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 When using the `#[simd]` attribute on a tuple struct, the components of the
831 tuple struct must all be of a concrete, nongeneric type so the compiler can
832 reason about how to use SIMD with them. This error will occur if the types
837 struct Bad<T>(T, T, T); // This will cause an error
840 struct Good(u32, u32, u32); // This will not
845 The `#[simd]` attribute can only be applied to non empty tuple structs, because
846 it doesn't make sense to try to use SIMD operations when there are no values to
851 struct Bad; // This will cause an error
854 struct Good(u32); // This will not
859 When using the `#[simd]` attribute to automatically use SIMD operations in tuple
860 struct, the types in the struct must all be of the same type, or the compiler
861 will trigger this error.
865 struct Bad(u16, u32, u32); // This will cause an error
868 struct Good(u32, u32, u32); // This will not
874 When using the `#[simd]` attribute on a tuple struct, the elements in the tuple
875 must be machine types so SIMD operations can be applied to them.
879 struct Bad(String); // This will cause an error
882 struct Good(u32, u32, u32); // This will not
887 Enum discriminants are used to differentiate enum variants stored in memory.
888 This error indicates that the same value was used for two or more variants,
889 making them impossible to tell apart.
907 Note that variants without a manually specified discriminant are numbered from
908 top to bottom starting from 0, so clashes can occur with seemingly unrelated
918 Here `X` will have already been assigned the discriminant 0 by the time `Y` is
919 encountered, so a conflict occurs.
923 The default type for enum discriminants is `isize`, but it can be adjusted by
924 adding the `repr` attribute to the enum declaration. This error indicates that
925 an integer literal given as a discriminant is not a member of the discriminant
936 Here, 1024 lies outside the valid range for `u8`, so the discriminant for `A` is
937 invalid. You may want to change representation types to fix this, or else change
938 invalid discriminant values so that they fit within the existing type.
940 Note also that without a representation manually defined, the compiler will
941 optimize by using the smallest integer type possible.
945 At present, it's not possible to define a custom representation for an enum with
946 a single variant. As a workaround you can add a `Dummy` variant.
948 See: https://github.com/rust-lang/rust/issues/10292
952 It is impossible to define an integer type to be used to represent zero-variant
953 enum values because there are no zero-variant enum values. There is no way to
954 construct an instance of the following type using only safe code:
962 Too many type parameters were supplied for a function. For example:
968 foo::<f64, bool>(); // error, expected 1 parameter, found 2 parameters
972 The number of supplied parameters much exactly match the number of defined type
977 You gave too many lifetime parameters. Erroneous code example:
983 f::<'static>() // error: too many lifetime parameters provided
987 Please check you give the right number of lifetime parameters. Example:
997 It's also important to note that the Rust compiler can generally
998 determine the lifetime by itself. Example:
1006 // it can be written like this
1007 fn get_value<'a>(&'a self) -> &'a str { &self.value }
1008 // but the compiler works fine with this too:
1009 fn without_lifetime(&self) -> &str { &self.value }
1013 let f = Foo { value: "hello".to_owned() };
1015 println!("{}", f.get_value());
1016 println!("{}", f.without_lifetime());
1022 Not enough type parameters were supplied for a function. For example:
1028 foo::<f64>(); // error, expected 2 parameters, found 1 parameter
1032 Note that if a function takes multiple type parameters but you want the compiler
1033 to infer some of them, you can use type placeholders:
1036 fn foo<T, U>(x: T) {}
1040 foo::<f64>(x); // error, expected 2 parameters, found 1 parameter
1041 foo::<_, f64>(x); // same as `foo::<bool, f64>(x)`
1047 You gave an unnecessary type parameter in a type alias. Erroneous code
1051 type Foo<T> = u32; // error: type parameter `T` is unused
1053 type Foo<A,B> = Box<A>; // error: type parameter `B` is unused
1056 Please check you didn't write too many type parameters. Example:
1059 type Foo = u32; // ok!
1060 type Foo<A> = Box<A>; // ok!
