✨ Add rustup completions (#689)

Hi!
It's been a long time using nushell completions, It's about time I can
open a PR 😄
I couldn't finish it in one sitting, but I got the know-how, so I won't
be late to complete it. Left a bunch of `TODO`s

More completions coming!

Keep being awesome 🚀

custom-completions/rustup/rustup.nu

@@ -0,0 +1,201 @@
+# ------------------ nu-complete commands ------------------
+ 
+def "nu-complete rustup" [] {
+  ^rustup --help 
+  | str replace --regex --multiline '(rustup[\s\S]*(?=SUBCOMMANDS:))' '' 
+  | str replace --regex --multiline '\n+DISCUSSION[\s\S]*' ''
+  | lines 
+  | where $it starts-with "   " 
+  | parse -r '\s*(?P<value>[^ ]+) \s*(?P<description>\w.*)'
+}
+
+def "nu-complete rustup toolchain" [] {
+  ^rustup toolchain 
+  | str replace --regex --multiline '(rustup[\s\S]*(?=SUBCOMMANDS:))' '' 
+  | str replace --regex --multiline '\n+DISCUSSION[\s\S]*' ''
+  | lines 
+  | where $it starts-with "   " 
+  | parse -r '\s*(?P<value>[^ ]+) \s*(?P<description>\w.*)'
+}
+
+
+def "nu-complete rustup toolchain list" [] {
+  ^rustup toolchain list
+  | lines
+  # TODO: add `stable` `nightly` and any other starndard toolchain
+}
+
+def "nu-complete rustup target" [] {
+  ^rustup target 
+  | str replace --regex --multiline '(rustup[\s\S]*(?=SUBCOMMANDS:))' '' 
+  | str replace --regex --multiline '\n+DISCUSSION[\s\S]*' ''
+  | lines 
+  | where $it starts-with "   " 
+  | parse -r '\s*(?P<value>[^ ]+) \s*(?P<description>\w.*)'
+}
+
+def "nu-complete rustup target list" [] {
+  ^rustup target list
+  | lines
+}
+
+def "nu-complete rustup target list --installed" [] {
+  ^rustup target list --installed
+  | lines
+}
+
+def "nu-complete rustup update" [] {
+  ^rustup toolchain list
+  | lines
+}
+
+def "nu-complete rustup component" [] {
+  ^rustup component 
+  | str replace --regex --multiline '(rustup[\s\S]*(?=SUBCOMMANDS:))' '' 
+  | str replace --regex --multiline '\n+DISCUSSION[\s\S]*' ''
+  | lines 
+  | where $it starts-with "   " 
+  | parse -r '\s*(?P<value>[^ ]+) \s*(?P<description>\w.*)'
+}
+
+def "nu-complete rustup component list" [] {
+  ^rustup component list
+  | lines
+}
+
+def "nu-complete rustup component list installed" [] {
+  ^rustup component list 
+  | lines 
+  | find --regex '^.+(?=\s\(installed)'
+}
+
+def "nu-complete rustup override" [] {
+  ^rustup override 
+  | str replace --regex --multiline '(rustup[\s\S]*(?=SUBCOMMANDS:))' '' 
+  | str replace --regex --multiline '\n+DISCUSSION[\s\S]*' ''
+  | lines 
+  | where $it starts-with "   " 
+  | parse -r '\s*(?P<value>[^ ]+) \s*(?P<description>\w.*)'
+}
+
+def "nu-complete rustup override list" [] {
+  ^rustup override list
+  | lines
+  # TODO: I don't have any override to match
+  # TODO: exclude the result "no overrides"
+}
+
+def "nu-complete rustup override list installed" [] {
+  ^rustup override list 
+  | lines 
+  # TODO: I don't have any override to match
+  # TODO: exclude the result "no overrides"
+}
+
+def "nu-complete rustup self" [] {
+  ^rustup self 
+  | str replace --regex --multiline '(rustup[\s\S]*(?=SUBCOMMANDS:))' '' 
+  | str replace --regex --multiline '\n+DISCUSSION[\s\S]*' ''
+  | lines 
+  | where $it starts-with "   " 
+  | parse -r '\s*(?P<value>[^ ]+) \s*(?P<description>\w.*)'
+}
+
+# ------------------ export extern commands ------------------
+
+export extern "rustup" [
+   command?: string@"nu-complete rustup"
+    --verbose(-v)    # Enable verbose output
+    --quiet(-q)      # Disable progress output
+    --help(-h)       # Print help information
+    --version(-V)    # Print version information
+]
+
+export extern "rustup update" [
+  command?: string@"nu-complete rustup update"
+  --help(-h)        # Print help information
+  --force           # Force an update, even if some components are missing
+  --force-non-host  # Install toolchains that require an emulator. See https://github.com/rust-lang/rustup/wiki/Non-host-toolchains
+  --no-self-update  # Don't perform self update when running the `rustup update` command
+]
+
+export extern "rustup default" [
+  command?: string@"nu-complete rustup toolchain list"
+]
+
+export extern "rustup toolchain" [
+  command?: string@"nu-complete rustup toolchain"
+  --help(-h)        # Print help information
+]
+
+export extern "rustup toolchain uninstall" [
+  command?: string@"nu-complete rustup toolchain list"
+]
+
+export extern "rustup target" [
+  command?: string@"nu-complete rustup target"
+  --help(-h)        # Print help information
+]
+
+export extern "rustup target add" [
+  command?: string@"nu-complete rustup target list"
+  --toolchain       # Toolchain name, such as 'stable', 'nightly', or '1.8.0'. For more information see `rustup help toolchain`
+  --help(-h)        # Print help information
+]
+
+export extern "rustup target remove" [
+  command?: string@"nu-complete rustup target list --installed"
+]
+
+export extern "rustup component" [
+  command?: string@"nu-complete rustup component"
+  --help(-h)        # Print help information
+]
+
+export extern "rustup component add" [
+  command?: string@"nu-complete rustup component list"
+  --toolchain       # Toolchain name, such as 'stable', 'nightly', or '1.8.0'. For more information see `rustup help toolchain`
+  --target          # <target> TODO: explain this command a bit further
+  --help(-h)        # Print help information
+]
+
+export extern "rustup component remove" [
+  command?: string@"nu-complete rustup component list installed"
+  --toolchain       # Toolchain name, such as 'stable', 'nightly', or '1.8.0'. For more information see `rustup help toolchain`
+  --target          # <target> TODO: explain this command a bit further
+  --help(-h)        # Print help information
+]
+
+export extern "rustup override" [
+  command?: string@"nu-complete rustup override"
+  --help(-h)        # Print help information
+]
+
+export extern "rustup override set" [
+  command?: string@"nu-complete rustup override list"
+  --path            # Path to the directory
+  --help(-h)        # Print help information
+]
+
+export extern "rustup override unset" [
+  command?: string@"nu-complete rustup override list installed"
+  --path            # Path to the directory
+  --nonexistent     # Remove override toolchain for all nonexistent directories
+  --help(-h)        # Print help information
+]
+
+export extern "rustup run" [
+  command?: string@"nu-complete rustup toolchain list"
+]
+
+export extern "rustup self" [
+  command?: string@"nu-complete rustup self"
+]
+
+export extern "rustup set" [
+  # TODO
+]
+
+export extern "rustup completions" [
+  # TODO
+]