eval: Improve ExprConcatStrings error messages and positions

"while evaluating a path segment" was just plain wrong

Co-authored-by: eldritch horrors <pennae@lix.systems>
Change-Id: I827374635df2887d17a8ee8b1755c078bf0b506b

lix/libexpr/eval-expr.cc

@@ -434,7 +434,7 @@ Value ExprConcatStrings::eval(EvalState & state, Env & env)
                 } else {
                     state.ctx.errors
                         .make<EvalError>("integer overflow in adding %1% + %2%", n, vTmp.integer())
-                        .atPos(i_pos)
+                        .atPos(isInterpolation ? i_pos : pos)
                         .debugThrow();
                 }
             } else if (vTmp.type() == nFloat) {
@@ -444,7 +444,7 @@ Value ExprConcatStrings::eval(EvalState & state, Env & env)
                 nf += vTmp.fpoint();
             } else {
                 state.ctx.errors.make<EvalError>("cannot add %1% to an integer", showType(vTmp))
-                    .atPos(i_pos)
+                    .atPos(isInterpolation ? i_pos : pos)
                     .withFrame(env, *this)
                     .debugThrow();
             }
@@ -455,7 +455,7 @@ Value ExprConcatStrings::eval(EvalState & state, Env & env)
                 nf += vTmp.fpoint();
             } else {
                 state.ctx.errors.make<EvalError>("cannot add %1% to a float", showType(vTmp))
-                    .atPos(i_pos)
+                    .atPos(isInterpolation ? i_pos : pos)
                     .withFrame(env, *this)
                     .debugThrow();
             }
@@ -471,13 +471,18 @@ Value ExprConcatStrings::eval(EvalState & state, Env & env)
                 : StringCoercionMode::Strict;
 
             /* skip canonization of first path, which would only be not
-            canonized in the first place if it's coming from a ./${foo} type
-            path */
+             canonized in the first place if it's coming from a ./${foo} type
+             path */
             auto part = state.coerceToString(
-                i_pos,
+                isInterpolation ? i_pos : pos,
                 vTmp,
                 context,
-                "while evaluating a path segment",
+                (isInterpolation && firstType == nPath)
+                    ? "while evaluating a path interpolation"
+                    : (isInterpolation ? "while evaluating a string interpolation"
+                                       // TODO: to the person who eventually cleans up all of this mess,
+                                       // please turn this into "cannot concatenate $type to a $type" instead.
+                                       : "while concatenating"),
                 coercionMode,
                 firstType == nString,
                 !first

tests/functional/repl_characterization/data/basic_repl.test

@@ -13,10 +13,10 @@ Expect the trace to not contain any traceback:
 
   nix-repl> f 2
   error:
-         … while evaluating a path segment
-           at «string»:1:13:
+         … while concatenating
+           at «string»:1:11:
               1| f = a: "" + a
-               |             ^
+               |           ^
 
          error: cannot coerce an integer to a string: 2
 
@@ -37,10 +37,10 @@ Expect the trace to have traceback:
               1| f = a: "" + a
                |     ^
 
-         … while evaluating a path segment
-           at «string»:1:13:
+         … while concatenating
+           at «string»:1:11:
               1| f = a: "" + a
-               |             ^
+               |           ^
 
          error: cannot coerce an integer to a string: 2
 
@@ -51,9 +51,9 @@ Turning it off should also work:
 
   nix-repl> f 2
   error:
-         … while evaluating a path segment
-           at «string»:1:13:
+         … while concatenating
+           at «string»:1:11:
               1| f = a: "" + a
-               |             ^
+               |           ^
 
          error: cannot coerce an integer to a string: 2

tests/functional2/lang/arithmetic/eval-fail-overflowing-add.err.exp

@@ -1,6 +1,6 @@
 error: integer overflow in adding 9223372036854775807 + 1
-       at /pwd/in.nix:4:8:
+       at /pwd/in.nix:4:6:
             3|   b = 1;
             4| in a + b
-             |        ^
+             |      ^
             5|

tests/functional2/lang/bad_string_interpolation/eval-fail-1.err.exp

@@ -1,5 +1,5 @@
 error:
-       … while evaluating a path segment
+       … while evaluating a string interpolation
          at /pwd/in.nix:1:2:
             1| "${x: x}"
              |  ^

tests/functional2/lang/bad_string_interpolation/eval-fail-3.err.exp

@@ -1,5 +1,5 @@
 error:
-       … while evaluating a path segment
+       … while evaluating a string interpolation
          at /pwd/in.nix:1:3:
             1| ''${x: x}''
              |   ^

tests/functional2/lang/bad_string_interpolation/eval-fail-4.err.exp

@@ -1,5 +1,5 @@
 error:
-       … while evaluating a path segment
+       … while evaluating a string interpolation
          at /pwd/in.nix:9:3:
             8| # The error message should not be too long.
             9| ''${pkgs}''

tests/functional2/lang/string-coerce-integers/eval-fail-accidental-coerce.err.exp

@@ -5,10 +5,10 @@ error:
              |                     ^
             2|
 
-       … while evaluating a path segment
-         at /pwd/in.nix:1:14:
+       … while concatenating
+         at /pwd/in.nix:1:12:
             1| let x = "" + 42; in x == "42"
-             |              ^
+             |            ^
             2|
 
        error: cannot coerce an integer to a string: 42

tests/functional2/lang/string-coerce-integers/no-xp.err.exp

@@ -1,5 +1,5 @@
 error:
-       … while evaluating a path segment
+       … while evaluating a string interpolation
          at /pwd/in.nix:1:2:
             1| "${42}" == "42"
              |  ^