used before the
85 colon `:` so struct patterns should resemble the declaration of the struct type
95 let thing = Thing { x: 1, y: 2 };
97 Thing { x: xfield, y: yfield } => ...
101 If you are using shorthand field patterns but want to refer to the struct field
102 by a different name, you should rename it explicitly.
107 Thing { x, z } => ...
112 Thing { x, y: z } => ...
118 This error indicates that a pattern for a struct fails to specify a sub-pattern
119 for every one of the struct's fields. Ensure that each field from the struct's
120 definition is mentioned in the pattern, or use `..` to ignore unwanted fields.
130 let d = Dog { name: "Rusty".to_string(), age: 8 };
132 // This is incorrect.
134 Dog { age: x } => ...
137 // This is correct (explicit).
139 Dog { name: n, age: x } => ...
142 // This is also correct (ignore unused fields).
144 Dog { age: x, .. } => ...
150 In a match expression, only numbers and characters can be matched against a
151 range. This is because the compiler checks that the range is non-empty at
152 compile-time, and is unable to evaluate arbitrary comparison functions. If you
153 want to capture values of an orderable type between two end-points, you can use
157 // The ordering relation for strings can't be evaluated at compile time,
158 // so this doesn't work:
160 "hello" ... "world" => ...
164 // This is a more general version, using a guard:
166 s if s >= "hello" && s <= "world" => ...
173 This error indicates that a pointer to a trait type cannot be implicitly
174 dereferenced by a pattern. Every trait defines a type, but because the
175 size of trait implementors isn't fixed, this type has no compile-time size.
176 Therefore, all accesses to trait types must be through pointers. If you
177 encounter this error you should try to avoid dereferencing the pointer.
180 let trait_obj: &SomeTrait = ...;
182 // This tries to implicitly dereference to create an unsized local variable.
183 let &invalid = trait_obj;
185 // You can call methods without binding to the value being pointed at.
186 trait_obj.method_one();
187 trait_obj.method_two();
190 You can read more about trait objects in the Trait Object section of the
193 http://doc.rust-lang.org/reference.html#trait-objects
197 The compiler doesn't know what method to call because more than one method
198 has the same prototype. Example:
211 impl Trait1 for Test { fn foo() {} }
212 impl Trait2 for Test { fn foo() {} }
215 Test::foo() // error, which foo() to call?
219 To avoid this error, you have to keep only one of them and remove the others.
220 So let's take our example and fix it:
229 impl Trait1 for Test { fn foo() {} }
232 Test::foo() // and now that's good!
236 However, a better solution would be using fully explicit naming of type and
250 impl Trait1 for Test { fn foo() {} }
251 impl Trait2 for Test { fn foo() {} }
254 <Test as Trait1>::foo()
260 You tried to give a type parameter where it wasn't needed. Bad example:
272 x.method::<i32>(); // Error: Test::method doesn't need type parameter!
276 To fix this error, just remove the type parameter:
288 x.method(); // OK, we're good!
294 This error occurrs when you pass too many or not enough type parameters to
301 fn method<T>(&self, v: &[T]) -> usize {
310 x.method::<i32, i32>(v); // error: only one type parameter is expected!
314 To fix it, just specify a correct number of type parameters:
320 fn method<T>(&self, v: &[T]) -> usize {
329 x.method::<i32>(v); // OK, we're good!
333 Please note on the last example that we could have called `method` like this:
341 It is not allowed to manually call destructors in Rust. It is also not
342 necessary to do this since `drop` is called automatically whenever a value goes
345 Here's an example of this error:
359 let mut x = Foo { x: -7 };
360 x.drop(); // error: explicit use of destructor method
366 You can't use type parameters on foreign items. Example of erroneous code:
369 extern { fn some_func<T>(x: T); }
372 To fix this, replace the type parameter with the specializations that you
376 extern { fn some_func_i32(x: i32); }
377 extern { fn some_func_i64(x: i64); }
382 Rust only supports variadic parameters for interoperability with C code in its
383 FFI. As such, variadic parameters can only be used with functions which are
384 using the C ABI. Examples of erroneous code:
387 extern "rust-call" { fn foo(x: u8, ...); }
389 fn foo(x: u8, ...) {}
392 To fix such code, put them in an extern "C" block:
395 extern "C" fn foo (x: u8, ...);
404 When trying to make some type implement a trait `Foo`, you must, at minimum,
405 provide implementations for all of `Foo`'s required methods (meaning the
406 methods that do not have default implementations), as well as any required
407 trait items like associated types or constants.
411 This error indicates that an attempted implementation of a trait method
412 has the wrong number of type parameters.
414 For example, the trait below has a method `foo` with a type parameter `T`,
415 but the implementation of `foo` for the type `Bar` is missing this parameter:
419 fn foo<T: Default>(x: T) -> Self;
424 // error: method `foo` has 0 type parameters but its trait declaration has 1
427 fn foo(x: bool) -> Self { Bar }
433 This error indicates that an attempted implementation of a trait method
434 has the wrong number of function parameters.
436 For example, the trait below has a method `foo` with two function parameters
437 (`&self` and `u8`), but the implementation of `foo` for the type `Bar` omits
442 fn foo(&self, x: u8) -> bool;
447 // error: method `foo` has 1 parameter but the declaration in trait `Foo::foo`
450 fn foo(&self) -> bool { true }
456 The parameters of any trait method must match between a trait implementation
457 and the trait definition.
459 Here are a couple examples of this error:
470 // error, expected u16, found i16
473 // error, values differ in mutability
474 fn bar(&mut self) { }
480 It is not allowed to cast to a bool. If you are trying to cast a numeric type
481 to a bool, you can compare it with zero instead:
487 let x_is_nonzero = x != 0;
489 // Not allowed, won't compile
490 let x_is_nonzero = x as bool;
495 During a method call, a value is automatically dereferenced as many times as
496 needed to make the value's type match the method's receiver. The catch is that
497 the compiler will only attempt to dereference a number of times up to the
498 recursion limit (which can be set via the `recursion_limit` attribute).
