diff --git a/tests/functional/lang.sh b/tests/functional/lang.sh
index 8cb8e98fb..5a8cefd13 100755
--- a/tests/functional/lang.sh
+++ b/tests/functional/lang.sh
@@ -50,11 +50,22 @@ set +x
 badDiff=0
 badExitCode=0
 
+# Extra post-processing that's specific to each test case
+postprocess() {
+    if [[ -e "lang/$1.postprocess" ]]; then
+        (
+            set -x;
+            "lang/$1.postprocess" "lang/$1"
+        )
+    fi
+}
+
 for i in lang/parse-fail-*.nix; do
     echo "parsing $i (should fail)";
     i=$(basename "$i" .nix)
     if expectStderr 1 nix-instantiate --parse - < "lang/$i.nix" > "lang/$i.err"
     then
+        postprocess "$i"
         diffAndAccept "$i" err err.exp
     else
         echo "FAIL: $i shouldn't parse"
@@ -71,6 +82,7 @@ for i in lang/parse-okay-*.nix; do
             2> "lang/$i.err"
     then
         sed "s!$(pwd)!/pwd!g" "lang/$i.out" "lang/$i.err"
+        postprocess "$i"
         diffAndAccept "$i" out exp
         diffAndAccept "$i" err err.exp
     else
@@ -94,6 +106,7 @@ for i in lang/eval-fail-*.nix; do
         expectStderr 1 nix-instantiate $flags "lang/$i.nix" \
             | sed "s!$(pwd)!/pwd!g" > "lang/$i.err"
     then
+        postprocess "$i"
         diffAndAccept "$i" err err.exp
     else
         echo "FAIL: $i shouldn't evaluate"
@@ -109,6 +122,7 @@ for i in lang/eval-okay-*.nix; do
         if expect 0 nix-instantiate --eval --xml --no-location --strict \
                 "lang/$i.nix" > "lang/$i.out.xml"
         then
+            postprocess "$i"
             diffAndAccept "$i" out.xml exp.xml
         else
             echo "FAIL: $i should evaluate"
@@ -129,6 +143,7 @@ for i in lang/eval-okay-*.nix; do
                 2> "lang/$i.err"
         then
             sed -i "s!$(pwd)!/pwd!g" "lang/$i.out" "lang/$i.err"
+            postprocess "$i"
             diffAndAccept "$i" out exp
             diffAndAccept "$i" err err.exp
         else
diff --git a/tests/functional/lang/eval-fail-derivation-name.err.exp b/tests/functional/lang/eval-fail-derivation-name.err.exp
index ae7b47712..0ef98674d 100644
--- a/tests/functional/lang/eval-fail-derivation-name.err.exp
+++ b/tests/functional/lang/eval-fail-derivation-name.err.exp
@@ -1,26 +1,26 @@
 error:
        … while evaluating the attribute 'outPath'
-         at <nix/derivation-internal.nix>:44:9:
-           43|       value = commonAttrs // {
-           44|         outPath = builtins.getAttr outputName strict;
+         at <nix/derivation-internal.nix>:<number>:<number>:
+     <number>|       value = commonAttrs // {
+     <number>|         outPath = builtins.getAttr outputName strict;
              |         ^
-           45|         drvPath = strict.drvPath;
+     <number>|         drvPath = strict.drvPath;
 
        … while calling the 'getAttr' builtin
-         at <nix/derivation-internal.nix>:44:19:
-           43|       value = commonAttrs // {
-           44|         outPath = builtins.getAttr outputName strict;
+         at <nix/derivation-internal.nix>:<number>:<number>:
+     <number>|       value = commonAttrs // {
+     <number>|         outPath = builtins.getAttr outputName strict;
              |                   ^
-           45|         drvPath = strict.drvPath;
+     <number>|         drvPath = strict.drvPath;
 
        … while calling the 'derivationStrict' builtin
-         at <nix/derivation-internal.nix>:34:12:
-           33|
-           34|   strict = derivationStrict drvAttrs;
+         at <nix/derivation-internal.nix>:<number>:<number>:
+     <number>|
+     <number>|   strict = derivationStrict drvAttrs;
              |            ^
-           35|
+     <number>|
 
        … while evaluating derivation '~jiggle~'
-         whose name attribute is located at /pwd/lang/eval-fail-derivation-name.nix:2:3
+         whose name attribute is located at /pwd/lang/eval-fail-derivation-name.nix:<number>:<number>
 
        error: invalid derivation name: name '~jiggle~' contains illegal character '~'. Please pass a different 'name'.
diff --git a/tests/functional/lang/eval-fail-derivation-name.postprocess b/tests/functional/lang/eval-fail-derivation-name.postprocess
new file mode 100755
index 000000000..ab9fa5b5d
--- /dev/null
+++ b/tests/functional/lang/eval-fail-derivation-name.postprocess
@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+set -euo pipefail
+testcaseBasename=$1
+
+# Line numbers change when derivation.nix docs are updated.
+sed -i "$testcaseBasename.err" \
+  -e 's/[0-9 ][0-9 ][0-9 ][0-9 ][0-9 ][0-9 ][0-9 ][0-9]\([^0-9]\)/<number>\1/g' \
+  -e 's/[0-9][0-9]*/<number>/g' \
+  ;