8 files changed, 170 insertions, 14 deletions
diff --git a/compiler/rustc_error_codes/src/error_codes.rs b/compiler/rustc_error_codes/src/error_codes.rs
index 9d5f4ad75..df857be85 100644
--- a/compiler/rustc_error_codes/src/error_codes.rs
+++ b/compiler/rustc_error_codes/src/error_codes.rs
@@ -5,7 +5,7 @@
 // /!\ IMPORTANT /!\
 //
 // Error messages' format must follow the RFC 1567 available here:
-// https://github.com/rust-lang/rfcs/pull/1567
+// https://rust-lang.github.io/rfcs/1567-long-error-codes-explanation-normalization.html
 
 register_diagnostics! {
 E0001: include_str!("./error_codes/E0001.md"),
@@ -253,6 +253,7 @@ E0466: include_str!("./error_codes/E0466.md"),
 E0468: include_str!("./error_codes/E0468.md"),
 E0469: include_str!("./error_codes/E0469.md"),
 E0472: include_str!("./error_codes/E0472.md"),
+E0476: include_str!("./error_codes/E0476.md"),
 E0477: include_str!("./error_codes/E0477.md"),
 E0478: include_str!("./error_codes/E0478.md"),
 E0482: include_str!("./error_codes/E0482.md"),
@@ -286,6 +287,7 @@ E0519: include_str!("./error_codes/E0519.md"),
 E0520: include_str!("./error_codes/E0520.md"),
 E0521: include_str!("./error_codes/E0521.md"),
 E0522: include_str!("./error_codes/E0522.md"),
+E0523: include_str!("./error_codes/E0523.md"),
 E0524: include_str!("./error_codes/E0524.md"),
 E0525: include_str!("./error_codes/E0525.md"),
 E0527: include_str!("./error_codes/E0527.md"),
@@ -506,10 +508,14 @@ E0785: include_str!("./error_codes/E0785.md"),
 E0786: include_str!("./error_codes/E0786.md"),
 E0787: include_str!("./error_codes/E0787.md"),
 E0788: include_str!("./error_codes/E0788.md"),
+E0789: include_str!("./error_codes/E0789.md"),
 E0790: include_str!("./error_codes/E0790.md"),
 E0791: include_str!("./error_codes/E0791.md"),
 E0792: include_str!("./error_codes/E0792.md"),
-;
+E0793: include_str!("./error_codes/E0793.md"),
+}
+
+// Undocumented removed error codes. Note that many removed error codes are documented.
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard
 //  E0019, // merged into E0015
@@ -566,7 +572,7 @@ E0792: include_str!("./error_codes/E0792.md"),
 //  E0246, // invalid recursive type
 //  E0247,
 //  E0248, // value used as a type, now reported earlier during resolution
-           // as E0412
+//         // as E0412
 //  E0249,
 //  E0257,
 //  E0258,
@@ -608,7 +614,6 @@ E0792: include_str!("./error_codes/E0792.md"),
 //  E0473, // dereference of reference outside its lifetime
 //  E0474, // captured variable `..` does not outlive the enclosing closure
 //  E0475, // index of slice outside its lifetime
-    E0476, // lifetime of the source pointer does not outlive lifetime bound...
 //  E0479, // the type `..` (provided as the value of a type parameter) is...
 //  E0480, // lifetime of method receiver does not outlive the method call
 //  E0481, // lifetime of function argument does not outlive the function call
@@ -620,7 +625,6 @@ E0792: include_str!("./error_codes/E0792.md"),
 //  E0488, // lifetime of variable does not enclose its declaration
 //  E0489, // type/lifetime parameter not in scope here
 //  E0490, // removed: unreachable
-    E0523, // two dependencies have same (crate-name, disambiguator) but different SVH
 //  E0526, // shuffle indices are not constant
 //  E0540, // multiple rustc_deprecated attributes
 //  E0548, // replaced with a generic attribute input check
@@ -629,14 +633,14 @@ E0792: include_str!("./error_codes/E0792.md"),
 //  E0558, // replaced with a generic attribute input check
 //  E0563, // cannot determine a type for this `impl Trait` removed in 6383de15
 //  E0564, // only named lifetimes are allowed in `impl Trait`,
-           // but `{}` was found in the type `{}`
+//         // but `{}` was found in the type `{}`
 //  E0598, // lifetime of {} is too short to guarantee its contents can be...
