Support --config path_to_config.toml cli syntax.

src/cargo/util/config/mod.rs

@@ -864,20 +864,35 @@ impl Config {
         };
         let mut loaded_args = CV::Table(HashMap::new(), Definition::Cli);
         for arg in cli_args {
-            // TODO: This should probably use a more narrow parser, reject
-            // comments, blank lines, [headers], etc.
-            let toml_v: toml::Value = toml::de::from_str(arg)
-                .chain_err(|| format!("failed to parse --config argument `{}`", arg))?;
-            let toml_table = toml_v.as_table().unwrap();
-            if toml_table.len() != 1 {
-                bail!(
-                    "--config argument `{}` expected exactly one key=value pair, got {} keys",
-                    arg,
-                    toml_table.len()
-                );
-            }
-            let tmp_table = CV::from_toml(Definition::Cli, toml_v)
-                .chain_err(|| format!("failed to convert --config argument `{}`", arg))?;
+            let arg_as_path = self.cwd.join(arg);
+            let tmp_table = if !arg.is_empty() && arg_as_path.exists() {
+                // --config path_to_file
+                let str_path = arg_as_path
+                    .to_str()
+                    .ok_or_else(|| {
+                        anyhow::format_err!("config path {:?} is not utf-8", arg_as_path)
+                    })?
+                    .to_string();
+                let mut map = HashMap::new();
+                let value = CV::String(str_path, Definition::Cli);
+                map.insert("include".to_string(), value);
+                CV::Table(map, Definition::Cli)
+            } else {
+                // TODO: This should probably use a more narrow parser, reject
+                // comments, blank lines, [headers], etc.
+                let toml_v: toml::Value = toml::de::from_str(arg)
+                    .chain_err(|| format!("failed to parse --config argument `{}`", arg))?;
+                let toml_table = toml_v.as_table().unwrap();
+                if toml_table.len() != 1 {
+                    bail!(
+                        "--config argument `{}` expected exactly one key=value pair, got {} keys",
+                        arg,
+                        toml_table.len()
+                    );
+                }
+                CV::from_toml(Definition::Cli, toml_v)
+                    .chain_err(|| format!("failed to convert --config argument `{}`", arg))?
+            };
             let mut seen = HashSet::new();
             let tmp_table = self
                 .load_includes(tmp_table, &mut seen)

src/doc/src/reference/unstable.md

@@ -465,10 +465,11 @@ The config values are first loaded from the include path, and then the config
 file's own values are merged on top of it.
 
 This can be paired with [config-cli](#config-cli) to specify a file to load
-from the command-line:
+from the command-line. Pass a path to a config file as the argument to
+`--config`:
 
 ```console
-cargo +nightly -Zunstable-options -Zconfig-include --config 'include="somefile.toml"' build
+cargo +nightly -Zunstable-options -Zconfig-include --config somefile.toml build
 ```
 
 CLI paths are relative to the current working directory.

tests/testsuite/config_include.rs

@@ -3,7 +3,8 @@
 use super::config::{
     assert_error, assert_match, read_output, write_config, write_config_at, ConfigBuilder,
 };
-use cargo_test_support::NO_SUCH_FILE_ERR_MSG;
+use cargo_test_support::{paths, NO_SUCH_FILE_ERR_MSG};
+use std::fs;
 
 #[cargo_test]
 fn gated() {
@@ -208,3 +209,28 @@ Caused by:
   expected array, but found string",
     );
 }
+
+#[cargo_test]
+fn cli_path() {
+    // --config path_to_file
+    fs::write(paths::root().join("myconfig.toml"), "key = 123").unwrap();
+    let config = ConfigBuilder::new()
+        .cwd(paths::root())
+        .unstable_flag("config-include")
+        .config_arg("myconfig.toml")
+        .build();
+    assert_eq!(config.get::<u32>("key").unwrap(), 123);
+
+    let config = ConfigBuilder::new()
+        .unstable_flag("config-include")
+        .config_arg("missing.toml")
+        .build_err();
+    assert_error(
+        config.unwrap_err(),
+        "\
+failed to parse --config argument `missing.toml`
+
+Caused by:
+  expected an equals, found eof at line 1 column 13",
+    );
+}