Use context to create a chain of errors

There's an existing bug (#7782) in Cargo which exacerbates the issue
here but in general having a stack of errors is a bit easier to read and
work with than having a big long error message.

src/cargo/util/config/de.rs

@@ -468,9 +468,9 @@ impl<'de, 'config> de::MapAccess<'de> for ValueDeserializer<'config> {
         // If this is the first time around we deserialize the `value` field
         // which is the actual deserializer
         if self.hits == 1 {
-            return seed.deserialize(self.de.clone()).map_err(|e| {
-                e.with_key_context(&self.de.key, self.definition.clone())
-            });
+            return seed
+                .deserialize(self.de.clone())
+                .map_err(|e| e.with_key_context(&self.de.key, self.definition.clone()));
         }
 
         // ... otherwise we're deserializing the `definition` field, so we need

src/cargo/util/config/mod.rs

@@ -1236,7 +1236,8 @@ impl ConfigError {
 
     fn with_key_context(self, key: &ConfigKey, definition: Definition) -> ConfigError {
         ConfigError {
-            error: anyhow!("could not load config key `{}`: {}", key, self),
+            error: anyhow::Error::from(self)
+                .context(format!("could not load config key `{}`", key)),
             definition: Some(definition),
         }
     }

tests/testsuite/bad_config.rs

@@ -155,8 +155,10 @@ fn bad_cargo_config_jobs() {
         .with_stderr(
             "\
 [ERROR] error in [..].cargo/config: \
-could not load config key `build.jobs`: \
-invalid value: integer `-1`, expected u32
+could not load config key `build.jobs`
+
+Caused by:
+  invalid value: integer `-1`, expected u32
 ",
         )
         .run();
@@ -1102,8 +1104,12 @@ fn bad_source_config6() {
     p.cargo("build")
         .with_status(101)
         .with_stderr(
-            "[ERROR] error in [..]/foo/.cargo/config: `source.crates-io.replace-with` \
-             expected a string, but found a array",
+            "\
+[ERROR] error in [..]/foo/.cargo/config: could not load config key `source.crates-io.replace-with`
+
+Caused by:
+  error in [..]/foo/.cargo/config: `source.crates-io.replace-with` expected a string, but found a array
+"
         )
         .run();
 }
@@ -1342,11 +1348,15 @@ fn bad_target_cfg() {
         .with_stderr(
             "\
 [ERROR] error in [..]/foo/.cargo/config: \
-could not load config key `target.cfg(not(target_os = \"none\")).runner`: \
-error in [..]/foo/.cargo/config: \
-could not load config key `target.cfg(not(target_os = \"none\")).runner`: \
-failed to deserialize, expected a string or array of strings: \
-data did not match any variant of untagged enum Target
+could not load config key `target.cfg(not(target_os = \"none\")).runner`
+
+Caused by:
+  error in [..]/foo/.cargo/config: \
+  could not load config key `target.cfg(not(target_os = \"none\")).runner`
+
+Caused by:
+  failed to deserialize, expected a string or array of strings: \
+  data did not match any variant of untagged enum Target
 ",
         )
         .run();

tests/testsuite/config.rs

@@ -495,9 +495,11 @@ c = ['c']
     let config = ConfigBuilder::new().env("CARGO_A", "x y").build();
     assert_error(
         config.get::<VSOB>("a").unwrap_err(),
-        "error in environment variable `CARGO_A`: \
-         could not load config key `a`: \
-         invalid type: string \"x y\", expected a boolean or vector of strings",
+        "\
+error in environment variable `CARGO_A`: could not load config key `a`
+
+Caused by:
+  invalid type: string \"x y\", expected a boolean or vector of strings",
     );
 
     // Normal env.
@@ -570,9 +572,11 @@ opt-level = 'foo'
 
     assert_error(
         config.get::<toml::TomlProfile>("profile.dev").unwrap_err(),
-        "error in [..]/.cargo/config: \
-         could not load config key `profile.dev.opt-level`: \
-         must be an integer, `z`, or `s`, but found: foo",
+        "\
+error in [..]/.cargo/config: could not load config key `profile.dev.opt-level`
+
+Caused by:
+  must be an integer, `z`, or `s`, but found: foo",
     );
 
     let config = ConfigBuilder::new()
@@ -581,9 +585,11 @@ opt-level = 'foo'
 
     assert_error(
         config.get::<toml::TomlProfile>("profile.dev").unwrap_err(),
-        "error in environment variable `CARGO_PROFILE_DEV_OPT_LEVEL`: \
-         could not load config key `profile.dev.opt-level`: \
-         must be an integer, `z`, or `s`, but found: asdf",
+        "\
+error in environment variable `CARGO_PROFILE_DEV_OPT_LEVEL`: could not load config key `profile.dev.opt-level`
+
+Caused by:
+  must be an integer, `z`, or `s`, but found: asdf",
     );
 }
 
@@ -653,8 +659,11 @@ big = 123456789
     );
     assert_error(
         config.get::<u8>("S.big").unwrap_err(),
-        "error in [..].cargo/config: could not load config key `S.big`: \
-         invalid value: integer `123456789`, expected u8",
+        "\
+error in [..].cargo/config: could not load config key `S.big`
+
+Caused by:
+  invalid value: integer `123456789`, expected u8",
     );
 
     // Environment variable type errors.
@@ -664,9 +673,11 @@ big = 123456789
     );
     assert_error(
         config.get::<i8>("e.big").unwrap_err(),
-        "error in environment variable `CARGO_E_BIG`: \
-         could not load config key `e.big`: \
-         invalid value: integer `123456789`, expected i8",
+        "\
+error in environment variable `CARGO_E_BIG`: could not load config key `e.big`
+
+Caused by:
+  invalid value: integer `123456789`, expected i8",
     );
 
     #[derive(Debug, Deserialize)]
@@ -952,27 +963,35 @@ i64max = 9223372036854775807
 
     assert_error(
         config.get::<u32>("nneg").unwrap_err(),
-        "error in [..].cargo/config: \
-         could not load config key `nneg`: \
-         invalid value: integer `-123456789`, expected u32",
+        "\
+error in [..].cargo/config: could not load config key `nneg`
+
+Caused by:
+  invalid value: integer `-123456789`, expected u32",
     );
     assert_error(
         config.get::<u32>("eneg").unwrap_err(),
-        "error in environment variable `CARGO_ENEG`: \
-         could not load config key `eneg`: \
-         invalid value: integer `-1`, expected u32",
+        "\
+error in environment variable `CARGO_ENEG`: could not load config key `eneg`
+
+Caused by:
+  invalid value: integer `-1`, expected u32",
     );
     assert_error(
         config.get::<i8>("npos").unwrap_err(),
-        "error in [..].cargo/config: \
-         could not load config key `npos`: \
-         invalid value: integer `123456789`, expected i8",
+        "\
+error in [..].cargo/config: could not load config key `npos`
+
+Caused by:
+  invalid value: integer `123456789`, expected i8",
     );
     assert_error(
         config.get::<i8>("epos").unwrap_err(),
-        "error in environment variable `CARGO_EPOS`: \
-         could not load config key `epos`: \
-         invalid value: integer `123456789`, expected i8",
+        "\
+error in environment variable `CARGO_EPOS`: could not load config key `epos`
+
+Caused by:
+  invalid value: integer `123456789`, expected i8",
     );
 }