stage2: fix switch validation of handling all enum values

There were several problems, all fixed:
 * AstGen was storing field names as references to the original
   source code bytes. However, that data would be destroyed when the
   source file is updated. Now, it correctly stores the field names in
   the Decl arena for the enum. The same fix applies to error set field
   names.
 * Sema was missing a memset inside `analyzeSwitch`, leaving the "seen
   enum fields" array with undefined memory. Now that they are all
   properly set to null, the validation works.
 * Moved the "enum declared here" note to the end. It looked weird
   interrupting the notes for which enum values were missing.

src/Module.zig

@@ -4582,17 +4582,7 @@ pub fn optimizeMode(mod: Module) std.builtin.Mode {
 /// Otherwise, returns a reference to the source code bytes directly.
 /// See also `appendIdentStr` and `parseStrLit`.
 pub fn identifierTokenString(mod: *Module, scope: *Scope, token: ast.TokenIndex) InnerError![]const u8 {
-    return mod.identifierTokenStringTreeArena(scope, token, scope.tree(), scope.arena());
-}
-
-/// `scope` is only used for error reporting.
-pub fn identifierTokenStringTreeArena(
-    mod: *Module,
-    scope: *Scope,
-    token: ast.TokenIndex,
-    tree: *const ast.Tree,
-    arena: *Allocator,
-) InnerError![]const u8 {
+    const tree = scope.tree();
     const token_tags = tree.tokens.items(.tag);
     assert(token_tags[token] == .identifier);
     const ident_name = tree.tokenSlice(token);
@@ -4602,10 +4592,31 @@ pub fn identifierTokenStringTreeArena(
     var buf: ArrayListUnmanaged(u8) = .{};
     defer buf.deinit(mod.gpa);
     try parseStrLit(mod, scope, token, &buf, ident_name, 1);
-    const duped = try arena.dupe(u8, buf.items);
+    const duped = try scope.arena().dupe(u8, buf.items);
     return duped;
 }
 
+/// `scope` is only used for error reporting.
+/// The string is stored in `arena` regardless of whether it uses @"" syntax.
+pub fn identifierTokenStringTreeArena(
+    mod: *Module,
+    scope: *Scope,
+    token: ast.TokenIndex,
+    tree: *const ast.Tree,
+    arena: *Allocator,
+) InnerError![]u8 {
+    const token_tags = tree.tokens.items(.tag);
+    assert(token_tags[token] == .identifier);
+    const ident_name = tree.tokenSlice(token);
+    if (!mem.startsWith(u8, ident_name, "@")) {
+        return arena.dupe(u8, ident_name);
+    }
+    var buf: ArrayListUnmanaged(u8) = .{};
+    defer buf.deinit(mod.gpa);
+    try parseStrLit(mod, scope, token, &buf, ident_name, 1);
+    return arena.dupe(u8, buf.items);
+}
+
 /// Given an identifier token, obtain the string for it (possibly parsing as a string
 /// literal if it is @"" syntax), and append the string to `buf`.
 /// See also `identifierTokenString` and `parseStrLit`.

src/Sema.zig

@@ -2789,6 +2789,8 @@ fn analyzeSwitch(
             var seen_fields = try gpa.alloc(?AstGen.SwitchProngSrc, operand.ty.enumFieldCount());
             defer gpa.free(seen_fields);
 
+            mem.set(?AstGen.SwitchProngSrc, seen_fields, null);
+
             var extra_index: usize = special.end;
             {
                 var scalar_i: u32 = 0;
@@ -2849,12 +2851,6 @@ fn analyzeSwitch(
                                 .{},
                             );
                             errdefer msg.destroy(sema.gpa);
-                            try mod.errNoteNonLazy(
-                                operand.ty.declSrcLoc(),
-                                msg,
-                                "enum '{}' declared here",
-                                .{operand.ty},
-                            );
                             for (seen_fields) |seen_src, i| {
                                 if (seen_src != null) continue;
 
@@ -2865,10 +2861,16 @@ fn analyzeSwitch(
                                     &block.base,
                                     src,
                                     msg,
-                                    "unhandled enumeration value: '{s}",
+                                    "unhandled enumeration value: '{s}'",
                                     .{field_name},
                                 );
                             }
+                            try mod.errNoteNonLazy(
+                                operand.ty.declSrcLoc(),
+                                msg,
+                                "enum '{}' declared here",
+                                .{operand.ty},
+                            );
                             break :msg msg;
                         };
                         return mod.failWithOwnedErrorMsg(&block.base, msg);

test/stage2/cbe.zig

@@ -702,6 +702,21 @@ pub fn addCases(ctx: *TestContext) !void {
             ":3:15: error: enum 'E' has no tag with value 3",
             ":1:11: note: enum declared here",
         });
+
+        case.addError(
+            \\const E = enum { a, b, c };
+            \\export fn foo() void {
+            \\    var x: E = .a;
+            \\    switch (x) {
+            \\        .a => {},
+            \\        .c => {},
+            \\    }
+            \\}
+        , &.{
+            ":4:5: error: switch must handle all possibilities",
+            ":4:5: note: unhandled enumeration value: 'b'",
+            ":1:11: note: enum 'E' declared here",
+        });
     }
 
     ctx.c("empty start function", linux_x64,