lib.fileset: Make error messages more uniform

Just minor changes like:
- Always using "X is a Y, but it should be Z"
- "X is a path that does not exist" rather than "X does not exist"
- Always using multi-line strings for errors
- Always quoting string-like values and not quoting path-like values
  - But do quote filesystem roots. Even though they're paths, they might
    be very small, good to have quotes to know the start/end
- Capitalise the first word
- Distinguish root vs filesystem root more

3 files changed, 47 insertions, 45 deletions

diff --git a/lib/fileset/default.nix b/lib/fileset/default.nix
index 547809cf063..fe7b304ba69 100644
--- a/lib/fileset/default.nix
+++ b/lib/fileset/default.nix
@@ -154,7 +154,7 @@ in {
     if ! isPath root then
       if isStringLike root then
         throw ''
-          lib.fileset.toSource: `root` ("${toString root}") is a string-like value, but it should be a path instead.
+          lib.fileset.toSource: `root` (${toString root}) is a string-like value, but it should be a path instead.
               Paths in strings are not supported by `lib.fileset`, use `lib.sources` or derivations instead.''
       else
         throw ''
@@ -163,13 +163,13 @@ in {
     # See also ../path/README.md
     else if ! fileset._internalIsEmptyWithoutBase && rootFilesystemRoot != filesetFilesystemRoot then
       throw ''
-        lib.fileset.toSource: Filesystem roots are not the same for `fileset` and `root` ("${toString root}"):
-            `root`: root "${toString rootFilesystemRoot}"
-            `fileset`: root "${toString filesetFilesystemRoot}"
-            Different roots are not supported.''
+        lib.fileset.toSource: Filesystem roots are not the same for `fileset` and `root` (${toString root}):
+            `root`: Filesystem root is "${toString rootFilesystemRoot}"
+            `fileset`: Filesystem root is "${toString filesetFilesystemRoot}"
+            Different filesystem roots are not supported.''
     else if ! pathExists root then
       throw ''
-        lib.fileset.toSource: `root` (${toString root}) does not exist.''
+        lib.fileset.toSource: `root` (${toString root}) is a path that does not exist.''
     else if pathType root != "directory" then
       throw ''
         lib.fileset.toSource: `root` (${toString root}) is a file, but it should be a directory instead. Potential solutions:
@@ -221,11 +221,11 @@ in {
     _unionMany
       (_coerceMany "lib.fileset.union" [
         {
-          context = "first argument";
+          context = "First argument";
           value = fileset1;
         }
         {
-          context = "second argument";
+          context = "Second argument";
           value = fileset2;
         }
       ]);
@@ -267,12 +267,13 @@ in {
     # which get [implicitly coerced to file sets](#sec-fileset-path-coercion).
     filesets:
     if ! isList filesets then
-      throw "lib.fileset.unions: Expected argument to be a list, but got a ${typeOf filesets}."
+      throw ''
+        lib.fileset.unions: Argument is of type ${typeOf filesets}, but it should be a list instead.''
     else
       pipe filesets [
         # Annotate the elements with context, used by _coerceMany for better errors
         (imap0 (i: el: {
-          context = "element ${toString i}";
+          context = "Element ${toString i}";
           value = el;
         }))
         (_coerceMany "lib.fileset.unions")
@@ -323,10 +324,11 @@ in {
     # The file set to filter based on the predicate function
     fileset:
     if ! isFunction predicate then
-      throw "lib.fileset.fileFilter: Expected the first argument to be a function, but it's a ${typeOf predicate} instead."
+      throw ''
+        lib.fileset.fileFilter: First argument is of type ${typeOf predicate}, but it should be a function.''
     else
       _fileFilter predicate
-        (_coerce "lib.fileset.fileFilter: second argument" fileset);
+        (_coerce "lib.fileset.fileFilter: Second argument" fileset);
 
   /*
     The file set containing all files that are in both of two given file sets.
@@ -354,11 +356,11 @@ in {
     let
       filesets = _coerceMany "lib.fileset.intersection" [
         {
-          context = "first argument";
+          context = "First argument";
           value = fileset1;
         }
         {
-          context = "second argument";
+          context = "Second argument";
           value = fileset2;
         }
       ];
@@ -406,11 +408,11 @@ in {
     let
       filesets = _coerceMany "lib.fileset.difference" [
         {
-          context = "first argument (positive set)";
+          context = "First argument (positive set)";
           value = positive;
         }
         {
-          context = "second argument (negative set)";
+          context = "Second argument (negative set)";
           value = negative;
         }
       ];
@@ -454,7 +456,7 @@ in {
     let
       # "fileset" would be a better name, but that would clash with the argument name,
       # and we cannot change that because of https://github.com/nix-community/nixdoc/issues/76
-      actualFileset = _coerce "lib.fileset.trace: argument" fileset;
+      actualFileset = _coerce "lib.fileset.trace: Argument" fileset;
     in
     seq
       (_printFileset actualFileset)
@@ -501,7 +503,7 @@ in {
     let
       # "fileset" would be a better name, but that would clash with the argument name,
       # and we cannot change that because of https://github.com/nix-community/nixdoc/issues/76
-      actualFileset = _coerce "lib.fileset.traceVal: argument" fileset;
+      actualFileset = _coerce "lib.fileset.traceVal: Argument" fileset;
     in
     seq
       (_printFileset actualFileset)
diff --git a/lib/fileset/internal.nix b/lib/fileset/internal.nix
index b919a5de3ee..853115df9f6 100644
--- a/lib/fileset/internal.nix
+++ b/lib/fileset/internal.nix
@@ -179,7 +179,7 @@ rec {
           ${context} is of type ${typeOf value}, but it should be a file set or a path instead.''
     else if ! pathExists value then
       throw ''
-        ${context} (${toString value}) does not exist.''
+        ${context} (${toString value}) is a path that does not exist.''
     else
       _singleton value;
 
