From 173abec0bce03016b001c415822793a309c40e0b Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Fri, 20 Oct 2023 13:04:39 +0200
Subject: [PATCH] coerceToPath(): Handle __toString, add tests

---
 src/libexpr/eval.cc                           | 27 ++++++++++---------
 src/libexpr/tests/error_traces.cc             |  8 +++---
 .../functional/lang/eval-okay-pathexists.nix  |  2 ++
 3 files changed, 21 insertions(+), 16 deletions(-)

diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index 8235b83c5..e3ecb987c 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -2335,29 +2335,32 @@ SourcePath EvalState::coerceToPath(const PosIdx pos, Value & v, NixStringContext
 {
     try {
         forceValue(v, pos);
-
-        if (v.type() == nString) {
-            copyContext(v, context);
-            auto s = v.string_view();
-            if (!hasPrefix(s, "/"))
-                error("string '%s' doesn't represent an absolute path", s).atPos(pos).debugThrow<EvalError>();
-            return rootPath(CanonPath(s));
-        }
     } catch (Error & e) {
         e.addTrace(positions[pos], errorCtx);
         throw;
     }
 
+    /* Handle path values directly, without coercing to a string. */
     if (v.type() == nPath)
         return v.path();
 
+    /* Similarly, handle __toString where the result may be a path
+       value. */
     if (v.type() == nAttrs) {
-        auto i = v.attrs->find(sOutPath);
-        if (i != v.attrs->end())
-            return coerceToPath(pos, *i->value, context, errorCtx);
+        auto i = v.attrs->find(sToString);
+        if (i != v.attrs->end()) {
+            Value v1;
+            callFunction(*i->value, v, v1, pos);
+            return coerceToPath(pos, v1, context, errorCtx);
+        }
     }
 
-    error("cannot coerce %1% to a path", showType(v)).withTrace(pos, errorCtx).debugThrow<TypeError>();
+    /* Any other value should be coercable to a string, interpreted
+       relative to the root filesystem. */
+    auto path = coerceToString(pos, v, context, errorCtx, false, false, true).toOwned();
+    if (path == "" || path[0] != '/')
+        error("string '%1%' doesn't represent an absolute path", path).withTrace(pos, errorCtx).debugThrow<EvalError>();
+    return rootPath(CanonPath(path));
 }
 
 
diff --git a/src/libexpr/tests/error_traces.cc b/src/libexpr/tests/error_traces.cc
index 6da8a5954..9cd9f0a60 100644
--- a/src/libexpr/tests/error_traces.cc
+++ b/src/libexpr/tests/error_traces.cc
@@ -295,7 +295,7 @@ namespace nix {
     TEST_F(ErrorTraceTest, toPath) {
         ASSERT_TRACE2("toPath []",
                       TypeError,
-                      hintfmt("cannot coerce %s to a path", "a list"),
+                      hintfmt("cannot coerce %s to a string", "a list"),
                       hintfmt("while evaluating the first argument passed to builtins.toPath"));
 
         ASSERT_TRACE2("toPath \"foo\"",
@@ -309,7 +309,7 @@ namespace nix {
     TEST_F(ErrorTraceTest, storePath) {
         ASSERT_TRACE2("storePath true",
                       TypeError,
-                      hintfmt("cannot coerce %s to a path", "a Boolean"),
+                      hintfmt("cannot coerce %s to a string", "a Boolean"),
                       hintfmt("while evaluating the first argument passed to 'builtins.storePath'"));
 
     }
@@ -318,7 +318,7 @@ namespace nix {
     TEST_F(ErrorTraceTest, pathExists) {
         ASSERT_TRACE2("pathExists []",
                       TypeError,
-                      hintfmt("cannot coerce %s to a path", "a list"),
+                      hintfmt("cannot coerce %s to a string", "a list"),
                       hintfmt("while realising the context of a path"));
 
         ASSERT_TRACE2("pathExists \"zorglub\"",
@@ -377,7 +377,7 @@ namespace nix {
     TEST_F(ErrorTraceTest, filterSource) {
         ASSERT_TRACE2("filterSource [] []",
                       TypeError,
-                      hintfmt("cannot coerce %s to a path", "a list"),
+                      hintfmt("cannot coerce %s to a string", "a list"),
                       hintfmt("while evaluating the second argument (the path to filter) passed to 'builtins.filterSource'"));
 
         ASSERT_TRACE2("filterSource [] \"foo\"",
diff --git a/tests/functional/lang/eval-okay-pathexists.nix b/tests/functional/lang/eval-okay-pathexists.nix
index c5e7a62de..31697f66a 100644
--- a/tests/functional/lang/eval-okay-pathexists.nix
+++ b/tests/functional/lang/eval-okay-pathexists.nix
@@ -25,5 +25,7 @@ builtins.pathExists (./lib.nix)
 && builtins.pathExists (builtins.toString ./. + "/../lang/..//")
 && builtins.pathExists (builtins.toPath (builtins.toString ./lib.nix))
 && !builtins.pathExists (builtins.toPath (builtins.toString ./bla.nix))
+&& builtins.pathExists (builtins.toPath { __toString = x: builtins.toString ./lib.nix; })
+&& builtins.pathExists (builtins.toPath { outPath = builtins.toString ./lib.nix; })
 && builtins.pathExists ./lib.nix
 && !builtins.pathExists ./bla.nix