Tracking issue for `--extern noprelude`

[ehuss]

This is a tracking issue for the noprelude CLI option for --extern.

Docs: https://doc.rust-lang.org/nightly/unstable-book/compiler-flags/extern-options.html

This option prevents adding the given crate to the extern prelude.

The syntax is [opts ':'] name ['=' path] where opts is a comma separated list of options:

--extern noprelude:core=/path/to/libcore.rlib

This is used by the build-std project to simulate compatibility with sysroot-only crates.

See also #98399 and #98400 which share this syntax extension.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implementation: Add options to --extern flag. #67074
• Adjust documentation (see instructions on rustc-dev-guide)
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• This probably shouldn't be stabilized until build-std is closer to stabilization, or there are other users where this is a clear benefit.

• The syntax for adding options to --extern flags was added without much bikeshedding or discussion. It may need adjusting to be more flexible for more complex options.

Implementation history

• Add options to --extern flag. #67074

[petrochenkov]

Stabilization blocked on "Tracking Issue --extern crate modifiers".

[jsgf]

This probably shouldn't be stabilized until build-std is closer to stabilization, or there are other users where this is a clear benefit.

I have a use-case when Rust is built with a non-Cargo build system, where all dependencies - including the prebuilt standard library - must be explicitly specified. This is implemented by invoking rustc with --sysroot /empty/directory --extern std=/path/to/libstd-XXX/rlib. Certain prelude crates (eg alloc) are assumed to be available but are not in the prelude by default, hence requiring --extern noprelude:alloc=/path/to/liballoc-XXX.rlib.

(I describe this in more detail here)