500 For a somewhat artificial example:
503 #![recursion_limit="2"]
515 // error, reached the recursion limit while auto-dereferencing &&Foo
520 One fix may be to increase the recursion limit. Note that it is possible to
521 create an infinite recursion of dereferencing, in which case the only fix is to
522 somehow break the recursion.
526 When invoking closures or other implementations of the function traits `Fn`,
527 `FnMut` or `FnOnce` using call notation, the number of parameters passed to the
528 function must match its definition.
530 An example using a closure:
534 let a = f(); // invalid, too few parameters
535 let b = f(4); // this works!
536 let c = f(2, 3); // invalid, too many parameters
539 A generic function must be treated similarly:
542 fn foo<F: Fn()>(f: F) {
543 f(); // this is valid, but f(3) would not work
549 The built-in function traits are generic over a tuple of the function arguments.
550 If one uses angle-bracket notation (`Fn<(T,), Output=U>`) instead of parentheses
551 (`Fn(T) -> U`) to denote the function trait, the type parameter should be a
552 tuple. Otherwise function call notation cannot be used and the trait will not be
553 implemented by closures.
555 The most likely source of this error is using angle-bracket notation without
556 wrapping the function argument type into a tuple, for example:
559 fn foo<F: Fn<i32>>(f: F) -> F::Output { f(3) }
562 It can be fixed by adjusting the trait bound like this:
565 fn foo<F: Fn<(i32,)>>(f: F) -> F::Output { f(3) }
568 Note that `(T,)` always denotes the type of a 1-tuple containing an element of
569 type `T`. The comma is necessary for syntactic disambiguation.
573 External C functions are allowed to be variadic. However, a variadic function
574 takes a minimum number of arguments. For example, consider C's variadic `printf`
579 use libc::{ c_char, c_int };
582 fn printf(_: *const c_char, ...) -> c_int;
586 Using this declaration, it must be called with at least one argument, so
587 simply calling `printf()` is invalid. But the following uses are allowed:
591 use std::ffi::CString;
593 printf(CString::new("test\n").unwrap().as_ptr());
594 printf(CString::new("number = %d\n").unwrap().as_ptr(), 3);
595 printf(CString::new("%d, %d\n").unwrap().as_ptr(), 10, 5);
601 The number of arguments passed to a function must match the number of arguments
602 specified in the function signature.
604 For example, a function like
607 fn f(a: u16, b: &str) {}
610 must always be called with exactly two arguments, e.g. `f(2, "test")`.
612 Note, that Rust does not have a notion of optional function arguments or
613 variadic functions (except for its C-FFI).
617 This error indicates that during an attempt to build a struct or struct-like
618 enum variant, one of the fields was specified more than once. Each field should
619 be specified exactly one time.
623 This error indicates that during an attempt to build a struct or struct-like
624 enum variant, one of the fields was not provided. Each field should be
625 specified exactly once.
629 Box placement expressions (like C++'s "placement new") do not yet support any
630 place expression except the exchange heap (i.e. `std::boxed::HEAP`).
631 Furthermore, the syntax is changing to use `in` instead of `box`. See [RFC 470]
632 and [RFC 809] for more details.
634 [RFC 470]: https://github.com/rust-lang/rfcs/pull/470
635 [RFC 809]: https://github.com/rust-lang/rfcs/pull/809
639 The left-hand side of a compound assignment expression must be an lvalue
640 expression. An lvalue expression represents a memory location and includes
641 item paths (ie, namespaced variables), dereferences, indexing expressions,
642 and field references.
644 Let's start with some bad examples:
646 use std::collections::LinkedList;
648 // Bad: assignment to non-lvalue expression
649 LinkedList::new() += 1;
653 fn some_func(i: &mut i32) {
654 i += 12; // Error : '+=' operation cannot be applied on a reference !
657 And now some good examples:
665 fn some_func(i: &mut i32) {
673 The compiler found a function whose body contains a `return;` statement but
674 whose return type is not `()`. An example of this is:
683 Since `return;` is just like `return ();`, there is a mismatch between the
684 function's return type and the value being returned.
688 The left-hand side of an assignment operator must be an lvalue expression. An
689 lvalue expression represents a memory location and can be a variable (with
690 optional namespacing), a dereference, an indexing expression or a field
693 More details can be found here:
694 https://doc.rust-lang.org/reference.html#lvalues,-rvalues-and-temporaries
696 Now, we can go further. Here are some bad examples:
702 const SOME_CONST : i32 = 12;
704 fn some_other_func() {}
707 SOME_CONST = 14; // error : a constant value cannot be changed!
708 1 = 3; // error : 1 isn't a valid lvalue!
709 some_other_func() = 4; // error : we can't assign value to a function!
710 SomeStruct.x = 12; // error : SomeStruct a structure name but it is used
715 And now let's give good examples:
722 let mut s = SomeStruct {x: 0, y: 0};
724 s.x = 3; // that's good !
728 fn some_func(x: &mut i32) {
729 *x = 12; // that's good !
735 You tried to use a structure initialization with a non-structure type.
736 Example of erroneous code:
739 enum Foo { FirstValue };
741 let u = Foo::FirstValue { value: 0i32 }; // error: Foo::FirstValue
742 // isn't a structure!
743 // or even simpler, if the structure wasn't defined at all:
744 let u = RandomName { random_field: 0i32 }; // error: RandomName
745 // isn't a structure!
748 To fix this, please check:
749 * Did you spell it right?
750 * Did you accidentaly used an enum as a struct?
751 * Did you accidentaly make an enum when you intended to use a struct?
