Description

This PR adds an alternative alias implementation. Old aliases still work but you need to use old-alias instead of alias.

Instead of replacing spans in the original code and re-parsing, which proved to be extremely error-prone and a constant source of panics, the new implementation creates a new command that references the old command. Consider the new alias defined as alias ll = ls -l. The parser creates a new command called ll and remembers that it is actually a ls command called with the -l flag. Then, when the parser sees the ll command, it will translate it to ls -l and passes to it any parameters that were passed to the call to ll. It works quite similar to how known externals defined with extern are implemented.

The new alias implementation should work the same way as the old aliases, including exporting from modules, referencing both known and unknown externals. It seems to preserve custom completions and pipeline metadata. It is quite robust in most cases but there are some rough edges (see later).

Fixes #7648 #8026 #7512 #5780 #7754

No effect: #8122 (we might revisit the completions code after this PR)

Should use custom command instead: #6048

User-Facing Changes

Since aliases are now basically commands, it has some new implications:

1. alias spam = "spam" (requires command call)
   • workaround: use alias spam = echo "spam"
2. def foo [] { 'foo' }; alias foo = ls -l (foo defined more than once)
   • workaround: use different name (commands also have this limitation)
3. alias ls = (ls | sort-by type name -i)
   • workaround: Use custom command. The common issue with this is that it is currently not easy to pass flags through custom commands and command referencing itself will lead to stack overflow. Both of these issues are meant to be addressed.
4. TODO: Help messages, which command, $nu.scope.aliases, etc.
   • Should we treat the aliases as commands or should they be separated from regular commands?
5. Needs better error message and syntax highlight for recursed alias (alias f = f)
6. Can't create alias with the same name as existing command (alias ls = ls -a)
   • Might be possible to add support for it (not 100% sure)
7. Standalone alias doesn't list aliases anymore
8. Can't alias parser keywords (e.g., stuff like alias ou = overlay use won't work)
   • TODO: Needs a better error message when attempting to do so

Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo test --workspace to check that all tests pass

After Submitting

If your PR had any user-facing changes, update the documentation after the PR is merged, if necessary. This will help us keep the docs up to date.