Improve display of GitHub input filenames in error messages

E.g.

  … while evaluating the attribute 'buildCommand' of the derivation 'vm-test-run-github-flakes'

  at «github:NixOS/nixpkgs/2fa57ed190fd6c7c746319444f34b5917666e5c1»/pkgs/stdenv/generic/make-derivation.nix:278:7:

src/libexpr/eval.cc

@@ -496,6 +496,8 @@ EvalState::EvalState(
     , baseEnv(allocEnv(128))
     , staticBaseEnv{std::make_shared<StaticEnv>(false, nullptr)}
 {
+    corepkgsFS->setPathDisplay("<nix", ">");
+
     countCalls = getEnv("NIX_COUNT_CALLS").value_or("0") != "0";
 
     assert(gcInitialised);

src/libfetchers/github.cc

@@ -232,7 +232,11 @@ struct GitArchiveInputScheme : InputScheme
     {
         auto [storePath, input2] = downloadArchive(store, input);
 
-        return {makeZipInputAccessor(CanonPath(store->toRealPath(storePath))), input2};
+        auto accessor = makeZipInputAccessor(CanonPath(store->toRealPath(storePath)));
+
+        accessor->setPathDisplay("«" + input2.to_string() + "»");
+
+        return {accessor, input2};
     }
 };
 

src/libfetchers/input-accessor.cc

@@ -9,7 +9,9 @@ static std::atomic<size_t> nextNumber{0};
 
 InputAccessor::InputAccessor()
     : number(++nextNumber)
-{ }
+    , displayPrefix{"/virtual/" + std::to_string(number)}
+{
+}
 
 // FIXME: merge with archive.cc.
 void InputAccessor::dumpPath(
@@ -91,9 +93,15 @@ std::optional<InputAccessor::Stat> InputAccessor::maybeLstat(const CanonPath & p
     return lstat(path);
 }
 
+void InputAccessor::setPathDisplay(std::string displayPrefix, std::string displaySuffix)
+{
+    this->displayPrefix = std::move(displayPrefix);
+    this->displaySuffix = std::move(displaySuffix);
+}
+
 std::string InputAccessor::showPath(const CanonPath & path)
 {
-    return "/virtual/" + std::to_string(number) + path.abs();
+    return displayPrefix + path.abs() + displaySuffix;
 }
 
 struct FSInputAccessorImpl : FSInputAccessor
@@ -109,7 +117,9 @@ struct FSInputAccessorImpl : FSInputAccessor
         : root(root)
         , allowedPaths(std::move(allowedPaths))
         , makeNotAllowedError(std::move(makeNotAllowedError))
-    { }
+    {
+        displayPrefix = root.abs();
+    }
 
     std::string readFile(const CanonPath & path) override
     {
@@ -201,11 +211,6 @@ struct FSInputAccessorImpl : FSInputAccessor
     {
         return (bool) allowedPaths;
     }
-
-    std::string showPath(const CanonPath & path) override
-    {
-        return (root + path).abs();
-    }
 };
 
 ref<FSInputAccessor> makeFSInputAccessor(

src/libfetchers/input-accessor.hh

@@ -13,6 +13,8 @@ struct InputAccessor : public std::enable_shared_from_this<InputAccessor>
 {
     const size_t number;
 
+    std::string displayPrefix, displaySuffix;
+
     InputAccessor();
 
     virtual ~InputAccessor()
@@ -58,6 +60,8 @@ struct InputAccessor : public std::enable_shared_from_this<InputAccessor>
         return number < x.number;
     }
 
+    void setPathDisplay(std::string displayPrefix, std::string displaySuffix = "");
+
     virtual std::string showPath(const CanonPath & path);
 };