753 Here is the previous code with all missing information:
765 let u = Foo::FirstValue(Inner { value: 0i32 });
767 let t = Inner { value: 0i32 };
773 When defining a recursive struct or enum, any use of the type being defined
774 from inside the definition must occur behind a pointer (like `Box` or `&`).
775 This is because structs and enums must have a well-defined size, and without
776 the pointer the size of the type would need to be unbounded.
778 Consider the following erroneous definition of a type for a list of bytes:
781 // error, invalid recursive struct type
784 tail: Option<ListNode>,
788 This type cannot have a well-defined size, because it needs to be arbitrarily
789 large (since we would be able to nest `ListNode`s to any depth). Specifically,
792 size of `ListNode` = 1 byte for `head`
793 + 1 byte for the discriminant of the `Option`
797 One way to fix this is by wrapping `ListNode` in a `Box`, like so:
802 tail: Option<Box<ListNode>>,
806 This works because `Box` is a pointer, so its size is well-known.
810 You cannot define a struct (or enum) `Foo` that requires an instance of `Foo`
811 in order to make a new `Foo` value. This is because there would be no way a
812 first instance of `Foo` could be made to initialize another instance!
814 Here's an example of a struct that has this problem:
817 struct Foo { x: Box<Foo> } // error
820 One fix is to use `Option`, like so:
823 struct Foo { x: Option<Box<Foo>> }
826 Now it's possible to create at least one instance of `Foo`: `Foo { x: None }`.
830 Enum discriminants are used to differentiate enum variants stored in memory.
831 This error indicates that the same value was used for two or more variants,
832 making them impossible to tell apart.
850 Note that variants without a manually specified discriminant are numbered from
851 top to bottom starting from 0, so clashes can occur with seemingly unrelated
861 Here `X` will have already been assigned the discriminant 0 by the time `Y` is
862 encountered, so a conflict occurs.
866 The default type for enum discriminants is `isize`, but it can be adjusted by
867 adding the `repr` attribute to the enum declaration. This error indicates that
868 an integer literal given as a discriminant is not a member of the discriminant
879 Here, 1024 lies outside the valid range for `u8`, so the discriminant for `A` is
880 invalid. You may want to change representation types to fix this, or else change
881 invalid discriminant values so that they fit within the existing type.
883 Note also that without a representation manually defined, the compiler will
884 optimize by using the smallest integer type possible.
888 At present, it's not possible to define a custom representation for an enum with
889 a single variant. As a workaround you can add a `Dummy` variant.
891 See: https://github.com/rust-lang/rust/issues/10292
895 It is impossible to define an integer type to be used to represent zero-variant
896 enum values because there are no zero-variant enum values. There is no way to
897 construct an instance of the following type using only safe code:
905 Too many type parameters were supplied for a function. For example:
911 foo::<f64, bool>(); // error, expected 1 parameter, found 2 parameters
915 The number of supplied parameters much exactly match the number of defined type
920 You gave too many lifetime parameters. Erroneous code example:
926 f::<'static>() // error: too many lifetime parameters provided
930 Please check you give the right number of lifetime parameters. Example:
940 It's also important to note that the Rust compiler can generally
941 determine the lifetime by itself. Example:
949 // it can be written like this
950 fn get_value<'a>(&'a self) -> &'a str { &self.value }
951 // but the compiler works fine with this too:
952 fn without_lifetime(&self) -> &str { &self.value }
956 let f = Foo { value: "hello".to_owned() };
958 println!("{}", f.get_value());
959 println!("{}", f.without_lifetime());
965 Not enough type parameters were supplied for a function. For example:
971 foo::<f64>(); // error, expected 2 parameters, found 1 parameter
975 Note that if a function takes multiple type parameters but you want the compiler
976 to infer some of them, you can use type placeholders:
979 fn foo<T, U>(x: T) {}
983 foo::<f64>(x); // error, expected 2 parameters, found 1 parameter
984 foo::<_, f64>(x); // same as `foo::<bool, f64>(x)`
990 You gave an unnecessary type parameter in a type alias. Erroneous code
994 type Foo<T> = u32; // error: type parameter `T` is unused
996 type Foo<A,B> = Box<A>; // error: type parameter `B` is unused
999 Please check you didn't write too many type parameters. Example:
1002 type Foo = u32; // ok!
1003 type Foo<A> = Box<A>; // ok!
1008 You tried to declare an undefined atomic operation function.
1009 Erroneous code example:
1012 #![feature(intrinsics)]
1014 extern "rust-intrinsic" {
1015 fn atomic_foo(); // error: unrecognized atomic operation
1020 Please check you didn't make a mistake in the function's name. All intrinsic
1021 functions are defined in librustc_trans/trans/intrinsic.rs and in
1022 libcore/intrinsics.rs in the Rust source code. Example:
1025 #![feature(intrinsics)]
1027 extern "rust-intrinsic" {
1028 fn atomic_fence(); // ok!
1034 You declared an unknown intrinsic function. Erroneous code example:
1037 #![feature(intrinsics)]
1039 extern "rust-intrinsic" {
1040 fn foo(); // error: unrecognized intrinsic function: `foo`
1050 Please check you didn't make a mistake in the function's name. All intrinsic
1051 functions are defined in librustc_trans/trans/intrinsic.rs and in
1052 libcore/intrinsics.rs in the Rust source code. Example:
1055 #![feature(intrinsics)]
1057 extern "rust-intrinsic" {
1058 fn atomic_fence(); // ok!
1070 You gave an invalid number of type parameters to an intrinsic function.
1071 Erroneous code example:
1074 #![feature(intrinsics)]
1076 extern "rust-intrinsic" {
1077 fn size_of<T, U>() -> usize; // error: intrinsic has wrong number
1078 // of type parameters
1082 Please check that you provided the right number of lifetime parameters
1083 and verify with the function declaration in the Rust source code.
1087 #![feature(intrinsics)]
1089 extern "rust-intrinsic" {
1090 fn size_of<T>() -> usize; // ok!
