Merge pull request #9754 from 9999years/print-value-when-coercion-fails

Print the value in `error: cannot coerce` messages

(cherry picked from commit 5b7bfd2d6b89d7dd5f54c1ca6c8072358d31a84e)

===

test taken from 6e8d5983143ae576e3f4b1d2954a5267f2943a49; it was added
previously (and not backported because its pr was a mostly-revert), but
it's useful to have around.

Change-Id: Icbd14b55e3610ce7b774667bf14b82e6dc717982

doc/manual/rl-next/print-value-in-coercion-error.md

@@ -0,0 +1,24 @@
+---
+synopsis: Coercion errors include the failing value
+issues: #561
+prs: #9754
+---
+
+The `error: cannot coerce a <TYPE> to a string` message now includes the value
+which caused the error.
+
+Before:
+
+```
+       error: cannot coerce a set to a string
+```
+
+After:
+
+```
+       error: cannot coerce a set to a string: { aesSupport = «thunk»;
+       avx2Support = «thunk»; avx512Support = «thunk»; avxSupport = «thunk»;
+       canExecute = «thunk»; config = «thunk»; darwinArch = «thunk»; darwinMinVersion
+       = «thunk»; darwinMinVersionVariable = «thunk»; darwinPlatform = «thunk»; «84
+       attributes elided»}
+```

src/libexpr/eval.cc

@@ -2279,7 +2279,9 @@ BackedStringView EvalState::coerceToString(
             return std::move(*maybeString);
         auto i = v.attrs->find(sOutPath);
         if (i == v.attrs->end()) {
-            error("cannot coerce %1% to a string", showType(v))
+            error("cannot coerce %1% to a string: %2%",
+                  showType(v),
+                  ValuePrinter(*this, v, errorPrintOptions))
                 .withTrace(pos, errorCtx)
                 .debugThrow<TypeError>();
         }
@@ -2325,7 +2327,9 @@ BackedStringView EvalState::coerceToString(
         }
     }
 
-    error("cannot coerce %1% to a string", showType(v))
+    error("cannot coerce %1% to a string: %2%",
+          showType(v),
+          ValuePrinter(*this, v, errorPrintOptions))
         .withTrace(pos, errorCtx)
         .debugThrow<TypeError>();
 }
@@ -2657,7 +2661,7 @@ void EvalState::printStatistics()
 std::string ExternalValueBase::coerceToString(const Pos & pos, NixStringContext & context, bool copyMore, bool copyToStore) const
 {
     throw TypeError({
-        .msg = hintfmt("cannot coerce %1% to a string", showType())
+        .msg = hintfmt("cannot coerce %1% to a string: %2%", showType(), *this)
     });
 }
 

src/libexpr/print-options.hh

@@ -36,11 +36,17 @@ struct PrintOptions
      */
     size_t maxDepth = std::numeric_limits<size_t>::max();
     /**
-     * Maximum number of attributes in an attribute set to print.
+     * Maximum number of attributes in attribute sets to print.
+     *
+     * Note that this is a limit for the entire print invocation, not for each
+     * attribute set encountered.
      */
     size_t maxAttrs = std::numeric_limits<size_t>::max();
     /**
      * Maximum number of list items to print.
+     *
+     * Note that this is a limit for the entire print invocation, not for each
+     * list encountered.
      */
     size_t maxListItems = std::numeric_limits<size_t>::max();
     /**

src/libexpr/print.cc

@@ -159,6 +159,8 @@ private:
     EvalState & state;
     PrintOptions options;
     std::optional<ValuesSeen> seen;
+    size_t attrsPrinted = 0;
+    size_t listItemsPrinted = 0;
 
     void printRepeated()
     {
@@ -277,7 +279,6 @@ private:
             else
                 std::sort(sorted.begin(), sorted.end(), ImportantFirstAttrNameCmp());
 
