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.

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

@@ -0,0 +1,74 @@
+---
+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. This makes debugging much easier:
+
+```
+$ cat bad.nix
+let
+  pkgs = import <nixpkgs> {};
+  system = pkgs.lib.systems.elaborate "x86_64-linux";
+in
+  import <nixpkgs> {inherit system;}
+```
+
+Previously, attempting to evaluate this expression would produce a confusing error message:
+
+```
+$ nix-instantiate --eval bad.nix
+error:
+       … while evaluating a branch condition
+
+         at /nix/store/m8ah0r1ih2shq35vp3hj1k0m1c4hsfga-nixpkgs/nixpkgs/pkgs/stdenv/booter.nix:64:9:
+
+           63|       go = pred: n:
+           64|         if n == len
+             |         ^
+           65|         then rnul pred
+
+       … while calling the 'length' builtin
+
+         at /nix/store/m8ah0r1ih2shq35vp3hj1k0m1c4hsfga-nixpkgs/nixpkgs/pkgs/stdenv/booter.nix:62:13:
+
+           61|     let
+           62|       len = builtins.length list;
+             |             ^
+           63|       go = pred: n:
+
+       (stack trace truncated; use '--show-trace' to show the full trace)
+
+       error: cannot coerce a set to a string
+```
+
+Now, the error message includes the set itself. This makes debugging much
+simpler, especially when the trace doesn't show the failing expression:
+
+```
+$ nix-instantiate --eval bad.nix
+error:
+       … while evaluating a branch condition
+         at /nix/store/m8ah0r1ih2shq35vp3hj1k0m1c4hsfga-nixpkgs/nixpkgs/pkgs/stdenv/booter.nix:64:9:
+           63|       go = pred: n:
+           64|         if n == len
+             |         ^
+           65|         then rnul pred
+
+       … while calling the 'length' builtin
+         at /nix/store/m8ah0r1ih2shq35vp3hj1k0m1c4hsfga-nixpkgs/nixpkgs/pkgs/stdenv/booter.nix:62:13:
+           61|     let
+           62|       len = builtins.length list;
+             |             ^
+           63|       go = pred: n:
+
+       (stack trace truncated; use '--show-trace' to show the full trace)
+
+       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»}
+```

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

@@ -29,7 +29,6 @@
 #include <unistd.h>
 #include <sys/time.h>
 #include <sys/resource.h>
-#include <iostream>
 #include <fstream>
 #include <functional>
 
@@ -2233,7 +2232,9 @@ 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, debugPrintOptions))
                 .withTrace(pos, errorCtx)
                 .debugThrow<TypeError>();
         }
@@ -2279,7 +2280,9 @@ 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, debugPrintOptions))
         .withTrace(pos, errorCtx)
         .debugThrow<TypeError>();
 }
@@ -2639,7 +2642,7 @@ 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-ambiguous.cc

@@ -1,6 +1,7 @@
 #include "print-ambiguous.hh"
 #include "print.hh"
 #include "signals.hh"
+#include "eval.hh"
 
 namespace nix {
 

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();
     /**
@@ -49,4 +55,18 @@ struct PrintOptions
     size_t maxStringLength = std::numeric_limits<size_t>::max();
 };
 
+/**
+ * `PrintOptions` suitable for debugging.
+ *
+ * These options are used for printing values in error messages without
+ * printing "too much" output.
+ */
+static PrintOptions debugPrintOptions = PrintOptions {
+    .ansiColors = true,
+    .maxDepth = 10,
+    .maxAttrs = 10,
+    .maxListItems = 10,
+    .maxStringLength = 1024
+};
+
 }

src/libexpr/print.cc

@@ -7,6 +7,7 @@
 #include "store-api.hh"
 #include "terminal.hh"
 #include "english.hh"
+#include "eval.hh"
 
 namespace nix {
 
@@ -19,7 +20,7 @@ void printElided(
 {
     if (ansiColors)
         output << ANSI_FAINT;
-    output << " «";
+    output << "«";
     pluralize(output, value, single, plural);
     output << " elided»";
     if (ansiColors)
@@ -36,7 +37,7 @@ printLiteralString(std::ostream & str, const std::string_view string, size_t max
     str << "\"";
     for (auto i = string.begin(); i != string.end(); ++i) {
         if (charsPrinted >= maxLength) {
-            str << "\"";
+            str << "\" ";
             printElided(str, string.length() - charsPrinted, "byte", "bytes", ansiColors);
             return str;
         }
@@ -160,6 +161,8 @@ class Printer
     EvalState & state;
     PrintOptions options;
     std::optional<ValuesSeen> seen;
+    size_t attrsPrinted = 0;
+    size_t listItemsPrinted = 0;
 
     void printRepeated()
     {
@@ -278,7 +281,6 @@ class Printer
             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");
@@ -306,7 +308,6 @@ class Printer
 
         output << "[ ";
         if (depth < options.maxDepth) {
-            size_t listItemsPrinted = 0;
             for (auto elem : v.listItems()) {
                 if (listItemsPrinted >= options.maxListItems) {
                     printElided(v.listSize() - listItemsPrinted, "item", "items");
@@ -485,6 +486,9 @@ class Printer
 
     void print(Value & v)
     {
+        attrsPrinted = 0;
+        listItemsPrinted = 0;
+
         if (options.trackRepeated) {
             seen.emplace();
         } else {
@@ -501,4 +505,10 @@ void printValue(EvalState & state, std::ostream & output, Value & v, PrintOption
     Printer(output, state, options).print(v);
 }
 
+std::ostream & operator<<(std::ostream & output, const ValuePrinter & printer)
+{
+    printValue(printer.state, output, printer.value, printer.options);
+    return output;
+}
+
 }

src/libexpr/print.hh

@@ -9,11 +9,13 @@
 
 #include <iostream>
 
-#include "eval.hh"
 #include "print-options.hh"
 
 namespace nix {
 
+class EvalState;
+struct Value;
+
 /**
  * Print a string as a Nix string literal.
  *
@@ -59,4 +61,21 @@ std::ostream & printIdentifier(std::ostream & o, std::string_view s);
 
 void printValue(EvalState & state, std::ostream & str, Value & v, PrintOptions options = PrintOptions {});
 
+/**
+ * A partially-applied form of `printValue` which can be formatted using `<<`
+ * without allocating an intermediate string.
+ */
+class ValuePrinter {
+    friend std::ostream & operator << (std::ostream & output, const ValuePrinter & printer);
+private:
+    EvalState & state;
+    Value & value;
+    PrintOptions options;
+
+public:
+    ValuePrinter(EvalState & state, Value & value, PrintOptions options = PrintOptions {})
+        : state(state), value(value), options(options) { }
+};
+
+std::ostream & operator<<(std::ostream & output, const ValuePrinter & printer);
 }

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

@@ -6,4 +6,4 @@ error:
              |   ^
            10|
 
-       error: cannot coerce a set to a string
+       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»}