Put each error code long explanation into their own markdown file

Author: GuillaumeGomez

src/librustc_error_codes/error_codes.rs

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+#### Note: this error code is no longer emitted by the compiler.
+
+This error suggests that the expression arm corresponding to the noted pattern
+will never be reached as for all possible values of the expression being
+matched, one of the preceding patterns will match.
+
+This means that perhaps some of the preceding patterns are too general, this
+one is too specific or the ordering is incorrect.
+
+For example, the following `match` block has too many arms:
+
+```
+match Some(0) {
+    Some(bar) => {/* ... */}
+    x => {/* ... */} // This handles the `None` case
+    _ => {/* ... */} // All possible cases have already been handled
+}
+```
+
+`match` blocks have their patterns matched in order, so, for example, putting
+a wildcard arm above a more specific arm will make the latter arm irrelevant.
+
+Ensure the ordering of the match arm is correct and remove any superfluous
+arms.

src/librustc_error_codes/error_codes/E0001.md

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+#### Note: this error code is no longer emitted by the compiler.
+
+This error indicates that an empty match expression is invalid because the type
+it is matching on is non-empty (there exist values of this type). In safe code
+it is impossible to create an instance of an empty type, so empty match
+expressions are almost never desired. This error is typically fixed by adding
+one or more cases to the match expression.
+
+An example of an empty type is `enum Empty { }`. So, the following will work:
+
+```
+enum Empty {}
+
+fn foo(x: Empty) {
+    match x {
+        // empty
+    }
+}
+```
+
+However, this won't:
+
+```compile_fail
+fn foo(x: Option<String>) {
+    match x {
+        // empty
+    }
+}
+```

src/librustc_error_codes/error_codes/E0002.md

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+This error indicates that the compiler cannot guarantee a matching pattern for
+one or more possible inputs to a match expression. Guaranteed matches are
+required in order to assign values to match expressions, or alternatively,
+determine the flow of execution.
+
+Erroneous code example:
+
+```compile_fail,E0004
+enum Terminator {
+    HastaLaVistaBaby,
+    TalkToMyHand,
+}
+
+let x = Terminator::HastaLaVistaBaby;
+
+match x { // error: non-exhaustive patterns: `HastaLaVistaBaby` not covered
+    Terminator::TalkToMyHand => {}
+}
+```
+
+If you encounter this error you must alter your patterns so that every possible
+value of the input type is matched. For types with a small number of variants
+(like enums) you should probably cover all cases explicitly. Alternatively, the
+underscore `_` wildcard pattern can be added after all other patterns to match
+"anything else". Example:
+
+```
+enum Terminator {
+    HastaLaVistaBaby,
+    TalkToMyHand,
+}
+
+let x = Terminator::HastaLaVistaBaby;
+
+match x {
+    Terminator::TalkToMyHand => {}
+    Terminator::HastaLaVistaBaby => {}
+}
+
+// or:
+
+match x {
+    Terminator::TalkToMyHand => {}
+    _ => {}
+}
+```

src/librustc_error_codes/error_codes/E0004.md

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+Patterns used to bind names must be irrefutable, that is, they must guarantee
+that a name will be extracted in all cases.
+
+Erroneous code example:
+
+```compile_fail,E0005
+let x = Some(1);
+let Some(y) = x;
+// error: refutable pattern in local binding: `None` not covered
+```
+
+If you encounter this error you probably need to use a `match` or `if let` to
+deal with the possibility of failure. Example:
+
+```
+let x = Some(1);
+
+match x {
+    Some(y) => {
+        // do something
+    },
+    None => {}
+}
+
+// or:
+
+if let Some(y) = x {
+    // do something
+}
+```

src/librustc_error_codes/error_codes/E0005.md

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+This error indicates that the bindings in a match arm would require a value to
+be moved into more than one location, thus violating unique ownership. Code
+like the following is invalid as it requires the entire `Option<String>` to be
+moved into a variable called `op_string` while simultaneously requiring the
+inner `String` to be moved into a variable called `s`.
+
+Erroneous code example:
+
+```compile_fail,E0007
+let x = Some("s".to_string());
+
+match x {
+    op_string @ Some(s) => {}, // error: cannot bind by-move with sub-bindings
+    None => {},
+}
+```
+
+See also the error E0303.

src/librustc_error_codes/error_codes/E0007.md

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+In a pattern, all values that don't implement the `Copy` trait have to be bound
+the same way. The goal here is to avoid binding simultaneously by-move and
+by-ref.
+
+This limitation may be removed in a future version of Rust.
+
+Erroneous code example:
+
+```compile_fail,E0009
+struct X { x: (), }
+
+let x = Some((X { x: () }, X { x: () }));
+match x {
+    Some((y, ref z)) => {}, // error: cannot bind by-move and by-ref in the
+                            //        same pattern
+    None => panic!()
+}
+```
+
+You have two solutions:
+
+Solution #1: Bind the pattern's values the same way.
+
+```
+struct X { x: (), }
+
+let x = Some((X { x: () }, X { x: () }));
+match x {
+    Some((ref y, ref z)) => {},
+    // or Some((y, z)) => {}
+    None => panic!()
+}
+```
+
+Solution #2: Implement the `Copy` trait for the `X` structure.
+
+However, please keep in mind that the first solution should be preferred.
+
+```
+#[derive(Clone, Copy)]
+struct X { x: (), }
+
+let x = Some((X { x: () }, X { x: () }));
+match x {
+    Some((y, ref z)) => {},
+    None => panic!()
+}
+```

