Support for rustup category home mode

[Cloud0310]

Related to #247.

Summary

Rustup historically stores configuration, state, data, and caches under a single RUSTUP_HOME (defaulting to $HOME/.rustup).
This prevents rustup from adhering to platform-standard directories (XDG on Unix, Known Folders on Windows).

This PR introduces an opt-in category-home layout guarded by RUSTUP_USE_CATEGORY_HOME, while preserving the legacy single-directory layout by default.

When category mode is enabled (RUSTUP_USE_CATEGORY_HOME=1 or any non-empty value other than "0"):

Category	Contents	Platform Default
Cache	downloads/, tmp/, update-hashes/	$XDG_CACHE_HOME/rustup or ~/.cache/rustup
Config	settings.toml	$XDG_CONFIG_HOME/rustup or ~/.config/rustup
Data	toolchains/, fallback/	$XDG_DATA_HOME/rustup or ~/.local/share/rustup
State	state.toml	$XDG_STATE_HOME/rustup or ~/.local/state/rustup
Bin	Proxies (rustc, cargo, etc.)	Unix: ~/.local/bin Windows: %USERPROFILE%/.local/bin (tentative)

Important

Why gated?
Gating both category overrides and platform defaults avoids split-brain installations where external tools (e.g. older rust-analyzer) still look exclusively in RUSTUP_HOME for toolchains.

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Resolution Precedence

Note

This order still needs discussion, upon whether we should consider RUSTUP_HOME and CARGO_HOME. See open questions.

Design Guidance

We consider this as a breaking change, and RUSTUP_HOME as a purely legacy env
var, so in new mode, we don't use it anymore, so, on category mode available,
the fallback paths are ignored.

Categories (Cache / Config / Data / State)

1. RUSTUP_<CATEGORY>_HOME (if category mode is enabled)
2. Platform category default (if category mode is enabled)
3. RUSTUP_HOME (legacy shared override)
4. $HOME/.rustup (legacy fallback)

Note

On Unix, explicit absolute XDG_variables take precedence over $HOME-derived paths. Empty or relative XDG values are ignored.
Windows does not consult XDG variables.

Bin Directory

1. RUSTUP_BIN_HOME (if category mode is enabled)
2. Platform default bin directory (if category mode is enabled: ~/.local/bin)
3. $CARGO_HOME/bin
4. $HOME/.cargo/bin

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Important Commits

feat(home): resolve category homes

Implements the core path resolution logic structured for future home crate compatibility across three layers:

1. Underlying platform resolution: Handles Unix XDG environment variables and Windows native directories.
2. home crate facade (mod home): Re-exports standard home APIs, implements RUSTUP_<CATEGORY>_HOME resolution and RUSTUP_HOME fallback without rollout logic.
3. Rollout strategy: Implements the gating logic driven by RUSTUP_USE_CATEGORY_HOME.

feat(uninstall): remove legacy and category rustup homes

Removes rustup home directories (config, cache, data, state) for both legacy and category layouts. (Excludes bin directory).

feat(uninstall): clean legacy and category cargo bin homes

Cleans up rustup-managed proxy binaries and symlinks across both legacy and category bin directories, leaving unrelated user binaries untouched.

feat(installer): migrate bin and env paths to split homes

Completes the installer-side migration from Cargo-owned paths to Rustup's resolved homes. In category mode, rustup binaries, proxy links, self-update artifacts, executable checks, and child-process PATH setup now use rustup_bin_home, while generated shell environment scripts use the config home through rustup_env_home. Legacy mode preserves the existing $CARGO_HOME layout.

The commit also updates shell profile integration, Windows PATH handling, installer messages, and end-to-end coverage for the split layout.

Open Questions

1. Windows Bin Location:

   • Currently set to %USERPROFILE%/.local/bin. Needs community consensus on whether this is the appropriate platform default for Windows.
   • Requires validating PATH precedence and migration UX from %USERPROFILE%\.cargo\bin.

2. home Crate Compatibility:

   • Should category resolution eventually be upstreamed into the home crate, or should rustup maintain its own internal implementation permanently?

3. RUSTUP_HOME is deprecated or not

   • Should we consider RUSTUP_HOME as an overall category override var?
   • Another option: consider RUSTUP_HOME as an overall category override variable,
     then resolution order is:
   1. RUSTUP_<CATEGORY>_HOME
   2. RUSTUP_HOME
   3. Platform category default

   overall variable. Then the the resolution order would be:

Following tasks

• [] Create a cli interface for migration, rustup migration category-mode, for
  creating symlinks in legacy dir, pointing to coorresponding category dirs.
• [] Add warning message for asking user for migrating to new category mode.
• [] Update user-guide for new mode

[Cloud0310]

Fixed a minor CI/CD problem on Windows.