Treat missing package name error as an unsupported requirement (#1025)

## Summary

Based on user feedback. Calling it a "parse error" is misleading, since
this is really something we don't support, but that users can work
around.

crates/pep508-rs/src/lib.rs

@@ -71,6 +71,9 @@ pub enum Pep508ErrorSource {
     /// A URL parsing error.
     #[error(transparent)]
     UrlError(#[from] verbatim_url::VerbatimUrlError),
+    /// The version requirement is not supported.
+    #[error("{0}")]
+    UnsupportedRequirement(String),
 }
 
 impl Display for Pep508Error {
@@ -842,11 +845,9 @@ fn parse(cursor: &mut Cursor) -> Result<Requirement, Pep508Error> {
             // a package name. pip supports this in `requirements.txt`, but it doesn't adhere to
             // the PEP 508 grammar.
             let mut clone = cursor.clone().at(start);
-            return if let Ok(url) = parse_url(&mut clone) {
+            return if parse_url(&mut clone).is_ok() {
                 Err(Pep508Error {
-                    message: Pep508ErrorSource::String(format!(
-                        "URL requirement is missing a package name; expected: `package_name @ {url}`",
-                    )),
+                    message: Pep508ErrorSource::UnsupportedRequirement("URL requirement must be preceded by a package name. Add the name of the package before the URL (e.g., `package_name @ https://...`).".to_string()),
                     start,
                     len: clone.pos() - start,
                     input: clone.to_string(),
@@ -1305,7 +1306,7 @@ mod tests {
         assert_err(
             r#"git+https://github.com/pallets/flask.git"#,
             indoc! {"
-                URL requirement is missing a package name; expected: `package_name @ git+https://github.com/pallets/flask.git`
+                URL requirement must be preceded by a package name. Add the name of the package before the URL (e.g., `package_name @ https://...`).
                 git+https://github.com/pallets/flask.git
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^"
             },

crates/pep508-rs/src/verbatim_url.rs

@@ -5,18 +5,20 @@ use std::path::{Component, Path, PathBuf};
 
 use once_cell::sync::Lazy;
 use regex::Regex;
-use serde::{Deserialize, Serialize};
 use url::Url;
 
 /// A wrapper around [`Url`] that preserves the original string.
 #[derive(Debug, Clone, Eq, derivative::Derivative)]
 #[derivative(PartialEq, Hash)]
-#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct VerbatimUrl {
     /// The parsed URL.
-    #[serde(
-        serialize_with = "Url::serialize_internal",
-        deserialize_with = "Url::deserialize_internal"
+    #[cfg_attr(
+        feature = "serde",
+        serde(
+            serialize_with = "Url::serialize_internal",
+            deserialize_with = "Url::deserialize_internal"
+        )
     )]
     url: Url,
     /// The URL as it was provided by the user.

crates/puffin/tests/pip_compile.rs

@@ -3495,3 +3495,41 @@ fn missing_editable_requirement() -> Result<()> {
 
     Ok(())
 }
+
+/// Attempt to resolve a URL requirement without a package name.
+#[test]
+fn missing_package_name() -> Result<()> {
+    let temp_dir = TempDir::new()?;
+    let cache_dir = TempDir::new()?;
+    let venv = create_venv_py312(&temp_dir, &cache_dir);
+
+    let requirements_in = temp_dir.child("requirements.in");
+    requirements_in.write_str("https://files.pythonhosted.org/packages/36/42/015c23096649b908c809c69388a805a571a3bea44362fe87e33fc3afa01f/flask-3.0.0-py3-none-any.whl")?;
+
+    insta::with_settings!({
+        filters => INSTA_FILTERS.to_vec()
+    }, {
+        assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+            .arg("pip")
+            .arg("compile")
+            .arg("requirements.in")
+            .arg("--cache-dir")
+            .arg(cache_dir.path())
+            .arg("--exclude-newer")
+            .arg(EXCLUDE_NEWER)
+            .env("VIRTUAL_ENV", venv.as_os_str())
+            .current_dir(&temp_dir), @r###"
+        success: false
+        exit_code: 2
+        ----- stdout -----
+
+        ----- stderr -----
+        error: Unsupported requirement in requirements.in position 0 to 135
+          Caused by: URL requirement must be preceded by a package name. Add the name of the package before the URL (e.g., `package_name @ https://...`).
+        https://files.pythonhosted.org/packages/36/42/015c23096649b908c809c69388a805a571a3bea44362fe87e33fc3afa01f/flask-3.0.0-py3-none-any.whl
+        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+        "###);
+    });
+
+    Ok(())
+}