1065 You tried to declare an undefined atomic operation function.
1066 Erroneous code example:
1069 #![feature(intrinsics)]
1071 extern "rust-intrinsic" {
1072 fn atomic_foo(); // error: unrecognized atomic operation
1077 Please check you didn't make a mistake in the function's name. All intrinsic
1078 functions are defined in librustc_trans/trans/intrinsic.rs and in
1079 libcore/intrinsics.rs in the Rust source code. Example:
1082 #![feature(intrinsics)]
1084 extern "rust-intrinsic" {
1085 fn atomic_fence(); // ok!
1091 You declared an unknown intrinsic function. Erroneous code example:
1094 #![feature(intrinsics)]
1096 extern "rust-intrinsic" {
1097 fn foo(); // error: unrecognized intrinsic function: `foo`
1107 Please check you didn't make a mistake in the function's name. All intrinsic
1108 functions are defined in librustc_trans/trans/intrinsic.rs and in
1109 libcore/intrinsics.rs in the Rust source code. Example:
1112 #![feature(intrinsics)]
1114 extern "rust-intrinsic" {
1115 fn atomic_fence(); // ok!
1127 You gave an invalid number of type parameters to an intrinsic function.
1128 Erroneous code example:
1131 #![feature(intrinsics)]
1133 extern "rust-intrinsic" {
1134 fn size_of<T, U>() -> usize; // error: intrinsic has wrong number
1135 // of type parameters
1139 Please check that you provided the right number of lifetime parameters
1140 and verify with the function declaration in the Rust source code.
1144 #![feature(intrinsics)]
1146 extern "rust-intrinsic" {
1147 fn size_of<T>() -> usize; // ok!
1153 You hit this error because the compiler the compiler lacks information
1154 to determine a type for this expression. Erroneous code example:
1158 let x = |_| {}; // error: cannot determine a type for this expression
1162 You have two possibilities to solve this situation:
1163 * Give an explicit definition of the expression
1164 * Infer the expression
1170 let x = |_ : u32| {}; // ok!
1179 This error indicates that a lifetime is missing from a type. If it is an error
1180 inside a function signature, the problem may be with failing to adhere to the
1181 lifetime elision rules (see below).
1183 Here are some simple examples of where you'll run into this error:
1186 struct Foo { x: &bool } // error
1187 struct Foo<'a> { x: &'a bool } // correct
1189 enum Bar { A(u8), B(&bool), } // error
1190 enum Bar<'a> { A(u8), B(&'a bool), } // correct
1192 type MyStr = &str; // error
1193 type MyStr<'a> = &'a str; //correct
1197 Lifetime elision is a special, limited kind of inference for lifetimes in
1198 function signatures which allows you to leave out lifetimes in certain cases.
1199 For more background on lifetime elision see [the book][book-le].
1201 The lifetime elision rules require that any function signature with an elided
1202 output lifetime must either have
1204 - exactly one input lifetime
1205 - or, multiple input lifetimes, but the function must also be a method with a
1206 `&self` or `&mut self` receiver
1208 In the first case, the output lifetime is inferred to be the same as the unique
1209 input lifetime. In the second case, the lifetime is instead inferred to be the
1210 same as the lifetime on `&self` or `&mut self`.
1212 Here are some examples of elision errors:
1215 // error, no input lifetimes
1216 fn foo() -> &str { ... }
1218 // error, `x` and `y` have distinct lifetimes inferred
1219 fn bar(x: &str, y: &str) -> &str { ... }
1221 // error, `y`'s lifetime is inferred to be distinct from `x`'s
1222 fn baz<'a>(x: &'a str, y: &str) -> &str { ... }
1225 [book-le]: http://doc.rust-lang.org/nightly/book/lifetimes.html#lifetime-elision
1229 This error means that an incorrect number of lifetime parameters were provided
1230 for a type (like a struct or enum) or trait.
