Error when parsing `requirements.txt`-like packages in `requirements.txt` file (#1179)

## Summary

Like https://github.com/astral-sh/puffin/pull/1180, this PR adds logic
for `requirements.txt` parsing whereby if a requirement _looks like_ a
local requirements file or an editable directory, we prompt the user to
correct the error (typically, by adding `-r`).

crates/requirements-txt/src/lib.rs

@@ -34,6 +34,7 @@
 //! wrappable_whitespaces = whitespace ('\\\n' | whitespace)*
 //! ```
 
+use std::borrow::Cow;
 use std::fmt::{Display, Formatter};
 use std::io;
 use std::path::{Path, PathBuf};
@@ -573,8 +574,49 @@ fn parse_requirement_and_hashes(
             break (end, false);
         }
     };
+
+    let requirement = &content[start..end];
+
+    // If the requirement looks like a `requirements.txt` file (with a missing `-r`), raise an
+    // error.
+    //
+    // While `requirements.txt` is a valid package name (per the spec), PyPI disallows
+    // `requirements.txt` and some other variants anyway.
+    #[allow(clippy::case_sensitive_file_extension_comparisons)]
+    if requirement.ends_with(".txt") || requirement.ends_with(".in") {
+        let path = Path::new(requirement);
+        let path = if path.is_absolute() {
+            Cow::Borrowed(path)
+        } else {
+            Cow::Owned(working_dir.join(path))
+        };
+        if path.is_file() {
+            return Err(RequirementsTxtParserError::MissingRequirementPrefix(
+                requirement.to_string(),
+            ));
+        }
+    }
+
+    // If the requirement looks like an editable requirement (with a missing `-e`), raise an
+    // error.
+    //
+    // Slashes are not allowed in package names, so these would be rejected in the next step anyway.
+    if requirement.contains('/') || requirement.contains('\\') {
+        let path = Path::new(requirement);
+        let path = if path.is_absolute() {
+            Cow::Borrowed(path)
+        } else {
+            Cow::Owned(working_dir.join(path))
+        };
+        if path.is_dir() {
+            return Err(RequirementsTxtParserError::MissingEditablePrefix(
+                requirement.to_string(),
+            ));
+        }
+    }
+
     let requirement =
-        Requirement::parse(&content[start..end], working_dir).map_err(|err| match err.message {
+        Requirement::parse(requirement, working_dir).map_err(|err| match err.message {
             Pep508ErrorSource::String(_) | Pep508ErrorSource::UrlError(_) => {
                 RequirementsTxtParserError::Pep508 {
                     source: err,
@@ -664,6 +706,8 @@ pub enum RequirementsTxtParserError {
     },
     InvalidEditablePath(String),
     UnsupportedUrl(String),
+    MissingRequirementPrefix(String),
+    MissingEditablePrefix(String),
     Parser {
         message: String,
         location: usize,
@@ -708,6 +752,12 @@ impl RequirementsTxtParserError {
             RequirementsTxtParserError::UnsupportedUrl(url) => {
                 RequirementsTxtParserError::UnsupportedUrl(url)
             }
+            RequirementsTxtParserError::MissingRequirementPrefix(given) => {
+                RequirementsTxtParserError::MissingRequirementPrefix(given)
+            }
+            RequirementsTxtParserError::MissingEditablePrefix(given) => {
+                RequirementsTxtParserError::MissingEditablePrefix(given)
+            }
             RequirementsTxtParserError::Parser { message, location } => {
                 RequirementsTxtParserError::Parser {
                     message,
@@ -752,6 +802,15 @@ impl Display for RequirementsTxtParserError {
             RequirementsTxtParserError::UnsupportedUrl(url) => {
                 write!(f, "Unsupported URL (expected a `file://` scheme): `{url}`")
             }
+            RequirementsTxtParserError::MissingRequirementPrefix(given) => {
+                write!(f, "Requirement `{given}` looks like a requirements file but was passed as a package name. Did you mean `-r {given}`?")
+            }
+            RequirementsTxtParserError::MissingEditablePrefix(given) => {
+                write!(
+                    f,
+                    "Requirement `{given}` looks like a directory but was passed as a package name. Did you mean `-e {given}`?"
+                )
+            }
             RequirementsTxtParserError::Parser { message, location } => {
                 write!(f, "{message} at position {location}")
             }
@@ -775,6 +834,8 @@ impl std::error::Error for RequirementsTxtParserError {
             RequirementsTxtParserError::Url { source, .. } => Some(source),
             RequirementsTxtParserError::InvalidEditablePath(_) => None,
             RequirementsTxtParserError::UnsupportedUrl(_) => None,
+            RequirementsTxtParserError::MissingRequirementPrefix(_) => None,
+            RequirementsTxtParserError::MissingEditablePrefix(_) => None,
             RequirementsTxtParserError::UnsupportedRequirement { source, .. } => Some(source),
             RequirementsTxtParserError::Pep508 { source, .. } => Some(source),
             RequirementsTxtParserError::Subfile { source, .. } => Some(source.as_ref()),
@@ -808,6 +869,20 @@ impl Display for RequirementsTxtFileError {
                     self.file.normalized_display(),
                 )
             }
+            RequirementsTxtParserError::MissingRequirementPrefix(given) => {
+                write!(
+                    f,
+                    "Requirement `{given}` in `{}` looks like a requirements file but was passed as a package name. Did you mean `-r {given}`?",
+                    self.file.normalized_display(),
+                )
+            }
+            RequirementsTxtParserError::MissingEditablePrefix(given) => {
+                write!(
+                    f,
+                    "Requirement `{given}` in `{}` looks like a directory but was passed as a package name. Did you mean `-e {given}`?",
+                    self.file.normalized_display(),
+                )
+            }
             RequirementsTxtParserError::Parser { message, location } => {
                 write!(
                     f,
@@ -1062,6 +1137,34 @@ mod test {
         Ok(())
     }
 
+    #[test]
+    fn missing_r() -> Result<()> {
+        let temp_dir = assert_fs::TempDir::new()?;
+
+        let file_txt = temp_dir.child("file.txt");
+        file_txt.touch()?;
+
+        let requirements_txt = temp_dir.child("requirements.txt");
+        requirements_txt.write_str(indoc! {"
+            flask
+            file.txt
+        "})?;
+
+        let error = RequirementsTxt::parse(requirements_txt.path(), temp_dir.path()).unwrap_err();
+        let errors = anyhow::Error::new(error)
+            .chain()
+            .map(|err| err.to_string().replace('\\', "/"))
+            .join("\n");
+
+        insta::with_settings!({
+            filters => vec![(requirements_txt.path().to_str().unwrap(), "<REQUIREMENTS_TXT>")]
+        }, {
+            insta::assert_display_snapshot!(errors, @"Requirement `file.txt` in `<REQUIREMENTS_TXT>` looks like a requirements file but was passed as a package name. Did you mean `-r file.txt`?");
+        });
+
+        Ok(())
+    }
+
     #[test]
     fn editable_extra() {
         assert_eq!(