1096 You hit this error because the compiler the compiler lacks information
1097 to determine a type for this expression. Erroneous code example:
1101 let x = |_| {}; // error: cannot determine a type for this expression
1105 You have two possibilities to solve this situation:
1106 * Give an explicit definition of the expression
1107 * Infer the expression
1113 let x = |_ : u32| {}; // ok!
1122 This error indicates that a lifetime is missing from a type. If it is an error
1123 inside a function signature, the problem may be with failing to adhere to the
1124 lifetime elision rules (see below).
1126 Here are some simple examples of where you'll run into this error:
1129 struct Foo { x: &bool } // error
1130 struct Foo<'a> { x: &'a bool } // correct
1132 enum Bar { A(u8), B(&bool), } // error
1133 enum Bar<'a> { A(u8), B(&'a bool), } // correct
1135 type MyStr = &str; // error
1136 type MyStr<'a> = &'a str; //correct
1140 Lifetime elision is a special, limited kind of inference for lifetimes in
1141 function signatures which allows you to leave out lifetimes in certain cases.
1142 For more background on lifetime elision see [the book][book-le].
1144 The lifetime elision rules require that any function signature with an elided
1145 output lifetime must either have
1147 - exactly one input lifetime
1148 - or, multiple input lifetimes, but the function must also be a method with a
1149 `&self` or `&mut self` receiver
1151 In the first case, the output lifetime is inferred to be the same as the unique
1152 input lifetime. In the second case, the lifetime is instead inferred to be the
1153 same as the lifetime on `&self` or `&mut self`.
1155 Here are some examples of elision errors:
1158 // error, no input lifetimes
1159 fn foo() -> &str { ... }
1161 // error, `x` and `y` have distinct lifetimes inferred
1162 fn bar(x: &str, y: &str) -> &str { ... }
1164 // error, `y`'s lifetime is inferred to be distinct from `x`'s
1165 fn baz<'a>(x: &'a str, y: &str) -> &str { ... }
1168 [book-le]: http://doc.rust-lang.org/nightly/book/lifetimes.html#lifetime-elision
1172 This error means that an incorrect number of lifetime parameters were provided
1173 for a type (like a struct or enum) or trait.
1175 Some basic examples include:
1178 struct Foo<'a>(&'a str);
1179 enum Bar { A, B, C }
1182 foo: Foo, // error: expected 1, found 0
1183 bar: Bar<'a>, // error: expected 0, found 1
1187 Here's an example that is currently an error, but may work in a future version
1191 struct Foo<'a>(&'a str);
1194 impl Quux for Foo { } // error: expected 1, found 0
1197 Lifetime elision in implementation headers was part of the lifetime elision
1198 RFC. It is, however, [currently unimplemented][iss15872].
1200 [iss15872]: https://github.com/rust-lang/rust/issues/15872
1204 You can only define an inherent implementation for a type in the same crate
1205 where the type was defined. For example, an `impl` block as below is not allowed
1206 since `Vec` is defined in the standard library:
1209 impl Vec<u8> { ... } // error
1212 To fix this problem, you can do either of these things:
1214 - define a trait that has the desired associated functions/types/constants and
1215 implement the trait for the type in question
1216 - define a new type wrapping the type and define an implementation on the new
1219 Note that using the `type` keyword does not work here because `type` only
1220 introduces a type alias:
1223 type Bytes = Vec<u8>;
1225 impl Bytes { ... } // error, same as above
1230 This error indicates a violation of one of Rust's orphan rules for trait
1231 implementations. The rule prohibits any implementation of a foreign trait (a
1232 trait defined in another crate) where
1234 - the type that is implementing the trait is foreign
1235 - all of the parameters being passed to the trait (if there are any) are also
1238 Here's one example of this error:
1241 impl Drop for u32 {}
1244 To avoid this kind of error, ensure that at least one local type is referenced
1248 pub struct Foo; // you define your type in your crate
1250 impl Drop for Foo { // and you can implement the trait on it!
1251 // code of trait implementation here
1254 impl From<Foo> for i32 { // or you use a type from your crate as
1256 fn from(i: Foo) -> i32 {
1262 Alternatively, define a trait locally and implement that instead:
1266 fn get(&self) -> usize;
1270 fn get(&self) -> usize { 0 }
1274 For information on the design of the orphan rules, see [RFC 1023].
1276 [RFC 1023]: https://github.com/rust-lang/rfcs/pull/1023
1280 There are conflicting trait implementations for the same type.
1281 Example of erroneous code:
1285 fn get(&self) -> usize;
1288 impl<T> MyTrait for T {
1289 fn get(&self) -> usize { 0 }
1296 impl MyTrait for Foo { // error: conflicting implementations for trait
1298 fn get(&self) -> usize { self.value }
1302 When looking for the implementation for the trait, the compiler finds
1303 both the `impl<T> MyTrait for T` where T is all types and the `impl
1304 MyTrait for Foo`. Since a trait cannot be implemented multiple times,
1305 this is an error. So, when you write:
1308 impl<T> MyTrait for T {
1309 fn get(&self) -> usize { 0 }
1313 This makes the trait implemented on all types in the scope. So if you
1314 try to implement it on another one after that, the implementations will
1319 fn get(&self) -> usize;
1322 impl<T> MyTrait for T {
1323 fn get(&self) -> usize { 0 }
1331 f.get(); // the trait is implemented so we can use it
1337 An attempt was made to implement Drop on a trait, which is not allowed: only
1338 structs and enums can implement Drop. An example causing this error:
1343 impl Drop for MyTrait {
1344 fn drop(&mut self) {}
1348 A workaround for this problem is to wrap the trait up in a struct, and implement
1349 Drop on that. An example is shown below:
1353 struct MyWrapper<T: MyTrait> { foo: T }
1355 impl <T: MyTrait> Drop for MyWrapper<T> {
1356 fn drop(&mut self) {}
1361 Alternatively, wrapping trait objects requires something like the following:
1366 //or Box<MyTrait>, if you wanted an owned trait object
1367 struct MyWrapper<'a> { foo: &'a MyTrait }
1369 impl <'a> Drop for MyWrapper<'a> {
1370 fn drop(&mut self) {}
1376 In order to be consistent with Rust's lack of global type inference, type
1377 placeholders are disallowed by design in item signatures.
