servo: Merge #18944 - Stop relying on linking details of std’s defaul…

…t allocator (from servo:jemallocator2); r=nox

We’ve been bitten before by symbol names changing: servo/heapsize#46, and upstream is planning to stop using jemalloc by default: rust-lang/rust#33082 (comment)

So use the (relatively) new `#[global_allocator]` attribute to explicitly select the system allocator on Windows and jemalloc (now in an external crate) on other platforms. This choice matches current defaults.

Source-Repo: https://github.com/servo/servo
Source-Revision: 07e9794306d597afe5d90d192fd32a99572c3cc3

UltraBlame original commit: 921526150768f9a2e9ba1586a350583f9ad025c9

servo/components/allocator/Cargo.toml

@@ -0,0 +1,125 @@
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+=
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+"
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+=
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+=
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+]
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+.
+0
+"
+publish
+=
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+=
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+lib
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+[
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+unstable
+=
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+kernel32
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+"
+jemallocator
+"
+]
+[
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+'
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+not
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+dependencies
+]
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+=
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+[
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+'
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+(
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+sys
+=
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+optional
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+}