 //  E0611, // merged into E0616
 //  E0612, // merged into E0609
 //  E0613, // Removed (merged with E0609)
 //  E0629, // missing 'feature' (rustc_const_unstable)
 //  E0630, // rustc_const_unstable attribute must be paired with stable/unstable
-           // attribute
+//         // attribute
 //  E0645, // trait aliases not finished
 //  E0694, // an unknown tool name found in scoped attributes
 //  E0702, // replaced with a generic attribute input check
@@ -645,5 +649,3 @@ E0792: include_str!("./error_codes/E0792.md"),
 //  E0721, // `await` keyword
 //  E0723, // unstable feature in `const` context
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
-    E0789, // rustc_allowed_through_unstable_modules without stability attribute
-}
diff --git a/compiler/rustc_error_codes/src/error_codes/E0464.md b/compiler/rustc_error_codes/src/error_codes/E0464.md
index 9108d856c..209cbb00d 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0464.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0464.md
@@ -1,6 +1,21 @@
 The compiler found multiple library files with the requested crate name.
 
+```compile_fail
+// aux-build:crateresolve-1.rs
+// aux-build:crateresolve-2.rs
+// aux-build:crateresolve-3.rs
+
+extern crate crateresolve;
+//~^ ERROR multiple candidates for `rlib` dependency `crateresolve` found
+
+fn main() {}
+```
+
 This error can occur in several different cases -- for example, when using
 `extern crate` or passing `--extern` options without crate paths. It can also be
 caused by caching issues with the build directory, in which case `cargo clean`
 may help.
+
+In the above example, there are three different library files, all of which
+define the same crate name. Without providing a full path, there is no way for
+the compiler to know which crate it should use.
diff --git a/compiler/rustc_error_codes/src/error_codes/E0476.md b/compiler/rustc_error_codes/src/error_codes/E0476.md
new file mode 100644
index 000000000..fc141ba77
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0476.md
@@ -0,0 +1,21 @@
+The coerced type does not outlive the value being coerced to.
+
+Example of erroneous code:
+
+```compile_fail,E0476
+#![feature(coerce_unsized)]
+#![feature(unsize)]
+
+use std::marker::Unsize;
+use std::ops::CoerceUnsized;
+
+// error: lifetime of the source pointer does not outlive lifetime bound of the
+//        object type
+impl<'a, 'b, T, S> CoerceUnsized<&'a T> for &'b S where S: Unsize<T> {}
+```
+
+During a coercion, the "source pointer" (the coerced type) did not outlive the
+"object type" (value being coerced to). In the above example, `'b` is not a
+subtype of `'a`. This error can currently only be encountered with the unstable
+`CoerceUnsized` trait which allows custom coercions of unsized types behind a
+smart pointer to be implemented.
diff --git a/compiler/rustc_error_codes/src/error_codes/E0523.md b/compiler/rustc_error_codes/src/error_codes/E0523.md
new file mode 100644
index 000000000..0ddf70386
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0523.md
@@ -0,0 +1,25 @@
+#### Note: this error code is no longer emitted by the compiler.
+
+The compiler found multiple library files with the requested crate name.
+
+```compile_fail
+// aux-build:crateresolve-1.rs
+// aux-build:crateresolve-2.rs
+// aux-build:crateresolve-3.rs
+
+extern crate crateresolve;
+//~^ ERROR multiple candidates for `rlib` dependency `crateresolve` found
+
+fn main() {}
+```
+
+This error can occur in several different cases -- for example, when using
+`extern crate` or passing `--extern` options without crate paths. It can also be
+caused by caching issues with the build directory, in which case `cargo clean`
+may help.
+
+In the above example, there are three different library files, all of which
+define the same crate name. Without providing a full path, there is no way for
+the compiler to know which crate it should use.
+
+*Note that E0523 has been merged into E0464.*
diff --git a/compiler/rustc_error_codes/src/error_codes/E0587.md b/compiler/rustc_error_codes/src/error_codes/E0587.md
index ee9031dc3..d7998af85 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0587.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0587.md
@@ -11,6 +11,6 @@ You cannot use `packed` and `align` hints on a same type. If you want to pack a
 type to a given size, you should provide a size to packed:
 
 ```
-#[repr(packed)] // ok!
+#[repr(packed(8))] // ok!
 struct Umbrella(i32);
 ```
diff --git a/compiler/rustc_error_codes/src/error_codes/E0789.md b/compiler/rustc_error_codes/src/error_codes/E0789.md
new file mode 100644
index 000000000..89b7cd422
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0789.md
@@ -0,0 +1,30 @@
+#### This error code is internal to the compiler and will not be emitted with normal Rust code.
