Auto merge of #77039 - ecstatic-morse:rollup-qv3jj4a, r=ecstatic-morse

Rollup of 13 pull requests

Successful merges:

 - #72734 (Reduce duplicate in liballoc reserve error handling)
 - #76131 (Don't use `zip` to compare iterators during pretty-print hack)
 - #76150 (Don't recommend ManuallyDrop to customize drop order)
 - #76275 (Implementation of Write for some immutable ref structs)
 - #76489 (Add explanation for E0756)
 - #76581 (do not ICE on bound variables, return `TooGeneric` instead)
 - #76655 (Make some methods of `Pin` unstable const)
 - #76783 (Only get ImplKind::Impl once)
 - #76807 (Use const-checking to forbid use of unstable features in const-stable functions)
 - #76888 (use if let instead of single match arm expressions)
 - #76914 (extend `Ty` and `TyCtxt` lints to self types)
 - #77022 (Reduce boilerplate for BytePos and CharPos)
 - #77032 (lint missing docs for extern items)

Failed merges:

r? `@ghost`

2 files changed, 30 insertions, 1 deletions

diff --git a/compiler/rustc_error_codes/src/error_codes.rs b/compiler/rustc_error_codes/src/error_codes.rs
index 23a7b08016e..a202736ea6c 100644
--- a/compiler/rustc_error_codes/src/error_codes.rs
+++ b/compiler/rustc_error_codes/src/error_codes.rs
@@ -441,6 +441,7 @@ E0752: include_str!("./error_codes/E0752.md"),
 E0753: include_str!("./error_codes/E0753.md"),
 E0754: include_str!("./error_codes/E0754.md"),
 E0755: include_str!("./error_codes/E0755.md"),
+E0756: include_str!("./error_codes/E0756.md"),
 E0758: include_str!("./error_codes/E0758.md"),
 E0759: include_str!("./error_codes/E0759.md"),
 E0760: include_str!("./error_codes/E0760.md"),
@@ -633,7 +634,6 @@ E0774: include_str!("./error_codes/E0774.md"),
     E0722, // Malformed `#[optimize]` attribute
     E0726, // non-explicit (not `'_`) elided lifetime in unsupported position
 //  E0738, // Removed; errored on `#[track_caller] fn`s in `extern "Rust" { ... }`.
-    E0756, // `#[ffi_const]` is only allowed on foreign functions
     E0757, // `#[ffi_const]` functions cannot be `#[ffi_pure]`
     E0772, // `'static' obligation coming from `impl dyn Trait {}` or `impl Foo for dyn Bar {}`.
 }
diff --git a/compiler/rustc_error_codes/src/error_codes/E0756.md b/compiler/rustc_error_codes/src/error_codes/E0756.md
new file mode 100644
index 00000000000..ffdc421aab5
--- /dev/null
+++ b/compiler/rustc_error_codes/src/error_codes/E0756.md
@@ -0,0 +1,29 @@
+The `ffi_const` attribute was used on something other than a foreign function
+declaration.
+
+Erroneous code example:
+
+```compile_fail,E0756
+#![feature(ffi_const)]
+
+#[ffi_const] // error!
+pub fn foo() {}
+# fn main() {}
+```
+
+The `ffi_const` attribute can only be used on foreign function declarations
+which have no side effects except for their return value:
+
+```
+#![feature(ffi_const)]
+
+extern "C" {
+    #[ffi_const] // ok!
+    pub fn strlen(s: *const i8) -> i32;
+}
+# fn main() {}
+```
+
+You can get more information about it in the [unstable Rust Book].
+
+[unstable Rust Book]: https://doc.rust-lang.org/nightly/unstable-book/language-features/ffi-const.html