1379 Examples of this error include:
1382 fn foo() -> _ { 5 } // error, explicitly write out the return type instead
1384 static BAR: _ = "test"; // error, explicitly write out the type instead
1389 You declared two fields of a struct with the same name. Erroneous code
1395 field1: i32 // error: field is already declared
1399 Please verify that the field names have been correctly spelled. Example:
1410 Type parameter defaults can only use parameters that occur before them.
1411 Erroneous code example:
1414 pub struct Foo<T=U, U=()> {
1418 // error: type parameters with a default cannot use forward declared
1422 Since type parameters are evaluated in-order, you may be able to fix this issue
1426 pub struct Foo<U=(), T=U> {
1432 Please also verify that this wasn't because of a name-clash and rename the type
1437 You declared a pattern as an argument in a foreign function declaration.
1438 Erroneous code example:
1442 fn foo((a, b): (u32, u32)); // error: patterns aren't allowed in foreign
1443 // function declarations
1447 Please replace the pattern argument with a regular one. Example:
1456 fn foo(s: SomeStruct); // ok!
1460 fn foo(a: (u32, u32)); // ok!
1466 It is not possible to define `main` with type parameters, or even with function
1467 parameters. When `main` is present, it must take no arguments and return `()`.
1471 It is not possible to declare type parameters on a function that has the `start`
1472 attribute. Such a function must have the following type signature:
1475 fn(isize, *const *const u8) -> isize
1480 You tried to use a trait as a struct constructor. Erroneous code example:
1483 trait TraitNotAStruct {}
1485 TraitNotAStruct{ value: 0 }; // error: use of trait `TraitNotAStruct` as a
1486 // struct constructor
1489 Please verify you used the correct type name or please implement the trait
1490 on a struct and use this struct constructor. Example:
1493 trait TraitNotAStruct {}
1499 Foo{ value: 0 }; // ok!
1504 This error means that the compiler found a return expression in a function
1505 marked as diverging. A function diverges if it has `!` in the place of the
1506 return type in its signature. For example:
1509 fn foo() -> ! { return; } // error
1512 For a function that diverges, every control path in the function must never
1513 return, for example with a `loop` that never breaks or a call to another
1514 diverging function (such as `panic!()`).
1518 This error means that an attempt was made to specify the type of a variable with
1519 a combination of a concrete type and a trait. Consider the following example:
1522 fn foo(bar: i32+std::fmt::Display) {}
1525 The code is trying to specify that we want to receive a signed 32-bit integer
1526 which also implements `Display`. This doesn't make sense: when we pass `i32`, a
1527 concrete type, it implicitly includes all of the traits that it implements.
1528 This includes `Display`, `Debug`, `Clone`, and a host of others.
1530 If `i32` implements the trait we desire, there's no need to specify the trait
1531 separately. If it does not, then we need to `impl` the trait for `i32` before
1532 passing it into `foo`. Either way, a fixed definition for `foo` will look like
1539 To learn more about traits, take a look at the Book:
1541 https://doc.rust-lang.org/book/traits.html
1545 In types, the `+` type operator has low precedence, so it is often necessary
1554 w: &'a Foo + Copy, // error, use &'a (Foo + Copy)
1555 x: &'a Foo + 'a, // error, use &'a (Foo + 'a)
1556 y: &'a mut Foo + 'a, // error, use &'a mut (Foo + 'a)
1557 z: fn() -> Foo + 'a, // error, use fn() -> (Foo + 'a)
1561 More details can be found in [RFC 438].
1563 [RFC 438]: https://github.com/rust-lang/rfcs/pull/438
1567 Explicitly implementing both Drop and Copy for a type is currently disallowed.
1568 This feature can make some sense in theory, but the current implementation is
1569 incorrect and can lead to memory unsafety (see [issue #20126][iss20126]), so
1570 it has been disabled for now.
1572 [iss20126]: https://github.com/rust-lang/rust/issues/20126
1576 An associated function for a trait was defined to be static, but an
1577 implementation of the trait declared the same function to be a method (i.e. to
1578 take a `self` parameter).
1580 Here's an example of this error:
1590 // error, method `foo` has a `&self` declaration in the impl, but not in
1597 An associated function for a trait was defined to be a method (i.e. to take a
1598 `self` parameter), but an implementation of the trait declared the same function
1601 Here's an example of this error:
1611 // error, method `foo` has a `&self` declaration in the trait, but not in
1619 Trait objects need to have all associated types specified. Erroneous code
1627 type Foo = Trait; // error: the value of the associated type `Bar` (from
1628 // the trait `Trait`) must be specified
1631 Please verify you specified all associated types of the trait and that you
1632 used the right trait. Example:
1639 type Foo = Trait<Bar=i32>; // ok!
1644 Negative impls are only allowed for traits with default impls. For more
1645 information see the [opt-in builtin traits RFC](https://github.com/rust-lang/
1646 rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
1650 Your method's lifetime parameters do not match the trait declaration.
1651 Erroneous code example:
1655 fn bar<'a,'b:'a>(x: &'a str, y: &'b str);
1660 impl Trait for Foo {
1661 fn bar<'a,'b>(x: &'a str, y: &'b str) {
1662 // error: lifetime parameters or bounds on method `bar`
1663 // do not match the trait declaration
1668 The lifetime constraint `'b` for bar() implementation does not match the
1669 trait declaration. Ensure lifetime declarations match exactly in both trait
1670 declaration and implementation. Example:
1674 fn t<'a,'b:'a>(x: &'a str, y: &'b str);
1679 impl Trait for Foo {
1680 fn t<'a,'b:'a>(x: &'a str, y: &'b str) { // ok!