@@ -208,9 +208,9 @@ rec {
     if firstWithBase != null && differentIndex != null then
       throw ''
         ${functionContext}: Filesystem roots are not the same:
-            ${(head list).context}: root "${toString firstBaseRoot}"
-            ${(elemAt list differentIndex).context}: root "${toString (elemAt filesets differentIndex)._internalBaseRoot}"
-            Different roots are not supported.''
+            ${(head list).context}: Filesystem root is "${toString firstBaseRoot}"
+            ${(elemAt list differentIndex).context}: Filesystem root is "${toString (elemAt filesets differentIndex)._internalBaseRoot}"
+            Different filesystem roots are not supported.''
     else
       filesets;
 
diff --git a/lib/fileset/tests.sh b/lib/fileset/tests.sh
index 2df0727bde3..c1c67800f5e 100755
--- a/lib/fileset/tests.sh
+++ b/lib/fileset/tests.sh
@@ -318,7 +318,7 @@ checkFileset() {
 #### Error messages #####
 
 # Absolute paths in strings cannot be passed as `root`
-expectFailure 'toSource { root = "/nix/store/foobar"; fileset = ./.; }' 'lib.fileset.toSource: `root` \("/nix/store/foobar"\) is a string-like value, but it should be a path instead.
+expectFailure 'toSource { root = "/nix/store/foobar"; fileset = ./.; }' 'lib.fileset.toSource: `root` \(/nix/store/foobar\) is a string-like value, but it should be a path instead.
 \s*Paths in strings are not supported by `lib.fileset`, use `lib.sources` or derivations instead.'
 
 # Only paths are accepted as `root`
@@ -328,14 +328,14 @@ expectFailure 'toSource { root = 10; fileset = ./.; }' 'lib.fileset.toSource: `r
 mkdir -p {foo,bar}/mock-root
 expectFailure 'with ((import <nixpkgs/lib>).extend (import <nixpkgs/lib/fileset/mock-splitRoot.nix>)).fileset;
   toSource { root = ./foo/mock-root; fileset = ./bar/mock-root; }
-' 'lib.fileset.toSource: Filesystem roots are not the same for `fileset` and `root` \("'"$work"'/foo/mock-root"\):
-\s*`root`: root "'"$work"'/foo/mock-root"
-\s*`fileset`: root "'"$work"'/bar/mock-root"
-\s*Different roots are not supported.'
+' 'lib.fileset.toSource: Filesystem roots are not the same for `fileset` and `root` \('"$work"'/foo/mock-root\):
+\s*`root`: Filesystem root is "'"$work"'/foo/mock-root"
+\s*`fileset`: Filesystem root is "'"$work"'/bar/mock-root"
+\s*Different filesystem roots are not supported.'
 rm -rf -- *
 
 # `root` needs to exist
-expectFailure 'toSource { root = ./a; fileset = ./.; }' 'lib.fileset.toSource: `root` \('"$work"'/a\) does not exist.'
+expectFailure 'toSource { root = ./a; fileset = ./.; }' 'lib.fileset.toSource: `root` \('"$work"'/a\) is a path that does not exist.'
 
 # `root` needs to be a file
 touch a
@@ -367,7 +367,7 @@ expectFailure 'toSource { root = ./.; fileset = "/some/path"; }' 'lib.fileset.to
 \s*Paths represented as strings are not supported by `lib.fileset`, use `lib.sources` or derivations instead.'
 
 # Path coercion errors for non-existent paths
-expectFailure 'toSource { root = ./.; fileset = ./a; }' 'lib.fileset.toSource: `fileset` \('"$work"'/a\) does not exist.'
+expectFailure 'toSource { root = ./.; fileset = ./a; }' 'lib.fileset.toSource: `fileset` \('"$work"'/a\) is a path that does not exist.'
 