1232 Some basic examples include:
1235 struct Foo<'a>(&'a str);
1236 enum Bar { A, B, C }
1239 foo: Foo, // error: expected 1, found 0
1240 bar: Bar<'a>, // error: expected 0, found 1
1244 Here's an example that is currently an error, but may work in a future version
1248 struct Foo<'a>(&'a str);
1251 impl Quux for Foo { } // error: expected 1, found 0
1254 Lifetime elision in implementation headers was part of the lifetime elision
1255 RFC. It is, however, [currently unimplemented][iss15872].
1257 [iss15872]: https://github.com/rust-lang/rust/issues/15872
1261 You can only define an inherent implementation for a type in the same crate
1262 where the type was defined. For example, an `impl` block as below is not allowed
1263 since `Vec` is defined in the standard library:
1266 impl Vec<u8> { ... } // error
1269 To fix this problem, you can do either of these things:
1271 - define a trait that has the desired associated functions/types/constants and
1272 implement the trait for the type in question
1273 - define a new type wrapping the type and define an implementation on the new
1276 Note that using the `type` keyword does not work here because `type` only
1277 introduces a type alias:
1280 type Bytes = Vec<u8>;
1282 impl Bytes { ... } // error, same as above
1287 You got this error because because you tried to implement a foreign
1288 trait for a foreign type (with maybe a foreign type parameter). Erroneous
1292 impl Drop for u32 {}
1295 The type, trait or the type parameter (or all of them) has to be defined
1296 in your crate. Example:
1299 pub struct Foo; // you define your type in your crate
1301 impl Drop for Foo { // and you can implement the trait on it!
1302 // code of trait implementation here
1305 trait Bar { // or define your trait in your crate
1306 fn get(&self) -> usize;
1309 impl Bar for u32 { // and then you implement it on a foreign type
1310 fn get(&self) -> usize { 0 }
1313 impl From<Foo> for i32 { // or you use a type from your crate as
1315 fn from(i: Foo) -> i32 {
1323 There are conflicting trait implementations for the same type.
1324 Example of erroneous code:
1328 fn get(&self) -> usize;
1331 impl<T> MyTrait for T {
1332 fn get(&self) -> usize { 0 }
1339 impl MyTrait for Foo { // error: conflicting implementations for trait
1341 fn get(&self) -> usize { self.value }
1345 When looking for the implementation for the trait, the compiler finds
1346 both the `impl<T> MyTrait for T` where T is all types and the `impl
1347 MyTrait for Foo`. Since a trait cannot be implemented multiple times,
1348 this is an error. So, when you write:
1351 impl<T> MyTrait for T {
1352 fn get(&self) -> usize { 0 }
1356 This makes the trait implemented on all types in the scope. So if you
1357 try to implement it on another one after that, the implementations will
1362 fn get(&self) -> usize;
1365 impl<T> MyTrait for T {
1366 fn get(&self) -> usize { 0 }
1374 f.get(); // the trait is implemented so we can use it
1380 An attempt was made to implement Drop on a trait, which is not allowed: only
1381 structs and enums can implement Drop. An example causing this error:
1386 impl Drop for MyTrait {
1387 fn drop(&mut self) {}
1391 A workaround for this problem is to wrap the trait up in a struct, and implement
1392 Drop on that. An example is shown below:
1396 struct MyWrapper<T: MyTrait> { foo: T }
1398 impl <T: MyTrait> Drop for MyWrapper<T> {
1399 fn drop(&mut self) {}
1404 Alternatively, wrapping trait objects requires something like the following:
1409 //or Box<MyTrait>, if you wanted an owned trait object
1410 struct MyWrapper<'a> { foo: &'a MyTrait }
1412 impl <'a> Drop for MyWrapper<'a> {
1413 fn drop(&mut self) {}
1419 In order to be consistent with Rust's lack of global type inference, type
1420 placeholders are disallowed by design in item signatures.
1422 Examples of this error include:
1425 fn foo() -> _ { 5 } // error, explicitly write out the return type instead
1427 static BAR: _ = "test"; // error, explicitly write out the type instead
1432 You declared two fields of a struct with the same name. Erroneous code
1438 field1: i32 // error: field is already declared
1442 Please verify that the field names have been correctly spelled. Example:
1453 Type parameter defaults can only use parameters that occur before them.
1454 Erroneous code example:
1457 pub struct Foo<T=U, U=()> {
1461 // error: type parameters with a default cannot use forward declared
1465 Since type parameters are evaluated in-order, you may be able to fix this issue
1469 pub struct Foo<U=(), T=U> {
1475 Please also verify that this wasn't because of a name-clash and rename the type
1480 You declared a pattern as an argument in a foreign function declaration.
1481 Erroneous code example:
1485 fn foo((a, b): (u32, u32)); // error: patterns aren't allowed in foreign
1486 // function declarations
1490 Please replace the pattern argument with a regular one. Example:
1499 fn foo(s: SomeStruct); // ok!