1687 Inherent implementations (one that do not implement a trait but provide
1688 methods associated with a type) are always safe because they are not
1689 implementing an unsafe trait. Removing the `unsafe` keyword from the inherent
1690 implementation will resolve this error.
1695 // this will cause this error
1697 // converting it to this will fix it
1704 A negative implementation is one that excludes a type from implementing a
1705 particular trait. Not being able to use a trait is always a safe operation,
1706 so negative implementations are always safe and never need to be marked as
1712 // unsafe is unnecessary
1713 unsafe impl !Clone for Foo { }
1714 // this will compile
1715 impl !Clone for Foo { }
1721 Safe traits should not have unsafe implementations, therefore marking an
1722 implementation for a safe trait unsafe will cause a compiler error. Removing the
1723 unsafe marker on the trait noted in the error will resolve this problem.
1730 // this won't compile because Bar is safe
1731 unsafe impl Bar for Foo { }
1732 // this will compile
1733 impl Bar for Foo { }
1739 Unsafe traits must have unsafe implementations. This error occurs when an
1740 implementation for an unsafe trait isn't marked as unsafe. This may be resolved
1741 by marking the unsafe implementation as unsafe.
1746 unsafe trait Bar { }
1748 // this won't compile because Bar is unsafe and impl isn't unsafe
1749 impl Bar for Foo { }
1750 // this will compile
1751 unsafe impl Bar for Foo { }
1757 It is an error to define two associated items (like methods, associated types,
1758 associated functions, etc.) with the same identifier.
1766 fn bar(&self) -> bool { self.0 > 5 }
1767 fn bar() {} // error: duplicate associated function
1772 fn baz(&self) -> bool;
1778 fn baz(&self) -> bool { true }
1780 // error: duplicate method
1781 fn baz(&self) -> bool { self.0 > 5 }
1783 // error: duplicate associated type
1790 Inherent associated types were part of [RFC 195] but are not yet implemented.
1791 See [the tracking issue][iss8995] for the status of this implementation.
1793 [RFC 195]: https://github.com/rust-lang/rfcs/pull/195
1794 [iss8995]: https://github.com/rust-lang/rust/issues/8995
1798 An attempt to implement the `Copy` trait for a struct failed because one of the
1799 fields does not implement `Copy`. To fix this, you must implement `Copy` for the
1800 mentioned field. Note that this may not be possible, as in the example of
1807 impl Copy for Foo { }
1810 This fails because `Vec<T>` does not implement `Copy` for any `T`.
1812 Here's another example that will fail:
1821 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
1822 differs from the behavior for `&T`, which is always `Copy`).
1826 An attempt to implement the `Copy` trait for an enum failed because one of the
1827 variants does not implement `Copy`. To fix this, you must implement `Copy` for
1828 the mentioned variant. Note that this may not be possible, as in the example of
1836 impl Copy for Foo { }
1839 This fails because `Vec<T>` does not implement `Copy` for any `T`.
1841 Here's another example that will fail:
1851 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
1852 differs from the behavior for `&T`, which is always `Copy`).
1856 You can only implement `Copy` for a struct or enum. Both of the following
1857 examples will fail, because neither `i32` (primitive type) nor `&'static Bar`
1858 (reference to `Bar`) is a struct or enum:
1862 impl Copy for Foo { } // error
1864 #[derive(Copy, Clone)]
1866 impl Copy for &'static Bar { } // error
1871 You declared an unused type parameter when implementing a trait on an object.
1872 Erroneous code example:
1876 fn get(&self) -> usize;
1881 impl<T> MyTrait for Foo {
1882 fn get(&self) -> usize {
1888 Please check your object definition and remove unused type
1889 parameter(s). Example:
1893 fn get(&self) -> usize;
1898 impl MyTrait for Foo {
1899 fn get(&self) -> usize {
1907 This error indicates a violation of one of Rust's orphan rules for trait
1908 implementations. The rule concerns the use of type parameters in an
1909 implementation of a foreign trait (a trait defined in another crate), and
1910 states that type parameters must be "covered" by a local type. To understand
1911 what this means, it is perhaps easiest to consider a few examples.
1913 If `ForeignTrait` is a trait defined in some external crate `foo`, then the
1914 following trait `impl` is an error:
1918 use foo::ForeignTrait;
1920 impl<T> ForeignTrait for T { ... } // error
1923 To work around this, it can be covered with a local type, `MyType`:
1926 struct MyType<T>(T);
1927 impl<T> ForeignTrait for MyType<T> { ... } // Ok
1930 For another example of an error, suppose there's another trait defined in `foo`
1931 named `ForeignTrait2` that takes two type parameters. Then this `impl` results
1932 in the same rule violation:
1936 impl<T> ForeignTrait2<T, MyType<T>> for MyType2 { ... } // error
1939 The reason for this is that there are two appearances of type parameter `T` in
1940 the `impl` header, both as parameters for `ForeignTrait2`. The first appearance
1941 is uncovered, and so runs afoul of the orphan rule.
1943 Consider one more example:
1946 impl<T> ForeignTrait2<MyType<T>, T> for MyType2 { ... } // Ok
1949 This only differs from the previous `impl` in that the parameters `T` and
1950 `MyType<T>` for `ForeignTrait2` have been swapped. This example does *not*
1951 violate the orphan rule; it is permitted.