 # File sets cannot be evaluated directly
 expectFailure 'union ./. ./.' 'lib.fileset: Directly evaluating a file set is not supported.
@@ -490,26 +490,26 @@ mkdir -p {foo,bar}/mock-root
 expectFailure 'with ((import <nixpkgs/lib>).extend (import <nixpkgs/lib/fileset/mock-splitRoot.nix>)).fileset;
   toSource { root = ./.; fileset = union ./foo/mock-root ./bar/mock-root; }
 ' 'lib.fileset.union: Filesystem roots are not the same:
-\s*first argument: root "'"$work"'/foo/mock-root"
-\s*second argument: root "'"$work"'/bar/mock-root"
-\s*Different roots are not supported.'
+\s*First argument: Filesystem root is "'"$work"'/foo/mock-root"
+\s*Second argument: Filesystem root is "'"$work"'/bar/mock-root"
+\s*Different filesystem roots are not supported.'
 
 expectFailure 'with ((import <nixpkgs/lib>).extend (import <nixpkgs/lib/fileset/mock-splitRoot.nix>)).fileset;
   toSource { root = ./.; fileset = unions [ ./foo/mock-root ./bar/mock-root ]; }
 ' 'lib.fileset.unions: Filesystem roots are not the same:
-\s*element 0: root "'"$work"'/foo/mock-root"
-\s*element 1: root "'"$work"'/bar/mock-root"
-\s*Different roots are not supported.'
+\s*Element 0: Filesystem root is "'"$work"'/foo/mock-root"
+\s*Element 1: Filesystem root is "'"$work"'/bar/mock-root"
+\s*Different filesystem roots are not supported.'
 rm -rf -- *
 
 # Coercion errors show the correct context
-expectFailure 'toSource { root = ./.; fileset = union ./a ./.; }' 'lib.fileset.union: first argument \('"$work"'/a\) does not exist.'
-expectFailure 'toSource { root = ./.; fileset = union ./. ./b; }' 'lib.fileset.union: second argument \('"$work"'/b\) does not exist.'
-expectFailure 'toSource { root = ./.; fileset = unions [ ./a ./. ]; }' 'lib.fileset.unions: element 0 \('"$work"'/a\) does not exist.'
-expectFailure 'toSource { root = ./.; fileset = unions [ ./. ./b ]; }' 'lib.fileset.unions: element 1 \('"$work"'/b\) does not exist.'
+expectFailure 'toSource { root = ./.; fileset = union ./a ./.; }' 'lib.fileset.union: First argument \('"$work"'/a\) is a path that does not exist.'
+expectFailure 'toSource { root = ./.; fileset = union ./. ./b; }' 'lib.fileset.union: Second argument \('"$work"'/b\) is a path that does not exist.'
+expectFailure 'toSource { root = ./.; fileset = unions [ ./a ./. ]; }' 'lib.fileset.unions: Element 0 \('"$work"'/a\) is a path that does not exist.'
+expectFailure 'toSource { root = ./.; fileset = unions [ ./. ./b ]; }' 'lib.fileset.unions: Element 1 \('"$work"'/b\) is a path that does not exist.'
 
 # unions needs a list
-expectFailure 'toSource { root = ./.; fileset = unions null; }' 'lib.fileset.unions: Expected argument to be a list, but got a null.'
+expectFailure 'toSource { root = ./.; fileset = unions null; }' 'lib.fileset.unions: Argument is of type null, but it should be a list instead.'
 
 # The tree of later arguments should not be evaluated if a former argument already includes all files
 tree=()
@@ -603,14 +603,14 @@ mkdir -p {foo,bar}/mock-root
 expectFailure 'with ((import <nixpkgs/lib>).extend (import <nixpkgs/lib/fileset/mock-splitRoot.nix>)).fileset;
   toSource { root = ./.; fileset = intersection ./foo/mock-root ./bar/mock-root; }
 ' 'lib.fileset.intersection: Filesystem roots are not the same:
-\s*first argument: root "'"$work"'/foo/mock-root"
-\s*second argument: root "'"$work"'/bar/mock-root"
-\s*Different roots are not supported.'
+\s*First argument: Filesystem root is "'"$work"'/foo/mock-root"
+\s*Second argument: Filesystem root is "'"$work"'/bar/mock-root"
+\s*Different filesystem roots are not supported.'
 rm -rf -- *
 
 # Coercion errors show the correct context
-expectFailure 'toSource { root = ./.; fileset = intersection ./a ./.; }' 'lib.fileset.intersection: first argument \('"$work"'/a\) does not exist.'
-expectFailure 'toSource { root = ./.; fileset = intersection ./. ./b; }' 'lib.fileset.intersection: second argument \('"$work"'/b\) does not exist.'
+expectFailure 'toSource { root = ./.; fileset = intersection ./a ./.; }' 'lib.fileset.intersection: First argument \('"$work"'/a\) is a path that does not exist.'
+expectFailure 'toSource { root = ./.; fileset = intersection ./. ./b; }' 'lib.fileset.intersection: Second argument \('"$work"'/b\) is a path that does not exist.'
 
 # The tree of later arguments should not be evaluated if a former argument already excludes all files
 tree=(