better error messages for 'relative import path not prefixed with / or ./ or ../' (#3405)

cli/deno_error.rs

@@ -140,7 +140,7 @@ impl GetErrorKind for ModuleResolutionError {
     match self {
       InvalidUrl(ref err) | InvalidBaseUrl(ref err) => err.kind(),
       InvalidPath(_) => ErrorKind::InvalidPath,
-      ImportPrefixMissing(_) => ErrorKind::ImportPrefixMissing,
+      ImportPrefixMissing(_, _) => ErrorKind::ImportPrefixMissing,
     }
   }
 }

cli/tests/error_011_bad_module_specifier.ts.out

-[WILDCARD]error: Uncaught ImportPrefixMissing: relative import path "bad-module.ts" not prefixed with / or ./ or ../
+[WILDCARD]error: Uncaught ImportPrefixMissing: relative import path "bad-module.ts" not prefixed with / or ./ or ../ Imported from "[WILDCARD]/error_011_bad_module_specifier.ts"
 [WILDCARD]dispatch_json.ts:[WILDCARD]
     at DenoError ([WILDCARD]errors.ts:[WILDCARD])
     at unwrapResponse ([WILDCARD]dispatch_json.ts:[WILDCARD])

cli/tests/error_012_bad_dynamic_import_specifier.ts.out

-[WILDCARD]error: Uncaught ImportPrefixMissing: relative import path "bad-module.ts" not prefixed with / or ./ or ../
+[WILDCARD]error: Uncaught ImportPrefixMissing: relative import path "bad-module.ts" not prefixed with / or ./ or ../ Imported from "[WILDCARD]/error_012_bad_dynamic_import_specifier.ts"
 [WILDCARD]dispatch_json.ts:[WILDCARD]
     at DenoError ([WILDCARD]errors.ts:[WILDCARD])
     at unwrapResponse ([WILDCARD]dispatch_json.ts:[WILDCARD])

cli/tests/error_014_catch_dynamic_import_error.js.out

 Caught direct dynamic import error.
-TypeError: relative import path "does not exist" not prefixed with / or ./ or ../
+TypeError: relative import path "does not exist" not prefixed with / or ./ or ../ Imported from "[WILDCARD]/error_014_catch_dynamic_import_error.js"
 
 Caught indirect direct dynamic import error.
-TypeError: relative import path "does not exist either" not prefixed with / or ./ or ../
+TypeError: relative import path "does not exist either" not prefixed with / or ./ or ../ Imported from "[WILDCARD]/indirect_import_error.js"
 
 Caught error thrown by dynamically imported module.
 Error: An error

cli/tests/error_type_definitions.ts.out

-[WILDCARD]error: Uncaught ImportPrefixMissing: relative import path "baz" not prefixed with / or ./ or ../
+[WILDCARD]error: Uncaught ImportPrefixMissing: relative import path "baz" not prefixed with / or ./ or ../ Imported from "[WILDCARD]/type_definitions/bar.d.ts"
 [WILDCARD]dispatch_json.ts:[WILDCARD]
     at DenoError ([WILDCARD]errors.ts:[WILDCARD])
     at unwrapResponse ([WILDCARD]dispatch_json.ts:[WILDCARD])

core/module_specifier.rs

@@ -11,7 +11,7 @@ pub enum ModuleResolutionError {
   InvalidUrl(ParseError),
   InvalidBaseUrl(ParseError),
   InvalidPath(PathBuf),
-  ImportPrefixMissing(String),
+  ImportPrefixMissing(String, Option<String>),
 }
 use ModuleResolutionError::*;
 
@@ -32,11 +32,19 @@ impl fmt::Display for ModuleResolutionError {
         write!(f, "invalid base URL for relative import: {}", err)
       }
       InvalidPath(ref path) => write!(f, "invalid module path: {:?}", path),
-      ImportPrefixMissing(ref specifier) => write!(
-        f,
+      ImportPrefixMissing(ref specifier, ref maybe_referrer) => {
+        let msg = format!(
           "relative import path \"{}\" not prefixed with / or ./ or ../",
           specifier
-      ),
+        );
+        let msg = if let Some(referrer) = maybe_referrer {
+          format!("{} Imported from \"{}\"", msg, referrer)
+        } else {
+          msg
+        };
+
+        write!(f, "{}", msg)
+      }
     }
   }
 }
@@ -78,7 +86,12 @@ impl ModuleSpecifier {
           || specifier.starts_with("./")
           || specifier.starts_with("../")) =>
       {
-        return Err(ImportPrefixMissing(specifier.to_string()))
+        let maybe_referrer = if base.is_empty() {
+          None
+        } else {
+          Some(base.to_string())
+        };
+        return Err(ImportPrefixMissing(specifier.to_string(), maybe_referrer));
       }
 
       // 3. Return the result of applying the URL parser to specifier with base
@@ -282,27 +295,42 @@ mod tests {
       (
         "005_more_imports.ts",
         "http://deno.land/core/tests/006_url_imports.ts",
-        ImportPrefixMissing("005_more_imports.ts".to_string()),
+        ImportPrefixMissing(
+          "005_more_imports.ts".to_string(),
+          Some("http://deno.land/core/tests/006_url_imports.ts".to_string()),
+        ),
       ),
       (
         ".tomato",
         "http://deno.land/core/tests/006_url_imports.ts",
-        ImportPrefixMissing(".tomato".to_string()),
+        ImportPrefixMissing(
+          ".tomato".to_string(),
+          Some("http://deno.land/core/tests/006_url_imports.ts".to_string()),
+        ),
       ),
       (
         "..zucchini.mjs",
         "http://deno.land/core/tests/006_url_imports.ts",
-        ImportPrefixMissing("..zucchini.mjs".to_string()),
+        ImportPrefixMissing(
+          "..zucchini.mjs".to_string(),
+          Some("http://deno.land/core/tests/006_url_imports.ts".to_string()),
+        ),
       ),
       (
         r".\yam.es",
         "http://deno.land/core/tests/006_url_imports.ts",
-        ImportPrefixMissing(r".\yam.es".to_string()),
+        ImportPrefixMissing(
+          r".\yam.es".to_string(),
+          Some("http://deno.land/core/tests/006_url_imports.ts".to_string()),
+        ),
       ),
       (
         r"..\yam.es",
         "http://deno.land/core/tests/006_url_imports.ts",
-        ImportPrefixMissing(r"..\yam.es".to_string()),
+        ImportPrefixMissing(
+          r"..\yam.es".to_string(),
+          Some("http://deno.land/core/tests/006_url_imports.ts".to_string()),
+        ),
       ),
       (
         "https://eggplant:b/c",