1953 To see why that last example was allowed, you need to understand the general
1954 rule. Unfortunately this rule is a bit tricky to state. Consider an `impl`:
1957 impl<P1, ..., Pm> ForeignTrait<T1, ..., Tn> for T0 { ... }
1960 where `P1, ..., Pm` are the type parameters of the `impl` and `T0, ..., Tn`
1961 are types. One of the types `T0, ..., Tn` must be a local type (this is another
1962 orphan rule, see the explanation for E0117). Let `i` be the smallest integer
1963 such that `Ti` is a local type. Then no type parameter can appear in any of the
1966 For information on the design of the orphan rules, see [RFC 1023].
1968 [RFC 1023]: https://github.com/rust-lang/rfcs/pull/1023
1972 You used an intrinsic function which doesn't correspond to its
1973 definition. Erroneous code example:
1976 #![feature(intrinsics)]
1978 extern "rust-intrinsic" {
1979 fn size_of<T>(); // error: intrinsic has wrong type
1983 Please check the function definition. Example:
1986 #![feature(intrinsics)]
1988 extern "rust-intrinsic" {
1989 fn size_of<T>() -> usize;
1995 You used an associated type which isn't defined in the trait.
1996 Erroneous code example:
2003 type Foo = Trait<F=i32>; // error: associated type `F` not found for
2007 Please verify you used the right trait or you didn't misspell the
2008 associated type name. Example:
2015 type Foo = Trait<Bar=i32>; // ok!
2020 An attempt was made to retrieve an associated type, but the type was ambiguous.
2024 trait MyTrait {type X; }
2027 let foo: MyTrait::X;
2031 The problem here is that we're attempting to take the type of X from MyTrait.
2032 Unfortunately, the type of X is not defined, because it's only made concrete in
2033 implementations of the trait. A working version of this code might look like:
2036 trait MyTrait {type X; }
2039 impl MyTrait for MyStruct {
2044 let foo: <MyStruct as MyTrait>::X;
2048 This syntax specifies that we want the X type from MyTrait, as made concrete in
2049 MyStruct. The reason that we cannot simply use `MyStruct::X` is that MyStruct
2050 might implement two different traits with identically-named associated types.
2051 This syntax allows disambiguation between the two.
2055 You attempted to use multiple types as bounds for a closure or trait object.
2056 Rust does not currently support this. A simple example that causes this error:
2060 let _: Box<std::io::Read+std::io::Write>;
2064 Builtin traits are an exception to this rule: it's possible to have bounds of
2065 one non-builtin type, plus any number of builtin types. For example, the
2066 following compiles correctly:
2070 let _: Box<std::io::Read+Copy+Sync>;
2076 The attribute must have a value. Erroneous code example:
2079 #[rustc_on_unimplemented] // error: this attribute must have a value
2083 Please supply the missing value of the attribute. Example:
2086 #[rustc_on_unimplemented = "foo"] // ok!
2092 This error indicates that not enough type parameters were found in a type or
2095 For example, the `Foo` struct below is defined to be generic in `T`, but the
2096 type parameter is missing in the definition of `Bar`:
2099 struct Foo<T> { x: T }
2101 struct Bar { x: Foo }
2106 This error indicates that too many type parameters were found in a type or
2109 For example, the `Foo` struct below has no type parameters, but is supplied
2110 with two in the definition of `Bar`:
2113 struct Foo { x: bool }
2115 struct Bar<S, T> { x: Foo<S, T> }
2120 This error indicates a constant expression for the array length was found, but
2121 it was not an integer (signed or unsigned) expression.
2123 Some examples of code that produces this error are:
2126 const A: [u32; "hello"] = []; // error
2127 const B: [u32; true] = []; // error
2128 const C: [u32; 0.0] = []; // error
2132 There was an error while evaluating the expression for the length of a fixed-
2135 Some examples of this error are:
2138 // divide by zero in the length expression
2139 const A: [u32; 1/0] = [];
2141 // Rust currently will not evaluate the function `foo` at compile time
2142 fn foo() -> usize { 12 }
2143 const B: [u32; foo()] = [];
2145 // it is an error to try to add `u8` and `f64`
2147 const C: [u32; u8::MAX + f64::EPSILON] = [];
2152 Default impls for a trait must be located in the same crate where the trait was
2153 defined. For more information see the [opt-in builtin traits RFC](https://github
2154 .com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
2158 The `Sized` trait is a special trait built-in to the compiler for types with a
2159 constant size known at compile-time. This trait is automatically implemented
2160 for types as needed by the compiler, and it is currently disallowed to
2161 explicitly implement it for a type.
2165 The types of any associated constants in a trait implementation must match the
2166 types in the trait definition. This error indicates that there was a mismatch.
2168 Here's an example of this error:
2178 const BAR: u32 = 5; // error, expected bool, found u32
2184 You cannot use associated items other than constant items as patterns. This
2185 includes method items. Example of erroneous code:
2191 fn bb() -> i32 { 0 }
2196 B::bb => {} // error: associated items in match patterns must
2202 Please check that you're not using a method as a pattern. Example:
2220 This error indicates that a binary assignment operator like `+=` or `^=` was
2221 applied to the wrong types. For example:
2225 x ^= true; // error, `^=` cannot be applied to types `u16` and `bool`
2226 x += (); // error, `+=` cannot be applied to types `u16` and `()`
2229 Another problem you might be facing is this: suppose you've overloaded the `+`
2230 operator for some type `Foo` by implementing the `std::ops::Add` trait for
2231 `Foo`, but you find that using `+=` does not work, as in this example:
2241 fn add(self, rhs: Foo) -> Foo {
2247 let mut x: Foo = Foo(5);
2248 x += Foo(7); // error, `+= cannot be applied to types `Foo` and `Foo`
2252 This is because the binary assignment operators currently do not work off of
2253 traits, so it is not possible to overload them. See [RFC 953] for a proposal
2256 [RFC 953]: https://github.com/rust-lang/rfcs/pull/953
2260 When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a
2261 definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement
2262 `Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by
2263 definition, so it is not useful to do this.