1503 fn foo(a: (u32, u32)); // ok!
1509 It is not possible to define `main` with type parameters, or even with function
1510 parameters. When `main` is present, it must take no arguments and return `()`.
1514 It is not possible to declare type parameters on a function that has the `start`
1515 attribute. Such a function must have the following type signature:
1518 fn(isize, *const *const u8) -> isize
1523 You tried to use a trait as a struct constructor. Erroneous code example:
1526 trait TraitNotAStruct {}
1528 TraitNotAStruct{ value: 0 }; // error: use of trait `TraitNotAStruct` as a
1529 // struct constructor
1532 Please verify you used the correct type name or please implement the trait
1533 on a struct and use this struct constructor. Example:
1536 trait TraitNotAStruct {}
1542 Foo{ value: 0 }; // ok!
1547 This error means that the compiler found a return expression in a function
1548 marked as diverging. A function diverges if it has `!` in the place of the
1549 return type in its signature. For example:
1552 fn foo() -> ! { return; } // error
1555 For a function that diverges, every control path in the function must never
1556 return, for example with a `loop` that never breaks or a call to another
1557 diverging function (such as `panic!()`).
1561 This error means that an attempt was made to specify the type of a variable with
1562 a combination of a concrete type and a trait. Consider the following example:
1565 fn foo(bar: i32+std::fmt::Display) {}
1568 The code is trying to specify that we want to receive a signed 32-bit integer
1569 which also implements `Display`. This doesn't make sense: when we pass `i32`, a
1570 concrete type, it implicitly includes all of the traits that it implements.
1571 This includes `Display`, `Debug`, `Clone`, and a host of others.
1573 If `i32` implements the trait we desire, there's no need to specify the trait
1574 separately. If it does not, then we need to `impl` the trait for `i32` before
1575 passing it into `foo`. Either way, a fixed definition for `foo` will look like
1582 To learn more about traits, take a look at the Book:
1584 https://doc.rust-lang.org/book/traits.html
1588 In types, the `+` type operator has low precedence, so it is often necessary
1597 w: &'a Foo + Copy, // error, use &'a (Foo + Copy)
1598 x: &'a Foo + 'a, // error, use &'a (Foo + 'a)
1599 y: &'a mut Foo + 'a, // error, use &'a mut (Foo + 'a)
1600 z: fn() -> Foo + 'a, // error, use fn() -> (Foo + 'a)
1604 More details can be found in [RFC 438].
1606 [RFC 438]: https://github.com/rust-lang/rfcs/pull/438
1610 Explicitly implementing both Drop and Copy for a type is currently disallowed.
1611 This feature can make some sense in theory, but the current implementation is
1612 incorrect and can lead to memory unsafety (see [issue #20126][iss20126]), so
1613 it has been disabled for now.
1615 [iss20126]: https://github.com/rust-lang/rust/issues/20126
1619 An associated function for a trait was defined to be static, but an
1620 implementation of the trait declared the same function to be a method (i.e. to
1621 take a `self` parameter).
1623 Here's an example of this error:
1633 // error, method `foo` has a `&self` declaration in the impl, but not in
1640 An associated function for a trait was defined to be a method (i.e. to take a
1641 `self` parameter), but an implementation of the trait declared the same function
1644 Here's an example of this error:
1654 // error, method `foo` has a `&self` declaration in the trait, but not in
1662 Trait objects need to have all associated types specified. Erroneous code
1670 type Foo = Trait; // error: the value of the associated type `Bar` (from
1671 // the trait `Trait`) must be specified
1674 Please verify you specified all associated types of the trait and that you
1675 used the right trait. Example:
1682 type Foo = Trait<Bar=i32>; // ok!
