nix-shell: Look for shell.nix when directory is specified

src/libexpr/eval.cc

@@ -2650,7 +2650,7 @@ void EvalState::printStatistics()
 }
 
 
-SourcePath resolveExprPath(SourcePath path)
+SourcePath resolveExprPath(SourcePath path, bool addDefaultNix)
 {
     unsigned int followCount = 0, maxFollow = 1024;
 
@@ -2666,7 +2666,7 @@ SourcePath resolveExprPath(SourcePath path)
     }
 
     /* If `path' refers to a directory, append `/default.nix'. */
-    if (path.resolveSymlinks().lstat().type == SourceAccessor::tDirectory)
+    if (addDefaultNix && path.resolveSymlinks().lstat().type == SourceAccessor::tDirectory)
         return path / "default.nix";
 
     return path;

src/libexpr/eval.hh

@@ -850,8 +850,10 @@ std::string showType(const Value & v);
 
 /**
  * If `path` refers to a directory, then append "/default.nix".
+ *
+ * @param addDefaultNix Whether to append "/default.nix" after resolving symlinks.
  */
-SourcePath resolveExprPath(SourcePath path);
+SourcePath resolveExprPath(SourcePath path, bool addDefaultNix = true);
 
 /**
  * Whether a URI is allowed, assuming restrictEval is enabled

src/nix-build/nix-build.cc

@@ -90,6 +90,26 @@ static std::vector<std::string> shellwords(const std::string & s)
     return res;
 }
 
+/**
+ * Like `resolveExprPath`, but prefers `shell.nix` instead of `default.nix`,
+ * and if `path` was a directory, it checks eagerly whether `shell.nix` or
+ * `default.nix` exist, throwing an error if they don't.
+ */
+static SourcePath resolveShellExprPath(SourcePath path)
+{
+    auto resolvedOrDir = resolveExprPath(path, false);
+    if (resolvedOrDir.resolveSymlinks().lstat().type == SourceAccessor::tDirectory) {
+        if ((resolvedOrDir / "shell.nix").pathExists()) {
+            return resolvedOrDir / "shell.nix";
+        }
+        if ((resolvedOrDir / "default.nix").pathExists()) {
+            return resolvedOrDir / "default.nix";
+        }
+        throw Error("neither '%s' nor '%s' found in '%s'", "shell.nix", "default.nix", resolvedOrDir);
+    }
+    return resolvedOrDir;
+}
+
 static void main_nix_build(int argc, char * * argv)
 {
     auto dryRun = false;
@@ -281,11 +301,12 @@ static void main_nix_build(int argc, char * * argv)
         joined << "]; } \"\"";
         fromArgs = true;
         remainingArgs = {joined.str()};
-    } else if (!fromArgs) {
-        if (remainingArgs.empty() && isNixShell && pathExists("shell.nix"))
-            remainingArgs = {"shell.nix"};
-        if (remainingArgs.empty())
-            remainingArgs = {"default.nix"};
+    } else if (!fromArgs && remainingArgs.empty()) {
+        remainingArgs = {"."};
+
+        // Instead of letting it throw later, we throw here to give a more relevant error message
+        if (isNixShell && !std::filesystem::exists("shell.nix") && !std::filesystem::exists("default.nix"))
+            throw Error("no argument specified and no '%s' or '%s' file found in the working directory", "shell.nix", "default.nix");
     }
 
     if (isNixShell)
@@ -317,7 +338,8 @@ static void main_nix_build(int argc, char * * argv)
 
                     auto sourcePath = lookupFileArg(*state,
                                     baseDir);
-                    auto resolvedPath = resolveExprPath(sourcePath);
+                    auto resolvedPath =
+                        isNixShell ? resolveShellExprPath(sourcePath) : resolveExprPath(sourcePath);
 
                     exprs.push_back(state->parseExprFromFile(resolvedPath));
                 }

tests/functional/nix-shell.sh

@@ -91,6 +91,50 @@ sed -e "s|@ENV_PROG@|$(type -P env)|" shell.shebang.nix > $TEST_ROOT/shell.sheba
 chmod a+rx $TEST_ROOT/shell.shebang.nix
 $TEST_ROOT/shell.shebang.nix
 
+mkdir $TEST_ROOT/lookup-test $TEST_ROOT/empty
+
+cp $shellDotNix $TEST_ROOT/lookup-test/shell.nix
+cp config.nix $TEST_ROOT/lookup-test/
+echo 'abort "do not load default.nix!"' > $TEST_ROOT/lookup-test/default.nix
+
+nix-shell $TEST_ROOT/lookup-test -A shellDrv --run 'echo "it works"' | grepQuiet "it works"
+# https://github.com/NixOS/nix/issues/4529
+nix-shell -I "testRoot=$TEST_ROOT" '<testRoot/lookup-test>' -A shellDrv --run 'echo "it works"' | grepQuiet "it works"
+
+(
+  cd $TEST_ROOT/empty;
+  expectStderr 1 nix-shell | \
+    grepQuiet "error.*no argument specified and no .*shell\.nix.* or .*default\.nix.* file found in the working directory"
+)
+
+expectStderr 1 nix-shell -I "testRoot=$TEST_ROOT" '<testRoot/empty>' |
+  grepQuiet "error.*neither .*shell\.nix.* nor .*default\.nix.* found in .*/empty"
+
+cat >$TEST_ROOT/lookup-test/shebangscript <<"EOF"
+#!/usr/bin/env nix-shell
+#!nix-shell -A shellDrv -i bash
+[[ $VAR_FROM_NIX == bar ]]
+echo "script works"
+EOF
+chmod +x $TEST_ROOT/lookup-test/shebangscript
+
+$TEST_ROOT/lookup-test/shebangscript | grepQuiet "script works"
+
+# https://github.com/NixOS/nix/issues/5431
+mkdir $TEST_ROOT/marco{,/polo}
+echo 'abort "marco/shell.nix must not be used, but its mere existence used to cause #5431"' > $TEST_ROOT/marco/shell.nix
+cat >$TEST_ROOT/marco/polo/default.nix <<EOF
+#!/usr/bin/env nix-shell
+(import $TEST_ROOT/lookup-test/shell.nix {}).polo
+EOF
+chmod a+x $TEST_ROOT/marco/polo/default.nix
+(cd $TEST_ROOT/marco && ./polo/default.nix | grepQuiet "Polo")
+
+
+#####################
+# Flake equivalents #
+#####################
+
 # Test 'nix develop'.
 nix develop -f "$shellDotNix" shellDrv -c bash -c '[[ -n $stdenv ]]'
 

tests/functional/shell.nix

@@ -45,6 +45,17 @@ let pkgs = rec {
     TEST_inNixShell = if inNixShell then "true" else "false";
     inherit stdenv;
     outputs = ["dev" "out"];
+  } // {
+    shellHook = abort "Ignore non-drv shellHook attr";
+  };
+
+  # https://github.com/NixOS/nix/issues/5431
+  # See nix-shell.sh
+  polo = mkDerivation {
+    name = "polo";
+    shellHook = ''
+      echo Polo
+    '';
   };
 
   # Used by nix-shell -p