replaceVars: fail when exemption can't be found

This also changes stdenv's substitute --replace-fail to error out when
the replacement is the same as the search pattern, but can't be found.
This should not cause any problems in existing code, from what I can
tell from grepping nixpkgs.

The exception for pattern==replacement was previously introduced all the
way back in 5ff872aa24, but this was
apparently only used to make the check for the warning "simpler".

pkgs/build-support/replace-vars/replace-vars-with.nix

@@ -58,13 +58,11 @@
 
 let
   # We use `--replace-fail` instead of `--subst-var-by` so that if the thing isn't there, we fail.
-  subst-var-by =
-    name: value:
-    lib.optionals (value != null) [
-      "--replace-fail"
-      (lib.escapeShellArg "@${name}@")
-      (lib.escapeShellArg value)
-    ];
+  subst-var-by = name: value: [
+    "--replace-fail"
+    (lib.escapeShellArg "@${name}@")
+    (lib.escapeShellArg (lib.defaultTo "@${name}@" value))
+  ];
 
   substitutions = lib.concatLists (lib.mapAttrsToList subst-var-by replacements);
 

pkgs/stdenv/generic/setup.sh

@@ -1042,19 +1042,15 @@ substituteStream() {
                 pattern="$2"
                 replacement="$3"
                 shift 3
-                local savedvar
-                savedvar="${!var}"
-                eval "$var"'=${'"$var"'//"$pattern"/"$replacement"}'
-                if [ "$pattern" != "$replacement" ]; then
-                    if [ "${!var}" == "$savedvar" ]; then
-                        if [ "$replace_mode" == --replace-warn ]; then
-                            printf "substituteStream() in derivation $name: WARNING: pattern %q doesn't match anything in %s\n" "$pattern" "$description" >&2
-                        elif [ "$replace_mode" == --replace-fail ]; then
-                            printf "substituteStream() in derivation $name: ERROR: pattern %q doesn't match anything in %s\n" "$pattern" "$description" >&2
-                            return 1
-                        fi
+                if ! [[ "${!var}" == *"$pattern"* ]]; then
+                    if [ "$replace_mode" == --replace-warn ]; then
+                        printf "substituteStream() in derivation $name: WARNING: pattern %q doesn't match anything in %s\n" "$pattern" "$description" >&2
+                    elif [ "$replace_mode" == --replace-fail ]; then
+                        printf "substituteStream() in derivation $name: ERROR: pattern %q doesn't match anything in %s\n" "$pattern" "$description" >&2
+                        return 1
                     fi
                 fi
+                eval "$var"'=${'"$var"'//"$pattern"/"$replacement"}'
                 ;;
 
             --subst-var)

pkgs/test/replace-vars/default.nix

@@ -119,6 +119,30 @@ let
             grep -F "@c@" $failed/testBuildFailure.log
             ! grep -F "@b@" $failed/testBuildFailure.log
 
+            touch $out
+          '';
+
+      fails-in-check-phase-with-bad-exemption =
+        runCommand "replaceVars-fails"
+          {
+            failed =
+              let
+                src = builtins.toFile "source.txt" ''
+                  @a@
+                  @b@
+                '';
+              in
+              testBuildFailure (
+                callReplaceVars src {
+                  a = "a";
+                  b = null;
+                  c = null;
+                }
+              );
+          }
+          ''
+            grep -e "ERROR: pattern @c@ doesn't match anything in file.*source.txt" $failed/testBuildFailure.log
+
             touch $out
           '';
     };