1687 Negative impls are only allowed for traits with default impls. For more
1688 information see the [opt-in builtin traits RFC](https://github.com/rust-lang/
1689 rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
1693 Your method's lifetime parameters do not match the trait declaration.
1694 Erroneous code example:
1698 fn bar<'a,'b:'a>(x: &'a str, y: &'b str);
1703 impl Trait for Foo {
1704 fn bar<'a,'b>(x: &'a str, y: &'b str) {
1705 // error: lifetime parameters or bounds on method `bar`
1706 // do not match the trait declaration
1711 The lifetime constraint `'b` for bar() implementation does not match the
1712 trait declaration. Ensure lifetime declarations match exactly in both trait
1713 declaration and implementation. Example:
1717 fn t<'a,'b:'a>(x: &'a str, y: &'b str);
1722 impl Trait for Foo {
1723 fn t<'a,'b:'a>(x: &'a str, y: &'b str) { // ok!
1730 Inherent implementations (one that do not implement a trait but provide
1731 methods associated with a type) are always safe because they are not
1732 implementing an unsafe trait. Removing the `unsafe` keyword from the inherent
1733 implementation will resolve this error.
1738 // this will cause this error
1740 // converting it to this will fix it
1747 A negative implementation is one that excludes a type from implementing a
1748 particular trait. Not being able to use a trait is always a safe operation,
1749 so negative implementations are always safe and never need to be marked as
1755 // unsafe is unnecessary
1756 unsafe impl !Clone for Foo { }
1757 // this will compile
1758 impl !Clone for Foo { }
1764 Safe traits should not have unsafe implementations, therefore marking an
1765 implementation for a safe trait unsafe will cause a compiler error. Removing the
1766 unsafe marker on the trait noted in the error will resolve this problem.
1773 // this won't compile because Bar is safe
1774 unsafe impl Bar for Foo { }
1775 // this will compile
1776 impl Bar for Foo { }
1782 Unsafe traits must have unsafe implementations. This error occurs when an
1783 implementation for an unsafe trait isn't marked as unsafe. This may be resolved
1784 by marking the unsafe implementation as unsafe.
1789 unsafe trait Bar { }
1791 // this won't compile because Bar is unsafe and impl isn't unsafe
1792 impl Bar for Foo { }
1793 // this will compile
1794 unsafe impl Bar for Foo { }
1800 It is an error to define two associated items (like methods, associated types,
1801 associated functions, etc.) with the same identifier.
1809 fn bar(&self) -> bool { self.0 > 5 }
1810 fn bar() {} // error: duplicate associated function
1815 fn baz(&self) -> bool;
1821 fn baz(&self) -> bool { true }
1823 // error: duplicate method
1824 fn baz(&self) -> bool { self.0 > 5 }
1826 // error: duplicate associated type
1833 Inherent associated types were part of [RFC 195] but are not yet implemented.
1834 See [the tracking issue][iss8995] for the status of this implementation.
1836 [RFC 195]: https://github.com/rust-lang/rfcs/pull/195
1837 [iss8995]: https://github.com/rust-lang/rust/issues/8995
1841 An attempt to implement the `Copy` trait for a struct failed because one of the
1842 fields does not implement `Copy`. To fix this, you must implement `Copy` for the
1843 mentioned field. Note that this may not be possible, as in the example of
1850 impl Copy for Foo { }
1853 This fails because `Vec<T>` does not implement `Copy` for any `T`.
1855 Here's another example that will fail:
1864 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
1865 differs from the behavior for `&T`, which is always `Copy`).
1869 An attempt to implement the `Copy` trait for an enum failed because one of the
1870 variants does not implement `Copy`. To fix this, you must implement `Copy` for
1871 the mentioned variant. Note that this may not be possible, as in the example of
1879 impl Copy for Foo { }
1882 This fails because `Vec<T>` does not implement `Copy` for any `T`.
1884 Here's another example that will fail:
1894 This fails because `&mut T` is not `Copy`, even when `T` is `Copy` (this
1895 differs from the behavior for `&T`, which is always `Copy`).
