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When spawning a linker rustc has historically been known to blow OS limits for
the command line being too large, notably on Windows. This is especially true of
incremental compilation where there can be dozens of object files per
compilation. The compiler currently has logic for detecting a failure to spawn
and instead passing arguments via a file instead, but this failure detection
only triggers if a process actually fails to spawn.
Unfortunately on Windows we've got something else to worry about which is
`cmd.exe`. The compiler may be running a linker through `cmd.exe` where
`cmd.exe` has a limit of 8192 on the command line vs 32k on `CreateProcess`.
Moreso rustc actually succeeds in spawning `cmd.exe` today, it's just that after
it's running `cmd.exe` fails to spawn its child, which rustc doesn't currently
detect.
Consequently this commit updates the logic for the spawning the linker on
Windows to instead have a heuristic to see if we need to pass arguments via a
file. This heuristic is an overly pessimistic and "inaccurate" calculation which
just calls `len` on a bunch of `OsString` instances (where `len` is not
precisely the length in u16 elements). This number, when exceeding the 6k
threshold, will force rustc to always pass arguments through a file.
This strategy should avoid us trying to parse the output on Windows of the
linker to see if it successfully spawned yet failed to actually sub-spawn the
linker. We may just be passing arguments through files a little more commonly
now...
The motivation for this commit was a recent bug in Gecko [1] when beta testing,
notably when incremental compilation was enabled it blew out the limit on
`cmd.exe`. This commit will also fix #46999 as well though as emscripten uses a
bat script as well (and we're blowing the limit there).
[1]: https://bugzilla.mozilla.org/show_bug.cgi?id=1430886
Closes #46999
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