-            size_t attrsPrinted = 0;
             for (auto & i : sorted) {
                 if (attrsPrinted >= options.maxAttrs) {
                     printElided(sorted.size() - attrsPrinted, "attribute", "attributes");
@@ -305,7 +306,6 @@ private:
 
         output << "[ ";
         if (depth < options.maxDepth) {
-            size_t listItemsPrinted = 0;
             for (auto elem : v.listItems()) {
                 if (listItemsPrinted >= options.maxListItems) {
                     printElided(v.listSize() - listItemsPrinted, "item", "items");
@@ -484,6 +484,9 @@ public:
 
     void print(Value & v)
     {
+        attrsPrinted = 0;
+        listItemsPrinted = 0;
+
         if (options.trackRepeated) {
             seen.emplace();
         } else {

tests/functional/lang/eval-fail-bad-string-interpolation-1.err.exp

@@ -5,4 +5,4 @@ error:
              |  ^
             2|
 
-       error: cannot coerce a function to a string
+       error: cannot coerce a function to a string: «lambda @ /pwd/lang/eval-fail-bad-string-interpolation-1.nix:1:4»

tests/functional/lang/eval-fail-bad-string-interpolation-3.err.exp

@@ -5,4 +5,4 @@ error:
              |   ^
             2|
 
-       error: cannot coerce a function to a string
+       error: cannot coerce a function to a string: «lambda @ /pwd/lang/eval-fail-bad-string-interpolation-3.nix:1:5»

tests/functional/lang/eval-fail-bad-string-interpolation-4.err.exp

@@ -0,0 +1,9 @@
+error:
+       … while evaluating a path segment
+         at /pwd/lang/eval-fail-bad-string-interpolation-4.nix:9:3:
+            8| # The error message should not be too long.
+            9| ''${pkgs}''
+             |   ^
+           10|
+
+       error: cannot coerce a set to a string: { a = { a = { a = { a = "ha"; b = "ha"; c = "ha"; d = "ha"; e = "ha"; f = "ha"; g = "ha"; h = "ha"; j = "ha"; }; «4294967295 attributes elided»}; «4294967294 attributes elided»}; «4294967293 attributes elided»}

tests/functional/lang/eval-fail-bad-string-interpolation-4.nix

@@ -0,0 +1,9 @@
+let
+  # Basically a "billion laughs" attack, but toned down to simulated `pkgs`.
+  ha = x: y: { a = x y; b = x y; c = x y; d = x y; e = x y; f = x y; g = x y; h = x y; j = x y; };
+  has = ha (ha (ha (ha (x: x)))) "ha";
+  # A large structure that has already been evaluated.
+  pkgs = builtins.deepSeq has has;
+in
+# The error message should not be too long.
+''${pkgs}''

tests/unit/libexpr/error_traces.cc

@@ -295,7 +295,7 @@ namespace nix {
     TEST_F(ErrorTraceTest, toPath) {
         ASSERT_TRACE2("toPath []",
                       TypeError,
-                      hintfmt("cannot coerce %s to a string", "a list"),
+                      hintfmt("cannot coerce %s to a string: %s", "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 string", "a Boolean"),
+                      hintfmt("cannot coerce %s to a string: %s", "a Boolean", ANSI_CYAN "true" ANSI_NORMAL),
                       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 string", "a list"),
+                      hintfmt("cannot coerce %s to a string: %s", "a list", "[ ]"),
                       hintfmt("while realising the context of a path"));
 
         ASSERT_TRACE2("pathExists \"zorglub\"",
@@ -332,7 +332,7 @@ namespace nix {
     TEST_F(ErrorTraceTest, baseNameOf) {
         ASSERT_TRACE2("baseNameOf []",
                       TypeError,
-                      hintfmt("cannot coerce %s to a string", "a list"),
+                      hintfmt("cannot coerce %s to a string: %s", "a list", "[ ]"),
                       hintfmt("while evaluating the first argument passed to builtins.baseNameOf"));
 
     }
@@ -377,7 +377,7 @@ namespace nix {
     TEST_F(ErrorTraceTest, filterSource) {
         ASSERT_TRACE2("filterSource [] []",
                       TypeError,
-                      hintfmt("cannot coerce %s to a string", "a list"),
+                      hintfmt("cannot coerce %s to a string: %s", "a list", "[ ]"),
                       hintfmt("while evaluating the second argument (the path to filter) passed to builtins.filterSource"));
 
