set --show: Show the originally inherited value, if any

This adds a line to `set --show`s output like

```
$PATH: originally inherited as |/home/alfa/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl:/var/lib/flatpak/exports/bin|
```

to help with debugging.

Note that this means keeping an additional copy of the original
environment around. At most this would be one ARG_MAX's worth, which
is about 2M.

src/builtins/set.cpp

@@ -556,14 +556,22 @@ static int builtin_set_show(const wchar_t *cmd, const set_cmd_opts_t &opts, int
                             const wchar_t **argv, parser_t &parser, io_streams_t &streams) {
     UNUSED(opts);
     const auto &vars = parser.vars();
+    auto inheriteds = env_get_inherited();
     if (argc == 0) {  // show all vars
-        wcstring_list_t names = parser.vars().get_names(ENV_USER);
+        wcstring_list_t names = vars.get_names(ENV_USER);
         sort(names.begin(), names.end());
         for (const auto &name : names) {
             if (name == L"history") continue;
             show_scope(name.c_str(), ENV_LOCAL, streams, vars);
             show_scope(name.c_str(), ENV_GLOBAL, streams, vars);
             show_scope(name.c_str(), ENV_UNIVERSAL, streams, vars);
+
+            // Show the originally imported value as a debugging aid.
+            auto inherited = inheriteds.find(name);
+            if (inherited != inheriteds.end()) {
+                const wcstring escaped_val = escape_string(inherited->second, ESCAPE_NO_QUOTED, STRING_STYLE_SCRIPT);
+                streams.out.append_format(_(L"$%ls: originally inherited as |%ls|\n"), name.c_str(), escaped_val.c_str());
+            }
         }
     } else {
         for (int i = 0; i < argc; i++) {
@@ -585,6 +593,11 @@ static int builtin_set_show(const wchar_t *cmd, const set_cmd_opts_t &opts, int
             show_scope(arg, ENV_LOCAL, streams, vars);
             show_scope(arg, ENV_GLOBAL, streams, vars);
             show_scope(arg, ENV_UNIVERSAL, streams, vars);
+            auto inherited = inheriteds.find(arg);
+            if (inherited != inheriteds.end()) {
+                const wcstring escaped_val = escape_string(inherited->second, ESCAPE_NO_QUOTED, STRING_STYLE_SCRIPT);
+                streams.out.append_format(_(L"$%ls: originally inherited as |%ls|\n"), arg, escaped_val.c_str());
+            }
         }
     }
 

src/env.cpp

@@ -13,6 +13,7 @@
 
 #include <algorithm>
 #include <iterator>
+#include <map>
 #include <mutex>
 #include <set>
 #include <utility>
@@ -284,6 +285,12 @@ static void setup_path() {
     }
 }
 
+static std::map<wcstring, wcstring> inheriteds;
+
+const std::map<wcstring, wcstring> &env_get_inherited() {
+    return inheriteds;
+}
+
 void env_init(const struct config_paths_t *paths, bool do_uvars, bool default_paths) {
     env_stack_t &vars = env_stack_t::principal();
     // Import environment variables. Walk backwards so that the first one out of any duplicates wins
@@ -300,9 +307,11 @@ void env_init(const struct config_paths_t *paths, bool do_uvars, bool default_pa
             if (!electric_var_t::for_name(key_and_val)) {
                 vars.set_empty(key_and_val, ENV_EXPORT | ENV_GLOBAL);
             }
+            inheriteds[key] = L"";
         } else {
             key.assign(key_and_val, 0, eql);
             val.assign(key_and_val, eql + 1, wcstring::npos);
+            inheriteds[key] = val;
             if (!electric_var_t::for_name(key)) {
                 // fish_user_paths should not be exported; attempting to re-import it from
                 // a value we previously (due to user error) exported will cause impossibly

src/env.h

@@ -5,6 +5,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
+#include <map>
 #include <memory>
 #include <string>
 #include <unordered_map>
@@ -311,4 +312,7 @@ wcstring env_get_runtime_path();
 void setenv_lock(const char *name, const char *value, int overwrite);
 void unsetenv_lock(const char *name);
 
+/// Returns the originally inherited variables and their values.
+/// This is a simple key->value map and not e.g. cut into paths.
+const std::map<wcstring, wcstring> &env_get_inherited();
 #endif

tests/checks/set.fish

@@ -719,6 +719,7 @@ set -S PWD
 #CHECK: $PWD: set in global scope, exported, with 1 elements
 #CHECK: Variable is read-only
 #CHECK: $PWD[1]: |{{.*}}|
+#CHECK: $PWD: originally inherited as |{{.*}}|
 
 set -ql history
 echo $status
@@ -913,3 +914,10 @@ set -S status
 # CHECK: Variable is read-only
 # CHECK: $status[1]: |2|
 
+# See that we show inherited variables correctly:
+foo=bar $FISH -c 'set foo 1 2 3; set --show foo'
+# CHECK: $foo: set in global scope, exported, with 3 elements
+# CHECK: $foo[1]: |1|
+# CHECK: $foo[2]: |2|
+# CHECK: $foo[3]: |3|
+# CHECK: $foo: originally inherited as |bar|