1899 You can only implement `Copy` for a struct or enum. Both of the following
1900 examples will fail, because neither `i32` (primitive type) nor `&'static Bar`
1901 (reference to `Bar`) is a struct or enum:
1905 impl Copy for Foo { } // error
1907 #[derive(Copy, Clone)]
1909 impl Copy for &'static Bar { } // error
1914 You declared an unused type parameter when implementing a trait on an object.
1915 Erroneous code example:
1919 fn get(&self) -> usize;
1924 impl<T> MyTrait for Foo {
1925 fn get(&self) -> usize {
1931 Please check your object definition and remove unused type
1932 parameter(s). Example:
1936 fn get(&self) -> usize;
1941 impl MyTrait for Foo {
1942 fn get(&self) -> usize {
1950 You used an intrinsic function which doesn't correspond to its
1951 definition. Erroneous code example:
1954 #![feature(intrinsics)]
1956 extern "rust-intrinsic" {
1957 fn size_of<T>(); // error: intrinsic has wrong type
1961 Please check the function definition. Example:
1964 #![feature(intrinsics)]
1966 extern "rust-intrinsic" {
1967 fn size_of<T>() -> usize;
1973 You used an associated type which isn't defined in the trait.
1974 Erroneous code example:
1981 type Foo = Trait<F=i32>; // error: associated type `F` not found for
1985 Please verify you used the right trait or you didn't misspell the
1986 associated type name. Example:
1993 type Foo = Trait<Bar=i32>; // ok!
1998 An attempt was made to retrieve an associated type, but the type was ambiguous.
2002 trait MyTrait {type X; }
2005 let foo: MyTrait::X;
2009 The problem here is that we're attempting to take the type of X from MyTrait.
2010 Unfortunately, the type of X is not defined, because it's only made concrete in
2011 implementations of the trait. A working version of this code might look like:
2014 trait MyTrait {type X; }
2017 impl MyTrait for MyStruct {
2022 let foo: <MyStruct as MyTrait>::X;
2026 This syntax specifies that we want the X type from MyTrait, as made concrete in
2027 MyStruct. The reason that we cannot simply use `MyStruct::X` is that MyStruct
2028 might implement two different traits with identically-named associated types.
2029 This syntax allows disambiguation between the two.
2033 You attempted to use multiple types as bounds for a closure or trait object.
2034 Rust does not currently support this. A simple example that causes this error:
2038 let _: Box<std::io::Read+std::io::Write>;
2042 Builtin traits are an exception to this rule: it's possible to have bounds of
2043 one non-builtin type, plus any number of builtin types. For example, the
2044 following compiles correctly:
2048 let _: Box<std::io::Read+Copy+Sync>;
2054 The attribute must have a value. Erroneous code example:
2057 #[rustc_on_unimplemented] // error: this attribute must have a value
2061 Please supply the missing value of the attribute. Example:
2064 #[rustc_on_unimplemented = "foo"] // ok!
2070 This error indicates that not enough type parameters were found in a type or
2073 For example, the `Foo` struct below is defined to be generic in `T`, but the
2074 type parameter is missing in the definition of `Bar`:
2077 struct Foo<T> { x: T }
2079 struct Bar { x: Foo }
2084 This error indicates that too many type parameters were found in a type or
2087 For example, the `Foo` struct below has no type parameters, but is supplied
2088 with two in the definition of `Bar`:
2091 struct Foo { x: bool }
2093 struct Bar<S, T> { x: Foo<S, T> }
2098 This error indicates a constant expression for the array length was found, but
2099 it was not an integer (signed or unsigned) expression.
2101 Some examples of code that produces this error are:
2104 const A: [u32; "hello"] = []; // error
2105 const B: [u32; true] = []; // error
2106 const C: [u32; 0.0] = []; // error
2110 There was an error while evaluating the expression for the length of a fixed-
2113 Some examples of this error are:
2116 // divide by zero in the length expression
2117 const A: [u32; 1/0] = [];
2119 // Rust currently will not evaluate the function `foo` at compile time
2120 fn foo() -> usize { 12 }
2121 const B: [u32; foo()] = [];
2123 // it is an error to try to add `u8` and `f64`
2125 const C: [u32; u8::MAX + f64::EPSILON] = [];
2130 Default impls for a trait must be located in the same crate where the trait was
2131 defined. For more information see the [opt-in builtin traits RFC](https://github
2132 .com/rust-lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
2136 The `Sized` trait is a special trait built-in to the compiler for types with a
2137 constant size known at compile-time. This trait is automatically implemented
2138 for types as needed by the compiler, and it is currently disallowed to
2139 explicitly implement it for a type.