+
+The internal `rustc_allowed_through_unstable_modules` attribute must be used
+on an item with a `stable` attribute.
+
+Erroneous code example:
+
+```compile_fail,E0789
+// NOTE: both of these attributes are perma-unstable and should *never* be
+//       used outside of the compiler and standard library.
+#![feature(rustc_attrs)]
+#![feature(staged_api)]
+
+#![unstable(feature = "foo_module", reason = "...", issue = "123")]
+
+#[rustc_allowed_through_unstable_modules]
+// #[stable(feature = "foo", since = "1.0")]
+struct Foo;
+// ^^^ error: `rustc_allowed_through_unstable_modules` attribute must be
+//            paired with a `stable` attribute
+```
+
+Typically when an item is marked with a `stable` attribute, the modules that
+enclose the item must also be marked with `stable` attributes, otherwise the
+item becomes *de facto* unstable. `#[rustc_allowed_through_unstable_modules]`
+is a workaround which allows an item to "escape" its unstable parent modules.
+This error occurs when an item is marked with
+`#[rustc_allowed_through_unstable_modules]` but no supplementary `stable`
+attribute exists. See [#99288](https://github.com/rust-lang/rust/pull/99288)
+for an example of `#[rustc_allowed_through_unstable_modules]` in use.
diff --git a/compiler/rustc_error_codes/src/error_codes/E0793.md b/compiler/rustc_error_codes/src/error_codes/E0793.md
new file mode 100644
index 000000000..b2e51e24e
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0793.md
@@ -0,0 +1,64 @@
+An unaligned references to a field of a [packed] struct got created.
+
+Erroneous code example:
+
+```compile_fail,E0793
+#[repr(packed)]
+pub struct Foo {
+    field1: u64,
+    field2: u8,
+}
+
+unsafe {
+    let foo = Foo { field1: 0, field2: 0 };
+    // Accessing the field directly is fine.
+    let val = foo.field1;
+    // A reference to a packed field causes a error.
+    let val = &foo.field1; // ERROR
+    // An implicit `&` is added in format strings, causing the same error.
+    println!("{}", foo.field1); // ERROR
+}
+```
+
+Creating a reference to an insufficiently aligned packed field is
+[undefined behavior] and therefore disallowed. Using an `unsafe` block does not
+change anything about this. Instead, the code should do a copy of the data in
+the packed field or use raw pointers and unaligned accesses.
+
+```
+#[repr(packed)]
+pub struct Foo {
+    field1: u64,
+    field2: u8,
+}
+
+unsafe {
+    let foo = Foo { field1: 0, field2: 0 };
+
+    // Instead of a reference, we can create a raw pointer...
+    let ptr = std::ptr::addr_of!(foo.field1);
+    // ... and then (crucially!) access it in an explicitly unaligned way.
+    let val = unsafe { ptr.read_unaligned() };
+    // This would *NOT* be correct:
+    // let val = unsafe { *ptr }; // Undefined Behavior due to unaligned load!
+
+    // For formatting, we can create a copy to avoid the direct reference.
+    let copy = foo.field1;
+    println!("{}", copy);
+    // Creating a copy can be written in a single line with curly braces.
+    // (This is equivalent to the two lines above.)
+    println!("{}", { foo.field1 });
+}
+```
+
+### Additional information
+
+Note that this error is specifically about *references* to packed fields.
+Direct by-value access of those fields is fine, since then the compiler has
+enough information to generate the correct kind of access.
+
+See [issue #82523] for more information.
+
+[packed]: https://doc.rust-lang.org/reference/type-layout.html#the-alignment-modifiers
+[undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
+[issue #82523]: https://github.com/rust-lang/rust/issues/82523
diff --git a/compiler/rustc_error_codes/src/lib.rs b/compiler/rustc_error_codes/src/lib.rs
index bd424dd9d..d6b120e4d 100644
--- a/compiler/rustc_error_codes/src/lib.rs
+++ b/compiler/rustc_error_codes/src/lib.rs
@@ -5,10 +5,9 @@
 //! the goal being to make their maintenance easier.
 
 macro_rules! register_diagnostics {
-    ($($ecode:ident: $message:expr,)* ; $($code:ident,)*) => (
-        pub static DIAGNOSTICS: &[(&str, Option<&str>)] = &[
-            $( (stringify!($ecode), Some($message)), )*
-            $( (stringify!($code), None), )*
+    ($($ecode:ident: $message:expr,)*) => (
+        pub static DIAGNOSTICS: &[(&str, &str)] = &[
+            $( (stringify!($ecode), $message), )*
         ];
     )
 }