6 files changed, 40 insertions, 25 deletions

diff --git a/crates/parser/src/grammar/items/consts.rs b/crates/parser/src/grammar/items/consts.rs
index eb7d1f828..12130df40 100644
--- a/crates/parser/src/grammar/items/consts.rs
+++ b/crates/parser/src/grammar/items/consts.rs
@@ -13,7 +13,7 @@ pub(super) fn konst(p: &mut Parser, m: Marker) {
 fn const_or_static(p: &mut Parser, m: Marker, kw: SyntaxKind, def: SyntaxKind) {
     assert!(p.at(kw));
     p.bump(kw);
-    p.eat(T![mut]); // FIXME: validator to forbid const mut
+    p.eat(T![mut]);
 
     // Allow `_` in place of an identifier in a `const`.
     let is_const_underscore = kw == T![const] && p.eat(T![_]);
diff --git a/crates/syntax/src/validation.rs b/crates/syntax/src/validation.rs
index 7694e8834..3e216fb70 100644
--- a/crates/syntax/src/validation.rs
+++ b/crates/syntax/src/validation.rs
@@ -1,4 +1,6 @@
-//! FIXME: write short doc here
+//! This module implements syntax validation that the parser doesn't handle.
+//!
+//! A failed validation emits a diagnostic.
 
 mod block;
 
@@ -92,6 +94,7 @@ pub(crate) fn validate(root: &SyntaxNode) -> Vec<SyntaxError> {
         match_ast! {
             match node {
                 ast::Literal(it) => validate_literal(it, &mut errors),
+                ast::Const(it) => validate_const(it, &mut errors),
                 ast::BlockExpr(it) => block::validate_block_expr(it, &mut errors),
                 ast::FieldExpr(it) => validate_numeric_name(it.name_ref(), &mut errors),
                 ast::RecordExprField(it) => validate_numeric_name(it.name_ref(), &mut errors),
@@ -362,3 +365,14 @@ fn validate_macro_rules(mac: ast::MacroRules, errors: &mut Vec<SyntaxError>) {
         ));
     }
 }
+
+fn validate_const(const_: ast::Const, errors: &mut Vec<SyntaxError>) {
+    if let Some(mut_token) = const_
+        .const_token()
+        .and_then(|t| t.next_token())
+        .and_then(|t| algo::skip_trivia_token(t, Direction::Next))
+        .filter(|t| t.kind() == T![mut])
+    {
+        errors.push(SyntaxError::new("const globals cannot be mutable", mut_token.text_range()));
+    }
+}
diff --git a/crates/syntax/test_data/parser/err/0047_mutable_const_item.rast b/crates/syntax/test_data/parser/err/0047_mutable_const_item.rast
new file mode 100644
index 000000000..c7eb312c9
--- /dev/null
+++ b/crates/syntax/test_data/parser/err/0047_mutable_const_item.rast
@@ -0,0 +1,22 @@
+[email protected]
+  [email protected]
+    [email protected] "const"
+    [email protected] " "
+    [email protected] "mut"
+    [email protected] " "
+    [email protected]
+      [email protected] "FOO"
+    [email protected] ":"
+    [email protected] " "
+    [email protected]
+      [email protected] "("
+      [email protected] ")"
+    [email protected] " "
+    [email protected] "="
+    [email protected] " "
+    [email protected]
+      [email protected] "("
+      [email protected] ")"
+    [email protected] ";"
+  [email protected] "\n"
+error 6..9: const globals cannot be mutable
diff --git a/crates/syntax/test_data/parser/err/0047_mutable_const_item.rs b/crates/syntax/test_data/parser/err/0047_mutable_const_item.rs
new file mode 100644
index 000000000..b34336f3f
--- /dev/null
+++ b/crates/syntax/test_data/parser/err/0047_mutable_const_item.rs
@@ -0,0 +1 @@
+const mut FOO: () = ();

diff --git a/crates/syntax/test_data/parser/ok/0024_const_item.rast b/crates/syntax/test_data/parser/ok/0024_const_item.rast
index dd1b9c9a0..b89ed6f98 100644
--- a/crates/syntax/test_data/parser/ok/0024_const_item.rast
+++ b/crates/syntax/test_data/parser/ok/0024_const_item.rast
@@ -1,4 +1,4 @@
-SOURCE_FILE@0..64
+SOURCE_FILE@0..39
   [email protected]
     [email protected] "const"
     [email protected] " "
@@ -36,24 +36,3 @@ [email protected]
       [email protected] "92"
     [email protected] ";"
   [email protected] "\n"
-  [email protected]
-    [email protected] "const"
-    [email protected] " "
-    [email protected] "mut"
-    [email protected] " "
-    [email protected]
-      [email protected] "BAR"
-    [email protected] ":"
-    [email protected] " "
-    [email protected]
-      [email protected]
-        [email protected]
-          [email protected]
-            [email protected] "u32"
-    [email protected] " "
-    [email protected] "="
-    [email protected] " "
-    [email protected]
-      [email protected] "62"
-    [email protected] ";"
-  [email protected] "\n"
diff --git a/crates/syntax/test_data/parser/ok/0024_const_item.rs b/crates/syntax/test_data/parser/ok/0024_const_item.rs
index a806a209d..1f2ffa0da 100644
--- a/crates/syntax/test_data/parser/ok/0024_const_item.rs
+++ b/crates/syntax/test_data/parser/ok/0024_const_item.rs
@@ -1,3 +1,2 @@
 const _: u32 = 0;
 const FOO: u32 = 92;
-const mut BAR: u32 = 62;