libexpr: Support structured error classes

While preparing PRs like #9753, I've had to change error messages in
dozens of code paths. It would be nice if instead of

    EvalError("expected 'boolean' but found '%1%'", showType(v))

we could write

    TypeError(v, "boolean")

or similar. Then, changing the error message could be a mechanical
refactor with the compiler pointing out places the constructor needs to
be changed, rather than the error-prone process of grepping through the
codebase. Structured errors would also help prevent the "same" error
from having multiple slightly different messages, and could be a first
step towards error codes / an error index.

This PR reworks the exception infrastructure in `libexpr` to
support exception types with different constructor signatures than
`BaseError`. Actually refactoring the exceptions to use structured data
will come in a future PR (this one is big enough already, as it has to
touch every exception in `libexpr`).

The core design is in `eval-error.hh`. Generally, errors like this:

    state.error("'%s' is not a string", getAttrPathStr())
      .debugThrow<TypeError>()

are transformed like this:

    state.error<TypeError>("'%s' is not a string", getAttrPathStr())
      .debugThrow()

The type annotation has moved from `ErrorBuilder::debugThrow` to
`EvalState::error`.

(cherry picked from commit c6a89c1a1659b31694c0fbcd21d78a6dd521c732)
Change-Id: Iced91ba4e00ca9e801518071fb43798936cbd05a

5 files changed, 26 insertions, 11 deletions

diff --git a/tests/functional/lang/eval-fail-attr-name-type.err.exp b/tests/functional/lang/eval-fail-attr-name-type.err.exp
index c8d56ba7d..6848a35ed 100644
--- a/tests/functional/lang/eval-fail-attr-name-type.err.exp
+++ b/tests/functional/lang/eval-fail-attr-name-type.err.exp
@@ -14,3 +14,8 @@ error:
             8|
 
        error: expected a string but found an integer: 1
+       at /pwd/lang/eval-fail-attr-name-type.nix:7:17:
+            6| in
+            7|   attrs.puppy.${key}
+             |                 ^
+            8|
diff --git a/tests/functional/lang/eval-fail-fromTOML-timestamps.err.exp b/tests/functional/lang/eval-fail-fromTOML-timestamps.err.exp
index 73f9df8cc..9bbb251e1 100644
--- a/tests/functional/lang/eval-fail-fromTOML-timestamps.err.exp
+++ b/tests/functional/lang/eval-fail-fromTOML-timestamps.err.exp
@@ -5,4 +5,4 @@ error:
              | ^
             2|   key = "value"
 
-       error: while parsing a TOML string: Dates and times are not supported
+       error: while parsing TOML: Dates and times are not supported
diff --git a/tests/functional/lang/eval-fail-toJSON.err.exp b/tests/functional/lang/eval-fail-toJSON.err.exp
index 4f6003437..ad267711b 100644
--- a/tests/functional/lang/eval-fail-toJSON.err.exp
+++ b/tests/functional/lang/eval-fail-toJSON.err.exp
@@ -20,6 +20,11 @@ error:
             3|     true
 
        … while evaluating list element at index 3
+         at /pwd/lang/eval-fail-toJSON.nix:2:3:
+            1| builtins.toJSON {
+            2|   a.b = [
+             |   ^
+            3|     true
 
        … while evaluating attribute 'c'
          at /pwd/lang/eval-fail-toJSON.nix:7:7:
diff --git a/tests/functional/lang/eval-fail-using-set-as-attr-name.err.exp b/tests/functional/lang/eval-fail-using-set-as-attr-name.err.exp
index 94784c651..4326c9650 100644
--- a/tests/functional/lang/eval-fail-using-set-as-attr-name.err.exp
+++ b/tests/functional/lang/eval-fail-using-set-as-attr-name.err.exp
@@ -7,3 +7,8 @@ error:
             6|
 
        error: expected a string but found a set: { }
+       at /pwd/lang/eval-fail-using-set-as-attr-name.nix:5:10:
+            4| in
+            5|   attr.${key}
+             |          ^
+            6|
diff --git a/tests/unit/libexpr/error_traces.cc b/tests/unit/libexpr/error_traces.cc
index c3e88d95e..7649fa587 100644
--- a/tests/unit/libexpr/error_traces.cc
+++ b/tests/unit/libexpr/error_traces.cc
@@ -12,33 +12,33 @@ namespace nix {
 
     TEST_F(ErrorTraceTest, TraceBuilder) {
         ASSERT_THROW(
-            state.error("Not much").debugThrow<EvalError>(),
+            state.error<EvalError>("puppy").debugThrow(),
             EvalError
         );
 
         ASSERT_THROW(
-            state.error("Not much").withTrace(noPos, "No more").debugThrow<EvalError>(),
+            state.error<EvalError>("puppy").withTrace(noPos, "doggy").debugThrow(),
             EvalError
         );
 
         ASSERT_THROW(
             try {
                 try {
-                    state.error("Not much").withTrace(noPos, "No more").debugThrow<EvalError>();
+                    state.error<EvalError>("puppy").withTrace(noPos, "doggy").debugThrow();
                 } catch (Error & e) {
-                    e.addTrace(state.positions[noPos], "Something", "");
+                    e.addTrace(state.positions[noPos], "beans", "");
                     throw;
                 }
             } catch (BaseError & e) {
                 ASSERT_EQ(PrintToString(e.info().msg),
-                          PrintToString(hintfmt("Not much")));
+                          PrintToString(hintfmt("puppy")));
                 auto trace = e.info().traces.rbegin();
                 ASSERT_EQ(e.info().traces.size(), 2);
                 ASSERT_EQ(PrintToString(trace->hint),
-                          PrintToString(hintfmt("No more")));
+                          PrintToString(hintfmt("doggy")));
                 trace++;
                 ASSERT_EQ(PrintToString(trace->hint),
-                          PrintToString(hintfmt("Something")));
+                          PrintToString(hintfmt("beans")));
                 throw;
             }
             , EvalError
@@ -47,12 +47,12 @@ namespace nix {
 
     TEST_F(ErrorTraceTest, NestedThrows) {
         try {
-            state.error("Not much").withTrace(noPos, "No more").debugThrow<EvalError>();
+            state.error<EvalError>("puppy").withTrace(noPos, "doggy").debugThrow();
         } catch (BaseError & e) {
             try {
-                state.error("Not much more").debugThrow<EvalError>();
+                state.error<EvalError>("beans").debugThrow();
             } catch (Error & e2) {
-                e.addTrace(state.positions[noPos], "Something", "");
+                e.addTrace(state.positions[noPos], "beans2", "");
                 //e2.addTrace(state.positions[noPos], "Something", "");
                 ASSERT_TRUE(e.info().traces.size() == 2);
                 ASSERT_TRUE(e2.info().traces.size() == 0);