Add raw borrow operator as a contextual keyword (#586)

* Revert "Add keyword `raw` (#585)"

This reverts commit 6482f56.

The raw borrow operator is a weak keyword,
as such, it must only be highlighted in the right context.

Weak keywords:
https://doc.rust-lang.org/reference/keywords.html#weak-keywords

* Add `raw` borrow operator as a contextual keyword

The `raw` borrow operator is a contextual keyword.
Highlight it only in the context of borrows.
```rust
let mut x = 5;

// highlight "raw" here
let a = &raw const x;
let b = &raw mut x;

// do not highlight "raw" here
let raw = &x as *const _;
let raw_mut = &x as *mut _;
```

Raw borrow operator:
https://doc.rust-lang.org/reference/expressions/operator-expr.html#raw-borrow-operators
Rust weak keywords:
https://doc.rust-lang.org/reference/keywords.html#weak-keywords

Author: skr4n

rust-mode-tests.el

@@ -1979,6 +1979,28 @@ this_is_not_a_string();)"
      "union" font-lock-variable-name-face
      "bar" font-lock-type-face)))
 
+(ert-deftest rust-test-raw-context-sensitive ()
+  (rust-test-font-lock
+   "let raw = 7; let foo = &raw const raw; let bar = &raw mut raw;"
+   '("let" font-lock-keyword-face
+     ;; The first raw is a variable name.
+     "raw" font-lock-variable-name-face
+     "let" font-lock-keyword-face
+     "foo" font-lock-variable-name-face
+     "&" rust-ampersand-face
+     ;; The second raw is a contextual keyword.
+     "raw" font-lock-keyword-face
+     "const" font-lock-keyword-face
+     ;; The third raw is a value.
+     "let" font-lock-keyword-face
+     "bar" font-lock-variable-name-face
+     "&" rust-ampersand-face
+     ;; The fourth raw is a contextual keyword.
+     "raw" font-lock-keyword-face
+     "mut" font-lock-keyword-face
+     ;; The fifth raw is a value.
+     )))
+
 (ert-deftest indent-method-chains-no-align ()
   (let ((rust-indent-method-chain nil)) (test-indent
    "

rust-prog-mode.el

@@ -112,6 +112,14 @@ to the function arguments.  When nil, `->' will be indented one level."
      (or space line-start)
      (group symbol-start "union" symbol-end)
      (+ space) (regexp ,rust-re-ident))))
+(defconst rust-re-raw
+  (rx-to-string
+   `(seq
+     "&"
+     (zero-or-more space)
+     (group "raw")
+     (one-or-more space)
+     (or "const" "mut"))))
 
 (defun rust-re-item-def (itype)
   (concat (rust-re-word itype)
@@ -188,7 +196,7 @@ See `prettify-symbols-compose-predicate'."
     "let" "loop"
     "match" "mod" "move" "mut"
     "priv" "pub"
-    "raw" "ref" "return"
+    "ref" "return"
     "self" "static" "struct" "super"
     "true" "trait" "type" "try"
     "use"
@@ -269,6 +277,7 @@ Does not match type annotations of the form \"foo::<\"."
      ;; Contextual keywords
      ("\\_<\\(default\\)[[:space:]]+fn\\_>" 1 font-lock-keyword-face)
      (,rust-re-union 1 font-lock-keyword-face)
+     (,rust-re-raw 1 font-lock-keyword-face)
 
      ;; Special types
      (,(regexp-opt rust-special-types 'symbols) . font-lock-type-face)