From 14721592c87ca6a4b47016f62cb92bc481a0bbfe Mon Sep 17 00:00:00 2001
From: Alex Crichton <alex@alexcrichton.com>
Date: Mon, 23 Sep 2024 08:02:32 -0700
Subject: [PATCH] Fix testing for errors during fuzzing

Create a first-class error instead of testing for error strings to
ensure this is more resilient over time.
---
 crates/wit-parser/src/lib.rs     |  2 +-
 crates/wit-parser/src/resolve.rs | 32 ++++++++++++++++++++++++++++++--
 crates/wit-smith/src/lib.rs      |  7 ++-----
 3 files changed, 33 insertions(+), 8 deletions(-)

diff --git a/crates/wit-parser/src/lib.rs b/crates/wit-parser/src/lib.rs
index f26b87f01f..ad0f9af3d5 100644
--- a/crates/wit-parser/src/lib.rs
+++ b/crates/wit-parser/src/lib.rs
@@ -21,7 +21,7 @@ pub use ast::{parse_use_path, ParsedUsePath};
 mod sizealign;
 pub use sizealign::*;
 mod resolve;
-pub use resolve::{Package, PackageId, Remap, Resolve};
+pub use resolve::{InvalidTransitiveDependency, Package, PackageId, Remap, Resolve};
 mod live;
 pub use live::{LiveTypes, TypeIdVisitor};
 
diff --git a/crates/wit-parser/src/resolve.rs b/crates/wit-parser/src/resolve.rs
index 40379ae9a8..9799f1a294 100644
--- a/crates/wit-parser/src/resolve.rs
+++ b/crates/wit-parser/src/resolve.rs
@@ -1,4 +1,5 @@
 use std::collections::{BTreeMap, HashMap, HashSet};
+use std::fmt;
 use std::mem;
 use std::path::{Path, PathBuf};
 
@@ -1888,8 +1889,7 @@ package {name} is defined in two different locations:\n\
                     // more refactoring, so it's left to a future date in the
                     // hopes that most folks won't actually run into this for
                     // the time being.
-                    "interface `{name}` transitively depends on an interface in \
-                     incompatible ways",
+                    InvalidTransitiveDependency(name),
                 );
             }
         }
@@ -3218,6 +3218,34 @@ fn update_stability(from: &Stability, into: &mut Stability) -> Result<()> {
     bail!("mismatch in stability attributes")
 }
 
+/// An error that can be returned during "world elaboration" during various
+/// [`Resolve`] operations.
+///
+/// Methods on [`Resolve`] which mutate its internals, such as
+/// [`Resolve::push_dir`] or [`Resolve::importize`] can fail if `world` imports
+/// in WIT packages are invalid. This error indicates one of these situations
+/// where an invalid dependency graph between imports and exports are detected.
+///
+/// Note that at this time this error is subtle and not easy to understand, and
+/// work needs to be done to explain this better and additionally provide a
+/// better error message. For now though this type enables callers to test for
+/// the exact kind of error emitted.
+#[derive(Debug, Clone)]
+pub struct InvalidTransitiveDependency(String);
+
+impl fmt::Display for InvalidTransitiveDependency {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(
+            f,
+            "interface `{}` transitively depends on an interface in \
+             incompatible ways",
+            self.0
+        )
+    }
+}
+
+impl std::error::Error for InvalidTransitiveDependency {}
+
 #[cfg(test)]
 mod tests {
     use crate::Resolve;
diff --git a/crates/wit-smith/src/lib.rs b/crates/wit-smith/src/lib.rs
index b702c5903e..10e753547d 100644
--- a/crates/wit-smith/src/lib.rs
+++ b/crates/wit-smith/src/lib.rs
@@ -6,7 +6,7 @@
 //! type structures.
 
 use arbitrary::{Result, Unstructured};
-use wit_parser::Resolve;
+use wit_parser::{InvalidTransitiveDependency, Resolve};
 
 mod config;
 pub use self::config::Config;
@@ -25,10 +25,7 @@ pub fn smith(config: &Config, u: &mut Unstructured<'_>) -> Result<Vec<u8>> {
         let id = match resolve.push_group(group) {
             Ok(id) => id,
             Err(e) => {
-                if e.to_string().contains(
-                    "interface transitively depends on an interface in \
-                     incompatible ways",
-                ) {
+                if e.is::<InvalidTransitiveDependency>() {
                     return Err(arbitrary::Error::IncorrectFormat);
                 }
                 panic!("bad wit parse: {e:?}")