Feat: add mutable pointer type *mut T and *mut opaque

- Grammar: update pointer_type to support optional mut keyword;
  LL(1) verified (56 named rules, no conflicts)
- AST: update Type enum with mutable: bool field for Pointer and
  OpaquePointer variants
- Parser: consume optional mut token in parse_type; update all
  existing pointer tests; add 4 new mut pointer tests (85 pass)
- VSCode extension: add *mut capture-group pattern for syntax
  highlighting; update SYNTAX.md with pointer mutability table

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

GRAMMAR.ebnf

@@ -271,13 +271,20 @@ named_type = IDENT ;
 
 (* --- Pointer types ---                                            *)
 (*                                                                  *)
-(* "*" type    — typed pointer; the pointee type is known.          *)
-(* "*opaque"   — untyped/opaque pointer (no pointee type info).     *)
+(* "*" type       — immutable typed pointer (read-only through ptr) *)
+(* "*mut" type    — mutable typed pointer (read-write through ptr)  *)
+(* "*opaque"      — immutable untyped pointer (like C's const void* *)
+(* "*mut opaque"  — mutable untyped pointer (like C's void*         *)
 (*                                                                  *)
-(* LL(1) note: after "*", "opaque" is not in FIRST(type), so the    *)
-(* two alternatives are always distinguishable with one token.      *)
+(* LL(1) note: after "*", peek at next token:                       *)
+(*   "mut"    → mutable pointer; consume "mut", then parse          *)
+(*              "opaque" | type                                     *)
+(*   "opaque" → immutable opaque pointer                            *)
+(*   other    → immutable typed pointer; parse type directly        *)
+(* "mut" is a keyword; it is not in FIRST(type) and is distinct     *)
+(* from "opaque", so all three cases are unambiguous with one token.*)
 
-pointer_type = "*" , ( "opaque" | type ) ;
+pointer_type = "*" , [ "mut" ] , ( "opaque" | type ) ;
 
 
 (* --- Array types ---                                              *)

SYNTAX.md

@@ -291,20 +291,36 @@ Node         // struct Node { value: i64, next: *Node }
 
 ### Pointer Types
 
-A pointer type is written with a leading `*`.
+A pointer type is written with a leading `*`. The optional `mut` keyword following
+`*` marks the pointer as **mutable** — the pointee can be written through.
+Without `mut` the pointer is **immutable** — the pointee can only be read.
 
-| Syntax    | Description                                                                           |
-| --------- | ------------------------------------------------------------------------------------- |
-| `*T`      | Typed pointer — points to a value of type `T`                                         |
-| `*opaque` | Opaque pointer — no compile-time pointee type information; equivalent to C's `void *` |
+| Syntax        | C equivalent      | Description                                        |
+| ------------- | ----------------- | -------------------------------------------------- |
+| `*T`          | `const T *`       | Immutable typed pointer — read-only through ptr    |
+| `*mut T`      | `T *`             | Mutable typed pointer — read-write through ptr     |
+| `*opaque`     | `const void *`    | Immutable untyped pointer — no pointee type info   |
+| `*mut opaque` | `void *`          | Mutable untyped pointer — no pointee type info     |
 
-Pointer types may be nested: `**u8` is a pointer to a pointer to `u8`.
+Pointer mutability (`*mut`) and binding mutability (`let mut`) are **independent**:
+
+| Declaration               | Can reassign ptr? | Can write through ptr? |
+| ------------------------- | ----------------- | ---------------------- |
+| `let p: *T`               | no                | no                     |
+| `let mut p: *T`           | yes               | no                     |
+| `let p: *mut T`           | no                | yes                    |
+| `let mut p: *mut T`       | yes               | yes                    |
+
+Pointer types may be nested. The `mut` qualifier applies only to the outermost
+indirection; each `*` in the chain can independently carry `mut`.
 
 ```flux
-*u8          // pointer to u8
-**i32        // pointer to pointer to i32
-*opaque      // untyped pointer
-**opaque     // pointer to untyped pointer
+*u8              // immutable pointer to u8  (cannot write *p)
+*mut u8          // mutable pointer to u8    (can write *p)
+**u8             // immutable pointer to immutable pointer to u8
+*mut *mut u8     // mutable pointer to mutable pointer to u8
+*opaque          // immutable untyped pointer
+*mut opaque      // mutable untyped pointer
 ```
 