2268 trait Foo { fn foo(&self) { } }
2272 impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
2273 impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
2274 impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
2275 impl Baz for Bar { } // Note: This is OK
2280 Trying to implement a trait for a trait object (as in `impl Trait1 for
2281 Trait2 { ... }`) does not work if the trait is not object-safe. Please see the
2282 [RFC 255] for more details on object safety rules.
2284 [RFC 255]: https://github.com/rust-lang/rfcs/pull/255
2288 Trait methods cannot be declared `const` by design. For more information, see
2291 [RFC 911]: https://github.com/rust-lang/rfcs/pull/911
2295 Default impls are only allowed for traits with no methods or associated items.
2296 For more information see the [opt-in builtin traits RFC](https://github.com/rust
2297 -lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
2301 This error indicates that some types or traits depend on each other
2302 and therefore cannot be constructed.
2304 The following example contains a circular dependency between two traits:
2307 trait FirstTrait : SecondTrait {
2311 trait SecondTrait : FirstTrait {
2318 This error indicates that a type or lifetime parameter has been declared
2319 but not actually used. Here is an example that demonstrates the error:
2327 If the type parameter was included by mistake, this error can be fixed
2328 by simply removing the type parameter, as shown below:
2336 Alternatively, if the type parameter was intentionally inserted, it must be
2337 used. A simple fix is shown below:
2345 This error may also commonly be found when working with unsafe code. For
2346 example, when using raw pointers one may wish to specify the lifetime for
2347 which the pointed-at data is valid. An initial attempt (below) causes this
2356 We want to express the constraint that Foo should not outlive `'a`, because
2357 the data pointed to by `T` is only valid for that lifetime. The problem is
2358 that there are no actual uses of `'a`. It's possible to work around this
2359 by adding a PhantomData type to the struct, using it to tell the compiler
2360 to act as if the struct contained a borrowed reference `&'a T`:
2363 use std::marker::PhantomData;
2365 struct Foo<'a, T: 'a> {
2367 phantom: PhantomData<&'a T>
2371 PhantomData can also be used to express information about unused type
2372 parameters. You can read more about it in the API documentation:
2374 https://doc.rust-lang.org/std/marker/struct.PhantomData.html
2379 register_diagnostics! {
2401 E0173, // manual implementations of unboxed closure traits are experimental
2402 E0174, // explicit use of unboxed closure methods are experimental
2405 E0187, // can't infer the kind of the closure
2406 E0188, // can not cast a immutable reference to a mutable pointer
2407 E0189, // deprecated: can only cast a boxed pointer to a boxed object
2408 E0190, // deprecated: can only cast a &-pointer to an &-object
2409 E0193, // cannot bound type where clause bounds may only be attached to types
2410 // involving type parameters
2412 E0196, // cannot determine a type for this closure
2413 E0203, // type parameter has more than one relaxed default bound,
2414 // and only one is supported
2416 E0209, // builtin traits can only be implemented on structs or enums
2417 E0212, // cannot extract an associated type from a higher-ranked trait bound
2418 E0213, // associated types are not accepted in this context
2419 E0214, // parenthesized parameters may only be used with a trait
2420 // E0215, // angle-bracket notation is not stable with `Fn`
2421 // E0216, // parenthetical notation is only stable with `Fn`
2422 E0217, // ambiguous associated type, defined in multiple supertraits
2423 E0218, // no associated type defined
2424 E0219, // associated type defined in higher-ranked supertrait
2425 E0221, // ambiguous associated type in bounds
2426 // E0222, // Error code E0045 (variadic function must have C calling
2427 // convention) duplicate
2428 E0224, // at least one non-builtin train is required for an object type
2429 E0226, // only a single explicit lifetime bound is permitted
2430 E0227, // ambiguous lifetime bound, explicit lifetime bound required
2431 E0228, // explicit lifetime bound required
2432 E0229, // associated type bindings are not allowed here
2433 E0230, // there is no type parameter on trait
2434 E0231, // only named substitution parameters are allowed
2437 E0235, // structure constructor specifies a structure of type but
2438 E0236, // no lang item for range syntax
2439 E0237, // no lang item for range syntax
2440 E0238, // parenthesized parameters may only be used with a trait
2441 E0239, // `next` method of `Iterator` trait has unexpected type
2444 E0242, // internal error looking up a definition
2445 E0245, // not a trait
2446 E0246, // invalid recursive type
2447 E0247, // found module name used as a type
2448 E0248, // found value name used as a type
2449 E0319, // trait impls for defaulted traits allowed just for structs/enums
2450 E0320, // recursive overflow during dropck
2451 E0321, // extended coherence rules for defaulted traits violated
2452 E0323, // implemented an associated const when another trait item expected
2453 E0324, // implemented a method when another trait item expected
2454 E0325, // implemented an associated type when another trait item expected
2455 E0328, // cannot implement Unsize explicitly
2456 E0329, // associated const depends on type parameter or Self.
2457 E0366, // dropck forbid specialization to concrete type or region
2458 E0367, // dropck forbid specialization to predicate not in struct/enum
2459 E0369, // binary operation `<op>` cannot be applied to types
2460 E0374, // the trait `CoerceUnsized` may only be implemented for a coercion
2461 // between structures with one field being coerced, none found
2462 E0375, // the trait `CoerceUnsized` may only be implemented for a coercion
2463 // between structures with one field being coerced, but multiple
2464 // fields need coercions
2465 E0376, // the trait `CoerceUnsized` may only be implemented for a coercion
2466 // between structures
2467 E0377, // the trait `CoerceUnsized` may only be implemented for a coercion
2468 // between structures with the same definition
2469 E0390, // only a single inherent implementation marked with
2470 // `#[lang = \"{}\"]` is allowed for the `{}` primitive
2471 E0393, // the type parameter `{}` must be explicitly specified in an object
2472 // type because its default value `{}` references the type `Self`"
2473 E0399, // trait items need to be implemented because the associated
2474 // type `{}` was overridden
2475 E0436 // functional record update requires a struct