Resolve#6152
## Summary
## Test Plan
Execution result of `cargo run generate-shell-completion --help`
```bash
Generate shell completion
Usage: uv generate-shell-completion <SHELL>
Arguments:
<SHELL> The shell to generate the completion script for [possible values: bash, elvish, fish, nushell, powershell, zsh]
```
Execution result of `cargo run help generate-shell-completion`
```bash
Generate shell completion
Usage: uv generate-shell-completion <SHELL>
Arguments:
<SHELL>
The shell to generate the completion script for
[possible values: bash, elvish, fish, nushell, powershell, zsh]
```
## Summary
Resolves https://github.com/astral-sh/uv/issues/4537
- First commit avoids overwriting dependencies with different markers.
- Second commit supports adding from requirements files.
## Test Plan
`cargo test`
## Summary
We now persist the `ResolverInstallerOptions` when writing out a tool
receipt. When upgrading, we grab the saved options, and merge with the
command-line arguments and user-level filesystem settings (CLI > receipt
> filesystem).
The loose consensus is that "fetch" doesn't have much meaning and that a
boolean flag makes more sense from the command line.
1. Adds `--allow-python-downloads` (hidden, default) and
`--no-python-downloads` to the CLI to quickly enable or disable
downloads
2. Deprecates `--python-fetch` in favor of the options from (1)
3. Removes `python-fetch` in favor of a `python-downloads` setting
5. Adds a `never` variant to the enum, allowing even explicit installs
to be disabled via the configuration file
## Test plan
I tested this with various `pyproject.toml`-level settings and `uv venv
--preview --python 3.12.2` and `uv python install 3.12.2` with and
without the new CLI flags.
## Summary
I think this seems reasonable... Otherwise, we might not go back to PyPI
to revalidate the list of available versions despite the user passing
`--upgrade`.
## Summary
Resolves#5188. Most of the changes involve creating a new function in
`tool/common.rs` to contain the common functionality previously found in
`tool/install.rs`.
## Test Plan
`cargo test`
```console
❯ ./target/debug/uv tool upgrade black
warning: `uv tool upgrade` is experimental and may change without warning.
Resolved 6 packages in 25ms
Uninstalled 1 package in 3ms
Installed 1 package in 19ms
- black==23.1.0
+ black==24.4.2
Installed 2 executables: black, blackd
```
Following #5869, the documentation has some less-than-helpful
suggestions to use `uv help python` for details — we should link to the
`uv python` section instead.
## Summary
Partially resolves#5561. Haven't added overrides support yet but I can
add it tomorrow if the current approach for constraints is ok.
## Test Plan
`cargo test`
Manually checked trace logs after changing the constraints.