2143 The types of any associated constants in a trait implementation must match the
2144 types in the trait definition. This error indicates that there was a mismatch.
2146 Here's an example of this error:
2156 const BAR: u32 = 5; // error, expected bool, found u32
2162 You cannot use associated items other than constant items as patterns. This
2163 includes method items. Example of erroneous code:
2169 fn bb() -> i32 { 0 }
2174 B::bb => {} // error: associated items in match patterns must
2180 Please check that you're not using a method as a pattern. Example:
2198 This error indicates that a binary assignment operator like `+=` or `^=` was
2199 applied to the wrong types. For example:
2203 x ^= true; // error, `^=` cannot be applied to types `u16` and `bool`
2204 x += (); // error, `+=` cannot be applied to types `u16` and `()`
2207 Another problem you might be facing is this: suppose you've overloaded the `+`
2208 operator for some type `Foo` by implementing the `std::ops::Add` trait for
2209 `Foo`, but you find that using `+=` does not work, as in this example:
2219 fn add(self, rhs: Foo) -> Foo {
2225 let mut x: Foo = Foo(5);
2226 x += Foo(7); // error, `+= cannot be applied to types `Foo` and `Foo`
2230 This is because the binary assignment operators currently do not work off of
2231 traits, so it is not possible to overload them. See [RFC 953] for a proposal
2234 [RFC 953]: https://github.com/rust-lang/rfcs/pull/953
2238 When `Trait2` is a subtrait of `Trait1` (for example, when `Trait2` has a
2239 definition like `trait Trait2: Trait1 { ... }`), it is not allowed to implement
2240 `Trait1` for `Trait2`. This is because `Trait2` already implements `Trait1` by
2241 definition, so it is not useful to do this.
2246 trait Foo { fn foo(&self) { } }
2250 impl Bar for Baz { } // error, `Baz` implements `Bar` by definition
2251 impl Foo for Baz { } // error, `Baz` implements `Bar` which implements `Foo`
2252 impl Baz for Baz { } // error, `Baz` (trivially) implements `Baz`
2253 impl Baz for Bar { } // Note: This is OK
2258 Trying to implement a trait for a trait object (as in `impl Trait1 for
2259 Trait2 { ... }`) does not work if the trait is not object-safe. Please see the
2260 [RFC 255] for more details on object safety rules.
2262 [RFC 255]: https://github.com/rust-lang/rfcs/pull/255
2266 Trait methods cannot be declared `const` by design. For more information, see
2269 [RFC 911]: https://github.com/rust-lang/rfcs/pull/911
2273 Default impls are only allowed for traits with no methods or associated items.
2274 For more information see the [opt-in builtin traits RFC](https://github.com/rust
2275 -lang/rfcs/blob/master/text/0019-opt-in-builtin-traits.md).
2279 This error indicates that some types or traits depend on each other
2280 and therefore cannot be constructed.
