[rust] Update strategy for the default active target

Due to some updates in the Installation Profiles PR in Rustup
(rust-lang/rustup#1673), as of Rustup 1.20.0, there is no longer a
` (default)` ending indicator on the active target when calling `rustup
target list`. Looking at the update, it's not totally clear that this
was ever completely accurate so another strategy was in order.

This change, uses the knowledge of the current active toolchain (via
`rustup show active-toolchain`) to list all installed components for
that toolchain. The `rustc` component should be installed in an active
toolchain (although this is honestly a stated assumption which could be
invalidated), so if we strip off `rustc-` from the start of the
component name, we should be left with the full target triple of the
active toolchain, hence the "default active target".

Signed-off-by: Fletcher Nichol <[email protected]>

Author: fnichol

home/.vim/autoload/rust/detect.vim

@@ -1,7 +1,13 @@
 function! s:default_target() abort
-  return get(split(get(filter(systemlist(
-        \ 'rustup target list'), 'v:val =~? " (default)$"'), 0, ''), '\s\+'),
-        \ 0, '')
+  let l:active = rust#detect#RlsActiveToolchain()
+
+  " Use the current active toolchain to get a list of installed components, of
+  " which `rustc` should be installed. Knowing this, we'll strip off `rustc-`
+  " and what we're left with should be the full target triple of the active
+  " toolchain.
+  return substitute(get(filter(systemlist(
+        \ 'rustup component list --installed --toolchain '. l:active),
+        \ 'v:val =~? "^rustc-"'), 0, ''), 'rustc-', '', '')
 endfunction
 
 function! s:component_installed(toolchain, fq_component) abort