Ban duplicates of parity-uil-mem from being linked into the same program

parity-util-mem/CHANGELOG.md

@@ -5,6 +5,8 @@ The format is based on [Keep a Changelog].
 [Keep a Changelog]: http://keepachangelog.com/en/1.0.0/
 
 ## [Unreleased]
+### Breaking
+- Prevent multiple versions from being linked into the same program. [#363](https://github.com/paritytech/parity-common/pull/363)
 
 ## [0.6.0] - 2020-03-13
 - Updated dependencies. [#361](https://github.com/paritytech/parity-common/pull/361)

parity-util-mem/Cargo.toml

@@ -7,6 +7,13 @@ description = "Collection of memory related utilities"
 license = "MIT/Apache-2.0"
 edition = "2018"
 
+# Prevent multiple versions from being linked into the same program.
+links = "parity-util-mem-ban-duplicates"
+# `links` requires a build script to be present:
+# https://doc.rust-lang.org/cargo/reference/build-scripts.html#the-links-manifest-key
+# so we use an empty build script
+build = "build.rs"
+
 [dependencies]
 cfg-if = "0.1.10"
 dlmalloc = { version = "0.1.3", features = ["global"], optional = true }

parity-util-mem/README.md

@@ -2,6 +2,20 @@
 
 Collection of memory related utilities.
 
+## WARNING
+
+When `parity-util-mem` is used as a dependency with any of the global allocator features enabled,
+it must be the sole place where a global allocator is defined.
+The only exception to this rule is when used in a `no_std` context or when the `estimate-heapsize` feature is used.
+
+Because of that, it must be present in the dependency tree with a single version.
+Starting from version 0.7, having duplicate versions of `parity-util-mem` will lead
+to a compile-time error. It still will be possible to have 0.6 and 0.7 versions in the same binary though.
+
+Unless heeded you risk UB; see discussion in [issue 364].
+
+[issue 364]: https://github.com/paritytech/parity-common/issues/364
+
 ## Features
 
 - estimate-heapsize : Do not use allocator, but `size_of` or `size_of_val`.

parity-util-mem/build.rs

@@ -0,0 +1 @@
+fn main() {}