src/librustc_error_codes/error_codes/E0009.md

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+The value of statics and constants must be known at compile time, and they live
+for the entire lifetime of a program. Creating a boxed value allocates memory on
+the heap at runtime, and therefore cannot be done at compile time.
+
+Erroneous code example:
+
+```compile_fail,E0010
+#![feature(box_syntax)]
+
+const CON : Box<i32> = box 0;
+```

src/librustc_error_codes/error_codes/E0010.md

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+Static and const variables can refer to other const variables. But a const
+variable cannot refer to a static variable.
+
+Erroneous code example:
+
+```compile_fail,E0013
+static X: i32 = 42;
+const Y: i32 = X;
+```
+
+In this example, `Y` cannot refer to `X` here. To fix this, the value can be
+extracted as a const and then used:
+
+```
+const A: i32 = 42;
+static X: i32 = A;
+const Y: i32 = A;
+```

src/librustc_error_codes/error_codes/E0013.md

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+#### Note: this error code is no longer emitted by the compiler.
+
+Constants can only be initialized by a constant value or, in a future
+version of Rust, a call to a const function. This error indicates the use
+of a path (like a::b, or x) denoting something other than one of these
+allowed items.
+
+Erroneous code example:
+
+```
+const FOO: i32 = { let x = 0; x }; // 'x' isn't a constant nor a function!
+```
+
+To avoid it, you have to replace the non-constant value:
+
+```
+const FOO: i32 = { const X : i32 = 0; X };
+// or even:
+const FOO2: i32 = { 0 }; // but brackets are useless here
+```

src/librustc_error_codes/error_codes/E0014.md

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+The only functions that can be called in static or constant expressions are
+`const` functions, and struct/enum constructors. `const` functions are only
+available on a nightly compiler. Rust currently does not support more general
+compile-time function execution.
+
+```
+const FOO: Option<u8> = Some(1); // enum constructor
+struct Bar {x: u8}
+const BAR: Bar = Bar {x: 1}; // struct constructor
+```
+
+See [RFC 911] for more details on the design of `const fn`s.
+
+[RFC 911]: https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn.md

src/librustc_error_codes/error_codes/E0015.md

@@ -0,0 +1,20 @@
+References in statics and constants may only refer to immutable values.
+
+Erroneous code example:
+
+```compile_fail,E0017
+static X: i32 = 1;
+const C: i32 = 2;
+
+// these three are not allowed:
+const CR: &mut i32 = &mut C;
+static STATIC_REF: &'static mut i32 = &mut X;
+static CONST_REF: &'static mut i32 = &mut C;
+```
+
+Statics are shared everywhere, and if they refer to mutable data one might
+violate memory safety since holding multiple mutable references to shared data
+is not allowed.
+
+If you really want global mutable state, try using `static mut` or a global
+`UnsafeCell`.

src/librustc_error_codes/error_codes/E0017.md

@@ -0,0 +1,36 @@
+A function call isn't allowed in the const's initialization expression
+because the expression's value must be known at compile-time.
+
+Erroneous code example:
+
+```compile_fail,E0019
+#![feature(box_syntax)]
+
+fn main() {
+    struct MyOwned;
+
+    static STATIC11: Box<MyOwned> = box MyOwned; // error!
+}
+```
+
+Remember: you can't use a function call inside a const's initialization
+expression! However, you can totally use it anywhere else:
+
+```
+enum Test {
+    V1
+}
+
+impl Test {
+    fn func(&self) -> i32 {
+        12
+    }
+}
+
+fn main() {
+    const FOO: Test = Test::V1;
+
+    FOO.func(); // here is good
+    let x = FOO.func(); // or even here!
+}
+```

src/librustc_error_codes/error_codes/E0019.md

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+A pattern used to match against an enum variant must provide a sub-pattern for
+each field of the enum variant. This error indicates that a pattern attempted to
+extract an incorrect number of fields from a variant.
+
+```
+enum Fruit {
+    Apple(String, String),
+    Pear(u32),
+}
+```
+
+Here the `Apple` variant has two fields, and should be matched against like so:
+
+```
+enum Fruit {
+    Apple(String, String),
+    Pear(u32),
+}
+
+let x = Fruit::Apple(String::new(), String::new());
+
+// Correct.
+match x {
+    Fruit::Apple(a, b) => {},
+    _ => {}
+}
+```
+
+Matching with the wrong number of fields has no sensible interpretation:
+
+```compile_fail,E0023
+enum Fruit {
+    Apple(String, String),
+    Pear(u32),
+}
+
+let x = Fruit::Apple(String::new(), String::new());
+
+// Incorrect.
+match x {
+    Fruit::Apple(a) => {},
+    Fruit::Apple(a, b, c) => {},
+}
+```
+
+Check how many fields the enum was declared with and ensure that your pattern
+uses the same number.