[ruby/prism] String literal hash keys should be frozen

String literal hash keys can't be mutated by the user so we should mark
them as frozen. We were seeing instructions for hashes with string
literal keys using two `putstring` instructions when it should be a
`putobject` and `putstring`.

Code example:

```ruby
{ "a" => "b" }
```

Instructions before:

```
"********* Ruby *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(2,14)>
0000 putobject                              "a"                       (   2)[Li]
0002 putstring                              "b"
0004 newhash                                2
0006 leave
"********* PRISM *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(1,14)>
0000 putstring                              "a"                       (   1)[Li]
0002 putstring                              "b"
0004 newhash                                2
0006 leave
```

Instructions after:

```
"********* Ruby *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(2,14)>
0000 putobject                              "a"                       (   2)[Li]
0002 putstring                              "b"
0004 newhash                                2
0006 leave

"********* PRISM *************"
== disasm: #<ISeq:<compiled>@<compiled>:1 (1,0)-(1,14)>
0000 putobject                              "a"                       (   1)[Li]
0002 putstring                              "b"
0004 newhash                                2
0006 leave
```

https://github.com/ruby/prism/commit/b14ae55385

3 files changed, 12 insertions, 7 deletions

diff --git a/prism/prism.c b/prism/prism.c
index a382a6b18c..ec319f2cac 100644
--- a/prism/prism.c
+++ b/prism/prism.c
@@ -1340,6 +1340,11 @@ pm_assoc_node_create(pm_parser_t *parser, pm_node_t *key, const pm_token_t *oper
         flags = key->flags & value->flags & PM_NODE_FLAG_STATIC_LITERAL;
     }
 
+    // Hash string keys should be frozen
+    if (PM_NODE_TYPE_P(key, PM_STRING_NODE)) {
+        key->flags |= PM_STRING_FLAGS_FROZEN;
+    }
+
     *node = (pm_assoc_node_t) {
         {
             .type = PM_ASSOC_NODE,
diff --git a/test/prism/snapshots/unparser/corpus/literal/literal.txt b/test/prism/snapshots/unparser/corpus/literal/literal.txt
index 21ae9f670b..a26b925196 100644
--- a/test/prism/snapshots/unparser/corpus/literal/literal.txt
+++ b/test/prism/snapshots/unparser/corpus/literal/literal.txt
@@ -9,7 +9,7 @@
         │   │   ├── @ AssocNode (location: (1,2)-(1,21))
         │   │   │   ├── key:
         │   │   │   │   @ StringNode (location: (1,2)-(1,7))
-        │   │   │   │   ├── flags: ∅
+        │   │   │   │   ├── flags: frozen
         │   │   │   │   ├── opening_loc: (1,2)-(1,3) = "\""
         │   │   │   │   ├── content_loc: (1,3)-(1,6) = "foo"
         │   │   │   │   ├── closing_loc: (1,6)-(1,7) = "\""
@@ -39,7 +39,7 @@
         │   │   └── @ AssocNode (location: (1,23)-(1,36))
         │   │       ├── key:
         │   │       │   @ StringNode (location: (1,23)-(1,28))
-        │   │       │   ├── flags: ∅
+        │   │       │   ├── flags: frozen
         │   │       │   ├── opening_loc: (1,23)-(1,24) = "\""
         │   │       │   ├── content_loc: (1,24)-(1,27) = "bar"
         │   │       │   ├── closing_loc: (1,27)-(1,28) = "\""
@@ -59,7 +59,7 @@
         │   │   ├── @ AssocNode (location: (4,2)-(4,14))
         │   │   │   ├── key:
         │   │   │   │   @ StringNode (location: (4,2)-(4,7))
-        │   │   │   │   ├── flags: ∅
+        │   │   │   │   ├── flags: frozen
         │   │   │   │   ├── opening_loc: (4,2)-(4,3) = "\""
         │   │   │   │   ├── content_loc: (4,3)-(4,6) = "foo"
         │   │   │   │   ├── closing_loc: (4,6)-(4,7) = "\""
@@ -75,7 +75,7 @@
         │   │   └── @ AssocNode (location: (4,16)-(4,29))
         │   │       ├── key:
         │   │       │   @ StringNode (location: (4,16)-(4,21))
-        │   │       │   ├── flags: ∅
+        │   │       │   ├── flags: frozen
         │   │       │   ├── opening_loc: (4,16)-(4,17) = "\""
         │   │       │   ├── content_loc: (4,17)-(4,20) = "bar"
         │   │       │   ├── closing_loc: (4,20)-(4,21) = "\""
@@ -184,7 +184,7 @@
         │   │   ├── @ AssocNode (location: (10,2)-(10,21))
         │   │   │   ├── key:
         │   │   │   │   @ StringNode (location: (10,2)-(10,7))
-        │   │   │   │   ├── flags: ∅
+        │   │   │   │   ├── flags: frozen
         │   │   │   │   ├── opening_loc: (10,2)-(10,3) = "\""
         │   │   │   │   ├── content_loc: (10,3)-(10,6) = "foo"
         │   │   │   │   ├── closing_loc: (10,6)-(10,7) = "\""
@@ -231,7 +231,7 @@
         │   │   ├── @ AssocNode (location: (13,2)-(13,14))
         │   │   │   ├── key:
         │   │   │   │   @ StringNode (location: (13,2)-(13,7))
-        │   │   │   │   ├── flags: ∅
+        │   │   │   │   ├── flags: frozen
         │   │   │   │   ├── opening_loc: (13,2)-(13,3) = "\""
         │   │   │   │   ├── content_loc: (13,3)-(13,6) = "foo"
         │   │   │   │   ├── closing_loc: (13,6)-(13,7) = "\""
diff --git a/test/prism/snapshots/unparser/corpus/literal/send.txt b/test/prism/snapshots/unparser/corpus/literal/send.txt
index 673787f1f1..edfd0cc6ae 100644
--- a/test/prism/snapshots/unparser/corpus/literal/send.txt
+++ b/test/prism/snapshots/unparser/corpus/literal/send.txt
@@ -1310,7 +1310,7 @@
         │   │               └── @ AssocNode (location: (64,13)-(64,25))
         │   │                   ├── key:
         │   │                   │   @ StringNode (location: (64,13)-(64,18))
-        │   │                   │   ├── flags: ∅
+        │   │                   │   ├── flags: frozen
         │   │                   │   ├── opening_loc: (64,13)-(64,14) = "\""
         │   │                   │   ├── content_loc: (64,14)-(64,17) = "baz"
         │   │                   │   ├── closing_loc: (64,17)-(64,18) = "\""