Add semantic tag for unresolved references

This is a quick way to implement unresolved reference diagnostics.
For example, adding to VS Code config

    "editor.tokenColorCustomizationsExperimental": {
        "unresolvedReference": "#FF0000"
    },

will highlight all unresolved refs in red.

4 files changed, 23 insertions, 20 deletions

diff --git a/crates/ra_ide/src/snapshots/highlighting.html b/crates/ra_ide/src/snapshots/highlighting.html
index 214dcbb62..ccb1fc751 100644
--- a/crates/ra_ide/src/snapshots/highlighting.html
+++ b/crates/ra_ide/src/snapshots/highlighting.html
@@ -50,12 +50,12 @@ pre                 { color: #DCDCCC; background: #3F3F3F; font-size: 22px; padd
 <span class="keyword">fn</span> <span class="function declaration">main</span>() {
     <span class="macro">println!</span>(<span class="string_literal">"Hello, {}!"</span>, <span class="numeric_literal">92</span>);
 
-    <span class="keyword">let</span> <span class="keyword">mut</span> <span class="variable declaration mutable">vec</span> = Vec::new();
+    <span class="keyword">let</span> <span class="keyword">mut</span> <span class="variable declaration mutable">vec</span> = <span class="unresolved_reference">Vec</span>::<span class="unresolved_reference">new</span>();
     <span class="keyword control">if</span> <span class="keyword">true</span> {
         <span class="keyword">let</span> <span class="variable declaration">x</span> = <span class="numeric_literal">92</span>;
-        <span class="variable mutable">vec</span>.push(<span class="struct">Foo</span> { <span class="field">x</span>, <span class="field">y</span>: <span class="numeric_literal">1</span> });
+        <span class="variable mutable">vec</span>.<span class="unresolved_reference">push</span>(<span class="struct">Foo</span> { <span class="field">x</span>, <span class="field">y</span>: <span class="numeric_literal">1</span> });
     }
-    <span class="keyword unsafe">unsafe</span> { <span class="variable mutable">vec</span>.set_len(<span class="numeric_literal">0</span>); }
+    <span class="keyword unsafe">unsafe</span> { <span class="variable mutable">vec</span>.<span class="unresolved_reference">set_len</span>(<span class="numeric_literal">0</span>); }
 
     <span class="keyword">let</span> <span class="keyword">mut</span> <span class="variable declaration mutable">x</span> = <span class="numeric_literal">42</span>;
     <span class="keyword">let</span> <span class="variable declaration mutable">y</span> = &<span class="keyword">mut</span> <span class="variable mutable">x</span>;
diff --git a/crates/ra_ide/src/snapshots/rainbow_highlighting.html b/crates/ra_ide/src/snapshots/rainbow_highlighting.html
index dddbfc0dd..3df82c45f 100644
--- a/crates/ra_ide/src/snapshots/rainbow_highlighting.html
+++ b/crates/ra_ide/src/snapshots/rainbow_highlighting.html
@@ -28,11 +28,11 @@ pre                 { color: #DCDCCC; background: #3F3F3F; font-size: 22px; padd
 </style>
 <pre><code><span class="keyword">fn</span> <span class="function declaration">main</span>() {
     <span class="keyword">let</span> <span class="variable declaration" data-binding-hash="8121853618659664005" style="color: hsl(261,57%,61%);">hello</span> = <span class="string_literal">"hello"</span>;
-    <span class="keyword">let</span> <span class="variable declaration" data-binding-hash="2705725358298919760" style="color: hsl(17,51%,74%);">x</span> = <span class="variable" data-binding-hash="8121853618659664005" style="color: hsl(261,57%,61%);">hello</span>.to_string();
-    <span class="keyword">let</span> <span class="variable declaration" data-binding-hash="3365759661443752373" style="color: hsl(127,76%,66%);">y</span> = <span class="variable" data-binding-hash="8121853618659664005" style="color: hsl(261,57%,61%);">hello</span>.to_string();
+    <span class="keyword">let</span> <span class="variable declaration" data-binding-hash="2705725358298919760" style="color: hsl(17,51%,74%);">x</span> = <span class="variable" data-binding-hash="8121853618659664005" style="color: hsl(261,57%,61%);">hello</span>.<span class="unresolved_reference">to_string</span>();
+    <span class="keyword">let</span> <span class="variable declaration" data-binding-hash="3365759661443752373" style="color: hsl(127,76%,66%);">y</span> = <span class="variable" data-binding-hash="8121853618659664005" style="color: hsl(261,57%,61%);">hello</span>.<span class="unresolved_reference">to_string</span>();
 