 ### Array Types
@@ -333,7 +349,7 @@ primitive_type = "u8" | "u16" | "u32" | "u64"
                | "i8" | "i16" | "i32" | "i64"
                | "f32" | "f64" | "bool" | "char" ;
 named_type     = IDENT ;
-pointer_type   = "*" , ( "opaque" | type ) ;
+pointer_type   = "*" , [ "mut" ] , ( "opaque" | type ) ;
 array_type     = "[" , type , ";" , INT_LIT , "]" ;
 ```
 

fluxc/src/ast.rs

@@ -76,10 +76,10 @@ pub enum Type {
     Char,
     // User-defined named type (e.g. a struct)
     Named(String, Span),
-    // Typed pointer: `*type`
-    Pointer(Box<Type>),
-    // Opaque (untyped) pointer: `*opaque`
-    OpaquePointer,
+    // Typed pointer: `*type` (immutable) or `*mut type` (mutable)
+    Pointer { mutable: bool, pointee: Box<Type> },
+    // Opaque (untyped) pointer: `*opaque` (immutable) or `*mut opaque` (mutable)
+    OpaquePointer { mutable: bool },
     // Fixed-size array: `[type; INT_LIT]`
     Array { elem: Box<Type>, size: String },
     // Error placeholder for recovery

fluxc/src/parser.rs

@@ -223,13 +223,19 @@ impl<'src> Parser<'src> {
             // Named type (user-defined struct, etc.)
             TokenKind::Ident => Type::Named(tok.text.to_owned(), tok.span),
 
-            // Pointer: `*opaque` or `*<type>`
+            // Pointer: `*[mut] opaque` or `*[mut] <type>`
             TokenKind::Star => {
+                let mutable = if self.current().kind == TokenKind::Mut {
+                    self.advance();
+                    true
+                } else {
+                    false
+                };
                 if self.current().kind == TokenKind::Opaque {
                     self.advance();
-                    Type::OpaquePointer
+                    Type::OpaquePointer { mutable }
                 } else {
-                    Type::Pointer(Box::new(self.parse_type()))
+                    Type::Pointer { mutable, pointee: Box::new(self.parse_type()) }
                 }
             }
 
@@ -1177,12 +1183,34 @@ mod tests {
 
     #[test]
     fn type_pointer() {
-        assert!(matches!(parse_type_str("*i32"), Type::Pointer(_)));
+        assert!(matches!(
+            parse_type_str("*i32"),
+            Type::Pointer { mutable: false, .. }
+        ));
+    }
+
+    #[test]
+    fn type_mut_pointer() {
+        assert!(matches!(
+            parse_type_str("*mut i32"),
+            Type::Pointer { mutable: true, .. }
+        ));
     }
 
     #[test]
     fn type_opaque_pointer() {
-        assert!(matches!(parse_type_str("*opaque"), Type::OpaquePointer));
+        assert!(matches!(
+            parse_type_str("*opaque"),
+            Type::OpaquePointer { mutable: false }
+        ));
+    }
+
+    #[test]
+    fn type_mut_opaque_pointer() {
+        assert!(matches!(
+            parse_type_str("*mut opaque"),
+            Type::OpaquePointer { mutable: true }
+        ));
     }
 
     #[test]
@@ -1194,8 +1222,17 @@ mod tests {
 
     #[test]
     fn type_nested_pointer() {
-        // `**i32` → Pointer(Pointer(I32))
-        assert!(matches!(parse_type_str("**i32"), Type::Pointer(_)));
+        // `**i32` → Pointer { Pointer { I32 } }
+        assert!(matches!(parse_type_str("**i32"), Type::Pointer { mutable: false, .. }));
+    }
+
+    #[test]
+    fn type_nested_mut_pointer() {
+        // `*mut *mut i32` → Pointer { mutable: true, Pointer { mutable: true, I32 } }
+        assert!(matches!(
+            parse_type_str("*mut *mut i32"),
+            Type::Pointer { mutable: true, .. }
+        ));
     }
 
     // ── Statement tests ───────────────────────────────────────────────────────
@@ -1466,7 +1503,7 @@ mod tests {
         let d = top("fn foo(p: *i32) { }");
         match &d.kind {
             TopLevelDefKind::Func(f) => {
-                assert!(matches!(f.params[0].ty, Type::Pointer(_)));
+                assert!(matches!(f.params[0].ty, Type::Pointer { mutable: false, .. }));
             }
             _ => panic!("expected func def"),
         }
@@ -1506,7 +1543,7 @@ mod tests {
         let d = top("struct Node { value: i32, next: *Node }");
         match &d.kind {
             TopLevelDefKind::Struct(s) => {
-                assert!(matches!(s.fields[1].ty, Type::Pointer(_)));
+                assert!(matches!(s.fields[1].ty, Type::Pointer { mutable: false, .. }));
             }
             _ => panic!("expected struct def"),
         }

vscode-flux/syntaxes/flux.tmLanguage.json

@@ -180,6 +180,14 @@
           "name": "keyword.operator.assignment.flux",
           "match": "(?<![=!<>+\\-*/%&|^])=(?!=)"
         },
+        {
+          "comment": "Mutable pointer type prefix *mut — must precede arithmetic *",
+          "match": "(\\*)(mut)\\b",
+          "captures": {
+            "1": { "name": "keyword.operator.type.pointer.flux" },
+            "2": { "name": "storage.modifier.mut.flux" }
+          }
+        },
         {
           "comment": "Arithmetic: + - * / %",
           "name": "keyword.operator.arithmetic.flux",