crates/requirements-txt/src/lib.rs

@@ -47,7 +47,7 @@ use tracing::warn;
 use unscanny::{Pattern, Scanner};
 use url::Url;
 
-use pep508_rs::{Pep508Error, Requirement, VerbatimUrl};
+use pep508_rs::{Pep508Error, Pep508ErrorSource, Requirement, VerbatimUrl};
 
 /// We emit one of those for each requirements.txt entry
 enum RequirementsTxtStatement {
@@ -468,13 +468,26 @@ fn parse_requirement_and_hashes(
             break (end, false);
         }
     };
-    let requirement = Requirement::from_str(&content[start..end]).map_err(|err| {
-        RequirementsTxtParserError::Pep508 {
-            source: err,
-            start,
-            end,
-        }
-    })?;
+    let requirement =
+        Requirement::from_str(&content[start..end]).map_err(|err| match err.message {
+            Pep508ErrorSource::String(_) => RequirementsTxtParserError::Pep508 {
+                source: err,
+                start,
+                end,
+            },
+            Pep508ErrorSource::UrlError(_) => RequirementsTxtParserError::Pep508 {
+                source: err,
+                start,
+                end,
+            },
+            Pep508ErrorSource::UnsupportedRequirement(_) => {
+                RequirementsTxtParserError::UnsupportedRequirement {
+                    source: err,
+                    start,
+                    end,
+                }
+            }
+        })?;
     let hashes = if has_hashes {
         let hashes = parse_hashes(s)?;
         eat_trailing_line(s)?;
@@ -547,6 +560,11 @@ pub enum RequirementsTxtParserError {
         message: String,
         location: usize,
     },
+    UnsupportedRequirement {
+        source: Pep508Error,
+        start: usize,
+        end: usize,
+    },
     Pep508 {
         source: Pep508Error,
         start: usize,
@@ -572,6 +590,9 @@ impl Display for RequirementsTxtParserError {
             RequirementsTxtParserError::Parser { message, location } => {
                 write!(f, "{message} at position {location}")
             }
+            RequirementsTxtParserError::UnsupportedRequirement { start, end, .. } => {
+                write!(f, "Unsupported requirement in position {start} to {end}")
+            }
             RequirementsTxtParserError::Pep508 { start, end, .. } => {
                 write!(f, "Couldn't parse requirement in position {start} to {end}")
             }
@@ -591,6 +612,7 @@ impl std::error::Error for RequirementsTxtParserError {
             RequirementsTxtParserError::IO(err) => err.source(),
             RequirementsTxtParserError::InvalidEditablePath(_) => None,
             RequirementsTxtParserError::UnsupportedUrl(_) => None,
+            RequirementsTxtParserError::UnsupportedRequirement { source, .. } => Some(source),
             RequirementsTxtParserError::Pep508 { source, .. } => Some(source),
             RequirementsTxtParserError::Subfile { source, .. } => Some(source.as_ref()),
             RequirementsTxtParserError::Parser { .. } => None,
@@ -626,6 +648,15 @@ impl Display for RequirementsTxtFileError {
                     location
                 )
             }
+            RequirementsTxtParserError::UnsupportedRequirement { start, end, .. } => {
+                write!(
+                    f,
+                    "Unsupported requirement in {} position {} to {}",
+                    self.file.display(),
+                    start,
+                    end,
+                )
+            }
             RequirementsTxtParserError::Pep508 { start, end, .. } => {
                 write!(
                     f,