     <span class="keyword">let</span> <span class="variable declaration" data-binding-hash="794745962933817518" style="color: hsl(19,74%,76%);">x</span> = <span class="string_literal">"other color please!"</span>;
-    <span class="keyword">let</span> <span class="variable declaration" data-binding-hash="6717528807933952652" style="color: hsl(85,49%,84%);">y</span> = <span class="variable" data-binding-hash="794745962933817518" style="color: hsl(19,74%,76%);">x</span>.to_string();
+    <span class="keyword">let</span> <span class="variable declaration" data-binding-hash="6717528807933952652" style="color: hsl(85,49%,84%);">y</span> = <span class="variable" data-binding-hash="794745962933817518" style="color: hsl(19,74%,76%);">x</span>.<span class="unresolved_reference">to_string</span>();
 }
 
 <span class="keyword">fn</span> <span class="function declaration">bar</span>() {
diff --git a/crates/ra_ide/src/syntax_highlighting.rs b/crates/ra_ide/src/syntax_highlighting.rs
index 7b15b82bd..93d502875 100644
--- a/crates/ra_ide/src/syntax_highlighting.rs
+++ b/crates/ra_ide/src/syntax_highlighting.rs
@@ -239,20 +239,21 @@ fn highlight_element(
         NAME_REF if element.ancestors().any(|it| it.kind() == ATTR) => return None,
         NAME_REF => {
             let name_ref = element.into_node().and_then(ast::NameRef::cast).unwrap();
-            let name_kind = classify_name_ref(sema, &name_ref)?;
-
-            match name_kind {
-                NameRefClass::Definition(def) => {
-                    if let Definition::Local(local) = &def {
-                        if let Some(name) = local.name(db) {
-                            let shadow_count =
-                                bindings_shadow_count.entry(name.clone()).or_default();
-                            binding_hash = Some(calc_binding_hash(&name, *shadow_count))
-                        }
-                    };
-                    highlight_name(db, def)
-                }
-                NameRefClass::FieldShorthand { .. } => HighlightTag::Field.into(),
+            match classify_name_ref(sema, &name_ref) {
+                Some(name_kind) => match name_kind {
+                    NameRefClass::Definition(def) => {
+                        if let Definition::Local(local) = &def {
+                            if let Some(name) = local.name(db) {
+                                let shadow_count =
+                                    bindings_shadow_count.entry(name.clone()).or_default();
+                                binding_hash = Some(calc_binding_hash(&name, *shadow_count))
+                            }
+                        };
+                        highlight_name(db, def)
+                    }
+                    NameRefClass::FieldShorthand { .. } => HighlightTag::Field.into(),
+                },
+                None => HighlightTag::UnresolvedReference.into(),
             }
         }
 
diff --git a/crates/ra_ide/src/syntax_highlighting/tags.rs b/crates/ra_ide/src/syntax_highlighting/tags.rs
index e8b138e1a..f2c421654 100644
--- a/crates/ra_ide/src/syntax_highlighting/tags.rs
+++ b/crates/ra_ide/src/syntax_highlighting/tags.rs
@@ -38,6 +38,7 @@ pub enum HighlightTag {
     TypeParam,
     Union,
     Local,
+    UnresolvedReference,
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, Hash, PartialOrd, Ord)]
@@ -79,6 +80,7 @@ impl HighlightTag {
             HighlightTag::TypeParam => "type_param",
             HighlightTag::Union => "union",
             HighlightTag::Local => "variable",
+            HighlightTag::UnresolvedReference => "unresolved_reference",
         }
     }
 }