         ASSERT_TRACE2("filterSource [] \"foo\"",
@@ -1038,7 +1038,7 @@ namespace nix {
     TEST_F(ErrorTraceTest, toString) {
         ASSERT_TRACE2("toString { a = 1; }",
                       TypeError,
-                      hintfmt("cannot coerce %s to a string", "a set"),
+                      hintfmt("cannot coerce %s to a string: %s", "a set", "{ a = " ANSI_CYAN "1" ANSI_NORMAL "; }"),
                       hintfmt("while evaluating the first argument passed to builtins.toString"));
 
     }
@@ -1057,7 +1057,7 @@ namespace nix {
 
         ASSERT_TRACE2("substring 0 3 {}",
                       TypeError,
-                      hintfmt("cannot coerce %s to a string", "a set"),
+                      hintfmt("cannot coerce %s to a string: %s", "a set", "{ }"),
                       hintfmt("while evaluating the third argument (the string) passed to builtins.substring"));
 
         ASSERT_TRACE1("substring (-3) 3 \"sometext\"",
@@ -1070,7 +1070,7 @@ namespace nix {
     TEST_F(ErrorTraceTest, stringLength) {
         ASSERT_TRACE2("stringLength {} # TODO: context is missing ???",
                       TypeError,
-                      hintfmt("cannot coerce %s to a string", "a set"),
+                      hintfmt("cannot coerce %s to a string: %s", "a set", "{ }"),
                       hintfmt("while evaluating the argument passed to builtins.stringLength"));
 
     }
@@ -1143,7 +1143,7 @@ namespace nix {
 
         ASSERT_TRACE2("concatStringsSep \"foo\" [ 1 2 {} ] # TODO: coerce to string is buggy",
                       TypeError,
-                      hintfmt("cannot coerce %s to a string", "an integer"),
+                      hintfmt("cannot coerce %s to a string: %s", "an integer", ANSI_CYAN "1" ANSI_NORMAL),
                       hintfmt("while evaluating one element of the list of strings to concat passed to builtins.concatStringsSep"));
 
     }
@@ -1229,12 +1229,12 @@ namespace nix {
 
         ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = {}; }",
                       TypeError,
-                      hintfmt("cannot coerce %s to a string", "a set"),
+                      hintfmt("cannot coerce %s to a string: %s", "a set", "{ }"),
                       hintfmt("while evaluating the attribute 'system' of derivation 'foo'"));
 
         ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = {}; }",
                       TypeError,
-                      hintfmt("cannot coerce %s to a string", "a set"),
+                      hintfmt("cannot coerce %s to a string: %s", "a set", "{ }"),
                       hintfmt("while evaluating the attribute 'outputs' of derivation 'foo'"));
 
         ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = \"drv\"; }",
@@ -1279,17 +1279,17 @@ namespace nix {
 
         ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = \"out\"; args = [ {} ]; }",
                       TypeError,
-                      hintfmt("cannot coerce %s to a string", "a set"),
+                      hintfmt("cannot coerce %s to a string: %s", "a set", "{ }"),
                       hintfmt("while evaluating an element of the argument list"));
 
         ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = \"out\"; args = [ \"a\" {} ]; }",
                       TypeError,
-                      hintfmt("cannot coerce %s to a string", "a set"),
+                      hintfmt("cannot coerce %s to a string: %s", "a set", "{ }"),
                       hintfmt("while evaluating an element of the argument list"));
 
         ASSERT_TRACE2("derivationStrict { name = \"foo\"; builder = 1; system = 1; outputs = \"out\"; FOO = {}; }",
                       TypeError,
-                      hintfmt("cannot coerce %s to a string", "a set"),
+                      hintfmt("cannot coerce %s to a string: %s", "a set", "{ }"),
                       hintfmt("while evaluating the attribute 'FOO' of derivation 'foo'"));
 
     }