Make literal "{}" expand to itself

This caused major annoyances with e.g. `find -exec`, and it's utterly
useless - "{}" expands to nothing, so why have it at all?

Fixes #1109.

CHANGELOG.md

@@ -9,6 +9,7 @@ This section is for changes merged to the `major` branch that are not also merge
 - `read` now requires at least one var name (#4220).
 - `set x[1] x[2] a b` is no longer valid syntax (#4236).
 - For loop control variables are no longer local to the for block (#1935).
+- A literal `{}` now expands to itself, rather than nothing. This makes working with `find -exec` easier. (#1109, #4632)
 
 ## Notable fixes and improvements
 - `wait` builtin is added for waiting on processes (#4498).

doc_src/faq.hdr

@@ -24,7 +24,6 @@
 - <a href='#faq-titlebar'>I'm seeing weird output before each prompt when using screen. What's wrong?</a>
 - <a href='#faq-greeting'>How do I change the greeting message?</a>
 - <a href='#faq-history'>Why doesn't history substitution ("!$" etc.) work?</a>
-- <a href='#faq-find-braces'>Why do I get a missing argument error with `find ... {}`?</a>
 - <a href='#faq-cd-minus'>How can I use `-` as a shortcut for `cd -`?</a>
 - <a href='#faq-uninstalling'>How do I uninstall fish?</a>
 - <a href='#faq-third-party'>Where can I find extra tools for fish?</a>
@@ -244,21 +243,6 @@ Fish history recall is very simple yet effective:
 
 See <a href='index.html#editor'>documentation</a> for more details about line editing in fish.
 
-<hr>
-\section faq-find-braces Why do I get a missing argument error with `find ... {}`?
-
-Running `find ... -exec ... {}` produces an error:
-
-    find: missing argument to '-exec'
-
-The problem is caused by the empty braces, which are subject to <a href="index.html#expand-brace">brace expansion</a>.
-
-Quote the empty braces to achieve the desired effect:
-
-\fish{cli-dark}
-find ... -exec ... '{{}}'
-\endfish
-
 <hr>
 \section faq-cd-minus How can I use `-` as a shortcut for `cd -`?
 

doc_src/index.hdr.in

@@ -506,6 +506,14 @@ mv *.{c,h} src/
 # Moves all files with the suffix '.c' or '.h' to the subdirectory src.
 \endfish
 
+A literal "{}" will not be used as a brace expansion:
+
+\fish
+echo foo-{}
+# Outputs foo-{}
+
+echo foo-{$undefinedvar}
+# Output is an empty line - see <a href="#cartesian-product">the cartesian product section</a>
 
 \subsection expand-variable Variable expansion
 

src/expand.cpp

@@ -980,6 +980,15 @@ static expand_error_t expand_brackets(const wcstring &instr, expand_flags_t flag
         }
     }
 
+    // Expand a literal "{}" to itself because it is useless otherwise,
+    // and this eases e.g. `find -exec {}`. See #1109.
+    if (bracket_begin + 1 == bracket_end) {
+        wcstring newstr = instr;
+        newstr.at(bracket_begin - in) = L'{';
+        newstr.at(bracket_end - in) = L'}';
+        return expand_brackets(newstr, flags, out, errors);
+    }
+
     if (syntax_error) {
         append_syntax_error(errors, SOURCE_LOCATION_UNKNOWN, _(L"Mismatched brackets"));
         return EXPAND_ERROR;

tests/test1.in

@@ -15,6 +15,10 @@ logmsg Bracket expansion
 echo x-{1}
 echo x-{1,2}
 echo foo-{1,2{3,4}}
+echo foo-{} # literal "{}" expands to itself
+echo foo-{{},{}} # the inner "{}" expand to themselves, the outer pair expands normally.
+echo foo-{""} # still expands to foo-
+echo foo-{$undefinedvar} # still expands to nothing
 
 logmsg Escaped newlines
 echo foo\ bar

tests/test1.out

@@ -11,6 +11,11 @@
 x-1
 x-1 x-2
 foo-1 foo-23 foo-24
+foo-{}
+foo-{} foo-{}
+foo-
+
+foo- foo- foo- foo-
 
 ####################
 # Escaped newlines