Print the value in error: cannot coerce messages (#9553)

* Print the value in `error: cannot coerce` messages

This extends the `error: cannot coerce a TYPE to a string` message
to print the value that could not be coerced. This helps with debugging
by making it easier to track down where the value is being produced
from, especially in errors with deep or unhelpful stack traces.

Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>

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

@@ -0,0 +1,50 @@
+synopsis: Coercion errors include the failing value
+issues: #561
+prs: #9553
+description: {
+
+The `error: cannot coerce a <TYPE> to a string` message now includes the value which caused the error.
+
+Previously, a failed string coercion produced a confusing error message if the trace didn't show where the offending value was defined:
+
+```bash
+$ nix-instantiate --eval --expr '
+let x = { a = 1; }; in
+
+"${x}"
+'
+error:
+       … while evaluating a path segment
+
+         at «string»:4:2:
+
+            3|
+            4| "${x}"
+             |  ^
+            5|
+
+       error: cannot coerce a set to a string
+```
+
+Now, the error message includes the value itself:
+
+```bash
+$ nix-instantiate --eval --expr '
+let x = { a = 1; }; in
+
+"${x}"
+'
+error:
+       … while evaluating a path segment
+
+         at «string»:4:2:
+
+            3|
+            4| "${x}"
+             |  ^
+            5|
+
+       error: cannot coerce a set to a string: { a = 1; }
+```
+
+}

doc/manual/src/language/string-interpolation.md

@@ -189,7 +189,7 @@ If neither is present, an error is thrown.
 > "${a}"
 > ```
 >
->     error: cannot coerce a set to a string
+>     error: cannot coerce a set to a string: { }
 >
 >            at «string»:4:2:
 >

src/libexpr/eval.cc

@@ -26,9 +26,9 @@
 #include <unistd.h>
 #include <sys/time.h>
 #include <sys/resource.h>
-#include <iostream>
 #include <fstream>
 #include <functional>
+#include <strstream>
 
 #include <sys/resource.h>
 #include <nlohmann/json.hpp>
@@ -2286,7 +2286,7 @@ 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), printValue(*this, v))
                 .withTrace(pos, errorCtx)
                 .debugThrow<TypeError>();
         }
@@ -2332,7 +2332,7 @@ BackedStringView EvalState::coerceToString(
         }
     }
 
-    error("cannot coerce %1% to a string", showType(v))
+    error("cannot coerce %1% to a string: %2%", showType(v), printValue(*this, v))
         .withTrace(pos, errorCtx)
         .debugThrow<TypeError>();
 }
@@ -2691,8 +2691,10 @@ void EvalState::printStatistics()
 
 std::string ExternalValueBase::coerceToString(const Pos & pos, NixStringContext & context, bool copyMore, bool copyToStore) const
 {
+    std::strstream printed;
+    print(printed);
     throw TypeError({
-        .msg = hintfmt("cannot coerce %1% to a string", showType())
+        .msg = hintfmt("cannot coerce %1% to a string: %2%", showType(), printed.str())
     });
 }
 

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

@@ -7,4 +7,4 @@ error:
              |  ^
             2|
 
-       error: cannot coerce a function to a string
+       error: cannot coerce a function to a string: <LAMBDA>

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

@@ -7,4 +7,4 @@ error:
              |   ^
             2|
 
-       error: cannot coerce a function to a string
+       error: cannot coerce a function to a string: <LAMBDA>

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", "true"),
                       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 = 1; }"),
                       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", "1"),
                       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'"));
 
     }