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.

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>;

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="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>.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>.<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>() {

crates/ra_ide/src/syntax_highlighting.rs

@@ -239,9 +239,8 @@ 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 classify_name_ref(sema, &name_ref) {
-
+                Some(name_kind) => match name_kind {
-            match name_kind {
                     NameRefClass::Definition(def) => {
                         if let Definition::Local(local) = &def {
                             if let Some(name) = local.name(db) {
@@ -253,6 +252,8 @@ fn highlight_element(
                         highlight_name(db, def)
                     }
                     NameRefClass::FieldShorthand { .. } => HighlightTag::Field.into(),
+                },
+                None => HighlightTag::UnresolvedReference.into(),
             }
         }
 

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",
         }
     }
 }

crates/rust-analyzer/src/conv.rs

@@ -24,7 +24,9 @@ use crate::{
     world::WorldSnapshot,
     Result,
 };
-use semantic_tokens::{ATTRIBUTE, BUILTIN_TYPE, ENUM_MEMBER, LIFETIME, TYPE_ALIAS, UNION};
+use semantic_tokens::{
+    ATTRIBUTE, BUILTIN_TYPE, ENUM_MEMBER, LIFETIME, TYPE_ALIAS, UNION, UNRESOLVED_REFERENCE,
+};
 
 pub trait Conv {
     type Output;
@@ -373,6 +375,7 @@ impl Conv for Highlight {
             HighlightTag::Comment => SemanticTokenType::COMMENT,
             HighlightTag::Attribute => ATTRIBUTE,
             HighlightTag::Keyword => SemanticTokenType::KEYWORD,
+            HighlightTag::UnresolvedReference => UNRESOLVED_REFERENCE,
         };
 
         for modifier in self.modifiers.iter() {

crates/rust-analyzer/src/semantic_tokens.rs

@@ -10,6 +10,8 @@ pub(crate) const ENUM_MEMBER: SemanticTokenType = SemanticTokenType::new("enumMe
 pub(crate) const LIFETIME: SemanticTokenType = SemanticTokenType::new("lifetime");
 pub(crate) const TYPE_ALIAS: SemanticTokenType = SemanticTokenType::new("typeAlias");
 pub(crate) const UNION: SemanticTokenType = SemanticTokenType::new("union");
+pub(crate) const UNRESOLVED_REFERENCE: SemanticTokenType =
+    SemanticTokenType::new("unresolvedReference");
 
 pub(crate) const CONSTANT: SemanticTokenModifier = SemanticTokenModifier::new("constant");
 pub(crate) const CONTROL_FLOW: SemanticTokenModifier = SemanticTokenModifier::new("controlFlow");
@@ -43,6 +45,7 @@ pub(crate) const SUPPORTED_TYPES: &[SemanticTokenType] = &[
     LIFETIME,
     TYPE_ALIAS,
     UNION,
+    UNRESOLVED_REFERENCE,
 ];
 
 pub(crate) const SUPPORTED_MODIFIERS: &[SemanticTokenModifier] = &[

editors/code/package.json

@@ -517,6 +517,10 @@
                 "id": "union",
                 "description": "Style for C-style untagged unions",
                 "superType": "type"
+            },
+            {
+                "id": "unresolvedReference",
+                "description": "Style for names which can not be resolved due to compilation errors"
             }
         ],
         "semanticTokenModifiers": [