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Limit the size of cgu names when using the `-Zhuman-readable-cgu-name… #141558

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Limit the size of cgu names when using the `-Zhuman-readable-cgu-name… #141558

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1 change: 1 addition & 0 deletions compiler/rustc_middle/src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -51,6 +51,7 @@
#![feature(negative_impls)]
#![feature(never_type)]
#![feature(ptr_alignment_type)]
#![feature(round_char_boundary)]
#![feature(rustc_attrs)]
#![feature(rustdoc_internals)]
#![feature(trusted_len)]
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26 changes: 25 additions & 1 deletion compiler/rustc_middle/src/mir/mono.rs
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@fmease fmease May 25, 2025 •
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I'm not a Windows dev but wouldn't it make sense to "just" use extended-length paths on Windows (whose length limit is 216/2-1 = 32,767) — prefix is \\?\. Docs

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Extended paths seem like the better solution to me as well since that means the logic to name the file will be the same regardless of platform.

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@Diggsey Diggsey May 25, 2025 •
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I think you are confusing paths with filenames. The limit here is a filename limit and is a limitation of the file-system, not just the OS.

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I don't think it's practical to determine the actual limit, since there are so many factors involved (file-system, OS, etc.) not to mention that when the object files are combined into an archive, the archive file format may have its own limit.

IMO it's much better to impose a simple limit that will work everywhere. Especially given that it only affects users of this niche feature, and only CGUs with abormally long filenames.

Original file line number Diff line number Diff line change
@@ -1,3 +1,4 @@
use std::borrow::Cow;
use std::fmt;
use std::hash::Hash;

Expand Down Expand Up @@ -468,6 +469,29 @@ impl<'tcx> CodegenUnit<'tcx> {
hash.as_u128().to_base_fixed_len(CASE_INSENSITIVE)
}

pub fn shorten_name(human_readable_name: &str) -> Cow<'_, str> {
// Set a limit a somewhat below the common platform limits for file names.
const MAX_CGU_NAME_LENGTH: usize = 200;
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Is this something we can query from the OS?

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This needs to be less than the actual filename limit, since the extension and other bits are added later.

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Even that aside, while on Windows we could use GetVolumeInformationByHandleW to query the maximum component length for local filesystems, the answer for both NTFS and ReFs is always 255 so I don't really see it as worth it. And I think it's very unlikely that it'll change in the near future for API reasons (there are some APIs that have a fixed-sized buffer for component names).

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Browsing the limits it looks like 255 is by far the most common amongst filesystems https://en.wikipedia.org/wiki/Comparison_of_file_systems#Limits

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A cgu name of 200 characters will still cause errors on Windows if the path to the directory it is in is 60 or more characters.

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@ChrisDenton ChrisDenton May 26, 2025 •
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As per the below conversation, verbatim paths can be used to workaround that. IIRC llvm handles long paths now and rustc does by virtue of using std APIs.

const TRUNCATED_NAME_PREFIX: &str = "-trunc-";
if human_readable_name.len() > MAX_CGU_NAME_LENGTH {
let mangled_name = Self::mangle_name(human_readable_name);
// Determine a safe byte offset to truncate the name to
let truncate_to = human_readable_name.floor_char_boundary(
MAX_CGU_NAME_LENGTH - TRUNCATED_NAME_PREFIX.len() - mangled_name.len(),
);
format!(
"{}{}{}",
&human_readable_name[..truncate_to],
TRUNCATED_NAME_PREFIX,
mangled_name
)
.into()
} else {
// If the name is short enough, we can just return it as is.
human_readable_name.into()
}
}

pub fn compute_size_estimate(&mut self) {
// The size of a codegen unit as the sum of the sizes of the items
// within it.
Expand Down Expand Up @@ -604,7 +628,7 @@ impl<'tcx> CodegenUnitNameBuilder<'tcx> {
let cgu_name = self.build_cgu_name_no_mangle(cnum, components, special_suffix);

if self.tcx.sess.opts.unstable_opts.human_readable_cgu_names {
cgu_name
Symbol::intern(&CodegenUnit::shorten_name(cgu_name.as_str()))
} else {
Symbol::intern(&CodegenUnit::mangle_name(cgu_name.as_str()))
}
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10 changes: 5 additions & 5 deletions compiler/rustc_monomorphize/src/partitioning.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -461,15 +461,15 @@ fn merge_codegen_units<'tcx>(

for cgu in codegen_units.iter_mut() {
if let Some(new_cgu_name) = new_cgu_names.get(&cgu.name()) {
if cx.tcx.sess.opts.unstable_opts.human_readable_cgu_names {
cgu.set_name(Symbol::intern(new_cgu_name));
let new_cgu_name = if cx.tcx.sess.opts.unstable_opts.human_readable_cgu_names {
Symbol::intern(&CodegenUnit::shorten_name(new_cgu_name))
} else {
// If we don't require CGU names to be human-readable,
// we use a fixed length hash of the composite CGU name
// instead.
let new_cgu_name = CodegenUnit::mangle_name(new_cgu_name);
cgu.set_name(Symbol::intern(&new_cgu_name));
}
Symbol::intern(&CodegenUnit::mangle_name(new_cgu_name))
};
cgu.set_name(new_cgu_name);
}
}

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