Improve new CLI UX by supporting short -E flag for --expr

Change-Id: I55881c846da8416a92a14deedfa5bbbf09a122fb

doc/manual/rl-next/short-expr-flag.md

@@ -0,0 +1,14 @@
+---
+synopsis: reintroduce shortened `-E` form for `--expr` to new CLI
+# prs: cl 605
+---
+
+In the past, it was possible to supply a shorter `-E` flag instead of fully
+specifying `--expr` every time you wished to provide an expression that would
+be evaluated to produce the given command's input. This was retained for the
+`--file` flag when the new CLI utilities were written with `-f`, but `-E` was
+dropped.
+
+We now restore the `-E` short form for better UX. This is most useful for
+`nix eval` but most any command that takes an Installable argument should benefit
+from it as well.

src/libcmd/installables.cc

@@ -187,6 +187,7 @@ SourceExprCommand::SourceExprCommand()
 
     addFlag({
         .longName = "expr",
+        .shortName = 'E',
         .description = "Interpret [*installables*](@docroot@/command-ref/new-cli/nix.md#installables) as attribute paths relative to the Nix expression *expr*.",
         .category = installablesCategory,
         .labels = {"expr"},

src/nix/eval.md

@@ -8,10 +8,10 @@ R""(
   # nix eval --expr '1 + 2'
   ```
 
-* Evaluate a Nix expression to JSON:
+* Evaluate a Nix expression to JSON using the short-form expression flag:
 
   ```console
-  # nix eval --json --expr '{ x = 1; }'
+  # nix eval --json -E '{ x = 1; }'
   {"x":1}
   ```
 

tests/functional/eval.sh

@@ -12,6 +12,7 @@ EOF
 [[ $testStdinHeredoc == '{ bar = 4; foo = 4; }' ]]
 
 nix eval --expr 'assert 1 + 2 == 3; true'
+nix eval -E 'assert 1 + 2 == 3; true'
 
 [[ $(nix eval int -f "./eval.nix") == 123 ]]
 [[ $(nix eval str -f "./eval.nix") == '"foo"' ]]