1 files changed, 76 insertions, 33 deletions

diff --git a/core/module_specifier.rs b/core/module_specifier.rs
index 1bdd5beee..8b8ccf4a6 100644
--- a/core/module_specifier.rs
+++ b/core/module_specifier.rs
@@ -1,6 +1,40 @@
+use std::error::Error;
 use std::fmt;
+use url::ParseError;
 use url::Url;
 
+/// Error indicating the reason resolving a module specifier failed.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+pub enum ModuleResolutionError {
+  InvalidUrl(ParseError),
+  InvalidBaseUrl(ParseError),
+  ImportPathPrefixMissing,
+}
+use ModuleResolutionError::*;
+
+impl Error for ModuleResolutionError {
+  fn source(&self) -> Option<&(dyn Error + 'static)> {
+    match self {
+      InvalidUrl(ref err) | InvalidBaseUrl(ref err) => Some(err),
+      _ => None,
+    }
+  }
+}
+
+impl fmt::Display for ModuleResolutionError {
+  fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+    match self {
+      InvalidUrl(ref err) => write!(f, "invalid URL: {}", err),
+      InvalidBaseUrl(ref err) => {
+        write!(f, "invalid base URL for relative import: {}", err)
+      }
+      ImportPathPrefixMissing => {
+        write!(f, "relative import path not prefixed with / or ./ or ../")
+      }
+    }
+  }
+}
+
 #[derive(Debug, Clone, PartialEq)]
 /// Resolved module specifier
 pub struct ModuleSpecifier(Url);
@@ -15,32 +49,39 @@ impl ModuleSpecifier {
   pub fn resolve(
     specifier: &str,
     base: &str,
-  ) -> Result<ModuleSpecifier, url::ParseError> {
-    // 1. Apply the URL parser to specifier. If the result is not failure, return
-    //    the result.
-    // let specifier = parse_local_or_remote(specifier)?.to_string();
-    if let Ok(url) = Url::parse(specifier) {
-      return Ok(ModuleSpecifier(url));
-    }
+  ) -> Result<ModuleSpecifier, ModuleResolutionError> {
+    let url = match Url::parse(specifier) {
+      // 1. Apply the URL parser to specifier.
+      //    If the result is not failure, return he result.
+      Ok(url) => url,
 
-    // 2. If specifier does not start with the character U+002F SOLIDUS (/), the
-    //    two-character sequence U+002E FULL STOP, U+002F SOLIDUS (./), or the
-    //    three-character sequence U+002E FULL STOP, U+002E FULL STOP, U+002F
-    //    SOLIDUS (../), return failure.
-    if !specifier.starts_with('/')
-      && !specifier.starts_with("./")
-      && !specifier.starts_with("../")
-    {
-      // TODO This is (probably) not the correct error to return here.
-      // TODO: This error is very not-user-friendly
-      return Err(url::ParseError::RelativeUrlWithCannotBeABaseBase);
-    }
+      // 2. If specifier does not start with the character U+002F SOLIDUS (/),
+      //    the two-character sequence U+002E FULL STOP, U+002F SOLIDUS (./),
+      //    or the three-character sequence U+002E FULL STOP, U+002E FULL STOP,
+      //    U+002F SOLIDUS (../), return failure.
+      Err(ParseError::RelativeUrlWithoutBase)
+        if !(specifier.starts_with('/')
+          || specifier.starts_with("./")
+          || specifier.starts_with("../")) =>
+      {
+        Err(ImportPathPrefixMissing)?
+      }
 
-    // 3. Return the result of applying the URL parser to specifier with base URL
-    //    as the base URL.
-    let base_url = Url::parse(base)?;
-    let u = base_url.join(&specifier)?;
-    Ok(ModuleSpecifier(u))
+      // 3. Return the result of applying the URL parser to specifier with base
+      //    URL as the base URL.
+      Err(ParseError::RelativeUrlWithoutBase) => {
+        let base = Url::parse(base).map_err(InvalidBaseUrl)?;
+        base.join(&specifier).map_err(InvalidUrl)?
+      }
+
+      // If parsing the specifier as a URL failed for a different reason than
+      // it being relative, always return the original error. We don't want to
+      // return `ImportPathPrefixMissing` or `InvalidBaseUrl` if the real
+      // problem lies somewhere else.
+      Err(err) => Err(InvalidUrl(err))?,
+    };
+
+    Ok(ModuleSpecifier(url))
   }
 
   /// Takes a string representing a path or URL to a module, but of the type
@@ -51,15 +92,17 @@ impl ModuleSpecifier {
   /// directory and returns an absolute URL.
   pub fn resolve_root(
     root_specifier: &str,
-  ) -> Result<ModuleSpecifier, url::ParseError> {
-    if let Ok(url) = Url::parse(root_specifier) {
-      Ok(ModuleSpecifier(url))
-    } else {
-      let cwd = std::env::current_dir().unwrap();
-      let base = Url::from_directory_path(cwd).unwrap();
-      let url = base.join(root_specifier)?;
-      Ok(ModuleSpecifier(url))
-    }
+  ) -> Result<ModuleSpecifier, ModuleResolutionError> {
+    let url = match Url::parse(root_specifier) {
+      Ok(url) => url,
+      Err(..) => {
+        let cwd = std::env::current_dir().unwrap();
+        let base = Url::from_directory_path(cwd).unwrap();
+        base.join(&root_specifier).map_err(InvalidUrl)?
+      }
+    };
+
+    Ok(ModuleSpecifier(url))
   }
 }