2282 The following example contains a circular dependency between two traits:
2285 trait FirstTrait : SecondTrait {
2289 trait SecondTrait : FirstTrait {
2296 This error indicates that a type or lifetime parameter has been declared
2297 but not actually used. Here is an example that demonstrates the error:
2305 If the type parameter was included by mistake, this error can be fixed
2306 by simply removing the type parameter, as shown below:
2314 Alternatively, if the type parameter was intentionally inserted, it must be
2315 used. A simple fix is shown below:
2323 This error may also commonly be found when working with unsafe code. For
2324 example, when using raw pointers one may wish to specify the lifetime for
2325 which the pointed-at data is valid. An initial attempt (below) causes this
2334 We want to express the constraint that Foo should not outlive `'a`, because
2335 the data pointed to by `T` is only valid for that lifetime. The problem is
2336 that there are no actual uses of `'a`. It's possible to work around this
2337 by adding a PhantomData type to the struct, using it to tell the compiler
2338 to act as if the struct contained a borrowed reference `&'a T`:
2341 use std::marker::PhantomData;
2343 struct Foo<'a, T: 'a> {
2345 phantom: PhantomData<&'a T>
2349 PhantomData can also be used to express information about unused type
2350 parameters. You can read more about it in the API documentation:
2352 https://doc.rust-lang.org/std/marker/struct.PhantomData.html
2357 register_diagnostics! {
2375 E0173, // manual implementations of unboxed closure traits are experimental
2376 E0174, // explicit use of unboxed closure methods are experimental
2379 E0187, // can't infer the kind of the closure
2380 E0188, // can not cast a immutable reference to a mutable pointer
2381 E0189, // deprecated: can only cast a boxed pointer to a boxed object
2382 E0190, // deprecated: can only cast a &-pointer to an &-object
2383 E0193, // cannot bound type where clause bounds may only be attached to types
2384 // involving type parameters
2386 E0196, // cannot determine a type for this closure
2387 E0203, // type parameter has more than one relaxed default bound,
2388 // and only one is supported
2390 E0209, // builtin traits can only be implemented on structs or enums
2391 E0210, // type parameter is not constrained by any local type
2392 E0212, // cannot extract an associated type from a higher-ranked trait bound
2393 E0213, // associated types are not accepted in this context
2394 E0214, // parenthesized parameters may only be used with a trait
2395 // E0215, // angle-bracket notation is not stable with `Fn`
2396 // E0216, // parenthetical notation is only stable with `Fn`
2397 E0217, // ambiguous associated type, defined in multiple supertraits
2398 E0218, // no associated type defined
2399 E0219, // associated type defined in higher-ranked supertrait
2400 E0221, // ambiguous associated type in bounds
2401 // E0222, // Error code E0045 (variadic function must have C calling
2402 // convention) duplicate
2403 E0224, // at least one non-builtin train is required for an object type
2404 E0226, // only a single explicit lifetime bound is permitted
2405 E0227, // ambiguous lifetime bound, explicit lifetime bound required
2406 E0228, // explicit lifetime bound required
2407 E0229, // associated type bindings are not allowed here
2408 E0230, // there is no type parameter on trait
2409 E0231, // only named substitution parameters are allowed
2412 E0235, // structure constructor specifies a structure of type but
2413 E0236, // no lang item for range syntax
2414 E0237, // no lang item for range syntax
2415 E0238, // parenthesized parameters may only be used with a trait
2416 E0239, // `next` method of `Iterator` trait has unexpected type
2419 E0242, // internal error looking up a definition
2420 E0245, // not a trait
2421 E0246, // invalid recursive type
2422 E0247, // found module name used as a type
2423 E0248, // found value name used as a type
2424 E0319, // trait impls for defaulted traits allowed just for structs/enums
2425 E0320, // recursive overflow during dropck
2426 E0321, // extended coherence rules for defaulted traits violated
2427 E0323, // implemented an associated const when another trait item expected
2428 E0324, // implemented a method when another trait item expected
2429 E0325, // implemented an associated type when another trait item expected
2430 E0328, // cannot implement Unsize explicitly
2431 E0329, // associated const depends on type parameter or Self.
2432 E0366, // dropck forbid specialization to concrete type or region
2433 E0367, // dropck forbid specialization to predicate not in struct/enum
2434 E0369, // binary operation `<op>` cannot be applied to types
2435 E0374, // the trait `CoerceUnsized` may only be implemented for a coercion
2436 // between structures with one field being coerced, none found
2437 E0375, // the trait `CoerceUnsized` may only be implemented for a coercion
2438 // between structures with one field being coerced, but multiple
2439 // fields need coercions
2440 E0376, // the trait `CoerceUnsized` may only be implemented for a coercion
2441 // between structures
2442 E0377, // the trait `CoerceUnsized` may only be implemented for a coercion
2443 // between structures with the same definition
2444 E0390, // only a single inherent implementation marked with
2445 // `#[lang = \"{}\"]` is allowed for the `{}` primitive
2446 E0393, // the type parameter `{}` must be explicitly specified in an object
2447 // type because its default value `{}` references the type `Self`"
2448 E0399, // trait items need to be implemented because the associated
2449 // type `{}` was overridden
2450 E0436 // functional record update requires a struct