rust: Prefer primary spans

This would be great improvement to the current situation, where the
first span is used, which is quite often not the right one.

See rust-lang/rust.vim#174

Related to: neomake#2075

autoload/neomake/makers/ft/rust.vim

@@ -37,6 +37,8 @@ function! neomake#makers#ft#rust#cargo() abort
     return maker
 endfunction
 
+" NOTE: does not use process_json, since cargo outputs multiple JSON root
+" elements per line.
 function! neomake#makers#ft#rust#CargoProcessOutput(context) abort
     let errors = []
     for line in a:context['output']
@@ -64,6 +66,13 @@ function! neomake#makers#ft#rust#CargoProcessOutput(context) abort
         endif
 
         let span = data.spans[0]
+        for candidate_span in data.spans
+            if candidate_span.is_primary
+                let span = candidate_span
+                break
+            endif
+        endfor
+
         let expanded = 0
         let has_expansion = type(span.expansion) == type({})
                     \ && type(span.expansion.span) == type({})

tests/fixtures/rust/cargo_error_primary_span.json

@@ -0,0 +1,3 @@
+{"message":{"children":[],"code":{"code":"E0061","explanation":"\nThe number of arguments passed to a function must match the number of arguments\nspecified in the function signature.\n\nFor example, a function like:\n\n```\nfn f(a: u16, b: &str) {}\n```\n\nMust always be called with exactly two arguments, e.g. `f(2, \"test\")`.\n\nNote that Rust does not have a notion of optional function arguments or\nvariadic functions (except for its C-FFI).\n"},"level":"error","message":"this function takes 1 parameter but 2 parameters were supplied","rendered":"error[E0061]: this function takes 1 parameter but 2 parameters were supplied\n --> build/main.rs:6:5\n  |\n1 | fn foo(i : i32) {\n  | --------------- defined here\n...\n6 |     foo(1, 1);\n  |     ^^^^^^^^^ expected 1 parameter\n\n","spans":[{"byte_end":15,"byte_start":0,"column_end":16,"column_start":1,"expansion":null,"file_name":"build/main.rs","is_primary":false,"label":"defined here","line_end":1,"line_start":1,"suggested_replacement":null,"suggestion_applicability":null,"text":[{"highlight_end":16,"highlight_start":1,"text":"fn foo(i : i32) {"}]},{"byte_end":67,"byte_start":58,"column_end":14,"column_start":5,"expansion":null,"file_name":"build/main.rs","is_primary":true,"label":"expected 1 parameter","line_end":6,"line_start":6,"suggested_replacement":null,"suggestion_applicability":null,"text":[{"highlight_end":14,"highlight_start":5,"text":"    foo(1, 1);"}]}]},"package_id":"testfoo 0.1.0 (path+file:///home/dpc/tmp/testfoo)","reason":"compiler-message","target":{"crate_types":["bin"],"edition":"2015","kind":["bin"],"name":"testfoo","src_path":"/home/dpc/tmp/testfoo/build/main.rs"}}
+{"message":{"children":[],"code":null,"level":"error","message":"aborting due to previous error","rendered":"error: aborting due to previous error\n\n","spans":[]},"package_id":"testfoo 0.1.0 (path+file:///home/dpc/tmp/testfoo)","reason":"compiler-message","target":{"crate_types":["bin"],"edition":"2015","kind":["bin"],"name":"testfoo","src_path":"/home/dpc/tmp/testfoo/build/main.rs"}}
+{"message":{"children":[],"code":null,"level":"","message":"For more information about this error, try `rustc --explain E0061`.","rendered":"For more information about this error, try `rustc --explain E0061`.\n","spans":[]},"package_id":"testfoo 0.1.0 (path+file:///home/dpc/tmp/testfoo)","reason":"compiler-message","target":{"crate_types":["bin"],"edition":"2015","kind":["bin"],"name":"testfoo","src_path":"/home/dpc/tmp/testfoo/build/main.rs"}}

tests/ft_rust.vader

@@ -37,6 +37,22 @@ Execute (cargo error without span):
   AssertEqual [], getloclist(0)
   bwipe
 
+Execute (cargo: uses primary span):
+  new
+  file build/main.rs
+  let cargo = NeomakeTestsGetMakerWithOutput('neomake#makers#ft#rust#cargo',
+    \ 'tests/fixtures/rust/cargo_error_primary_span.json')
+  let cargo.cwd = getcwd()
+
+  CallNeomake 1, [cargo]
+
+  AssertEqualQf getloclist(0), [{
+  \ 'lnum': 6, 'bufnr': bufnr('%'), 'col': 5, 'valid': 1, 'vcol': 0, 'nr': 49,
+  \ 'type': 'E', 'pattern': '',
+  \ 'text': 'this function takes 1 parameter but 2 parameters were supplied: expected 1 parameter',
+  \ }]
+  bwipe
+
 Execute (rust: cargo: uses directory with Cargo.toml as cwd):
   let maker = neomake#makers#ft#rust#cargo()
   AssertEqual maker.cwd, '%:p:h'