feat: substitute path that starts with ${configDir}/ in tsconfig.compilerOptions.paths

closes #129

NOTE: All tests cases are just a head replacement of `${configDir}`, so we are constrained as such.

Author: Boshen

Diff for: fixtures/tsconfig/cases/paths_template_variable/foo.js

@@ -0,0 +1,3 @@
+{
+  "extends":	"../../tsconfig_template_variable.json"
+}

Diff for: fixtures/tsconfig/cases/paths_template_variable/tsconfig.json

@@ -0,0 +1,7 @@
+{
+  "compilerOptions": {
+    "paths": {
+      "foo": ["${configDir}/foo.js"]
+    }
+  }
+}

Diff for: fixtures/tsconfig/tsconfig_template_variable.json

@@ -168,6 +168,31 @@ fn test_paths_and_base_url() {
     }
 }
 
+// Template variable ${configDir} for substitution of config files directory path
+// https://github.com/microsoft/TypeScript/pull/58042
+#[test]
+fn test_template_variable() {
+    let f = super::fixture_root().join("tsconfig");
+    let f2 = f.join("cases").join("paths_template_variable");
+
+    #[rustfmt::skip]
+    let pass = [
+        (f2.clone(), "foo", f2.join("foo.js")),
+    ];
+
+    for (dir, request, expected) in pass {
+        let resolver = Resolver::new(ResolveOptions {
+            tsconfig: Some(TsconfigOptions {
+                config_file: f2.join("tsconfig.json"),
+                references: TsconfigReferences::Auto,
+            }),
+            ..ResolveOptions::default()
+        });
+        let resolved_path = resolver.resolve(&dir, request).map(|f| f.full_path());
+        assert_eq!(resolved_path, Ok(expected), "{request} {dir:?}");
+    }
+}
+
 #[cfg(not(target_os = "windows"))] // MemoryFS's path separator is always `/` so the test will not pass in windows.
 mod windows_test {
     use std::path::{Path, PathBuf};

Diff for: src/tests/tsconfig_paths.rs

@@ -3,10 +3,11 @@ use std::{
     sync::Arc,
 };
 
-use crate::PathUtil;
 use serde::Deserialize;
 use typescript_tsconfig_json::{CompilerOptionsPathsMap, ExtendsField};
 
+use crate::PathUtil;
+
 #[derive(Debug, Deserialize)]
 #[serde(rename_all = "camelCase")]
 pub struct TsConfig {
@@ -71,24 +72,25 @@ impl TsConfig {
         Ok(tsconfig)
     }
 
-    /// Directory to `package.json`
+    fn base_path(&self) -> &Path {
+        self.compiler_options
+            .base_url
+            .as_ref()
+            .map_or_else(|| self.directory(), |path| path.as_ref())
+    }
+
+    /// Directory to `tsconfig.json`
     ///
     /// # Panics
     ///
-    /// * When the package.json path is misconfigured.
+    /// * When the `tsconfig.json` path is misconfigured.
     pub fn directory(&self) -> &Path {
         debug_assert!(self.path.file_name().is_some());
         self.path.parent().unwrap()
     }
 
-    fn base_path(&self) -> &Path {
-        self.compiler_options
-            .base_url
-            .as_ref()
-            .map_or_else(|| self.directory(), |path| path.as_ref())
-    }
-
     pub fn extend_tsconfig(&mut self, tsconfig: &Self) {
+        let dir = self.directory().to_path_buf();
         let compiler_options = &mut self.compiler_options;
         if compiler_options.paths.is_none() {
             compiler_options.paths_base = compiler_options
@@ -100,6 +102,29 @@ impl TsConfig {
         if compiler_options.base_url.is_none() {
             compiler_options.base_url = tsconfig.compiler_options.base_url.clone();
         }
+        if let Some(paths) = &mut compiler_options.paths {
+            Self::normalize_paths(&dir, paths);
+        }
+    }
+
+    fn normalize_paths(directory: &Path, paths: &mut CompilerOptionsPathsMap) {
+        for paths in paths.values_mut() {
+            for path in paths {
+                Self::substitute_template_variable(directory, path);
+            }
+        }
+    }
+
+    /// Template variable `${configDir}` for substitution of config files directory path
+    ///
+    /// NOTE: All tests cases are just a head replacement of `${configDir}`, so we are constrained as such.
+    ///
+    /// See <https://github.com/microsoft/TypeScript/pull/58042>
+    fn substitute_template_variable(directory: &Path, path: &mut String) {
+        const TEMPLATE_VARIABLE: &str = "${configDir}/";
+        if let Some(stripped_path) = path.strip_prefix(TEMPLATE_VARIABLE) {
+            *path = directory.join(stripped_path).to_string_lossy().to_string();
+        }
     }
 
     pub fn resolve(&self, path: &Path, specifier: &str) -> Vec<PathBuf> {