fix(config): make it clear which element in list is bad

Author: weihanglo

src/cargo/util/context/mod.rs

@@ -2236,7 +2236,7 @@ impl ConfigValue {
                         toml::Value::String(val) => Ok((val, def.clone())),
                         v => {
                             path.push(KeyOrIdx::Idx(i));
-                            bail!("expected string but found {} in list", v.type_str())
+                            bail!("expected string but found {} at index {i}", v.type_str())
                         }
                     })
                     .collect::<CargoResult<_>>()?,

tests/testsuite/cargo_alias_config.rs

@@ -90,7 +90,7 @@ Caused by:
   failed to parse config at `alias.b-cargo-test[0]`
 
 Caused by:
-  expected string but found integer in list
+  expected string but found integer at index 0
 
 "#]])
         .run();

tests/testsuite/config.rs

@@ -1366,7 +1366,7 @@ Caused by:
   failed to parse config at `foo[0]`
 
 Caused by:
-  expected string but found integer in list
+  expected string but found integer at index 0
 "#]],
     );
 }