fix(spirv-builder): error instead of panic when backend dylib is missing

crates/spirv-builder/src/lib.rs

@@ -110,6 +110,8 @@ pub enum SpirvBuilderError {
         "Without feature `rustc_codegen_spirv`, you need to set the path of the dylib with `rustc_codegen_spirv_location`"
     )]
     MissingRustcCodegenSpirvDylib,
+    #[error("could not find `{filename}` in the dynamic library search path")]
+    RustcCodegenSpirvDylibNotFound { filename: String },
     #[error("`rustc_codegen_spirv_location` path '{0}' is not a file")]
     RustcCodegenSpirvDylibDoesNotExist(PathBuf),
     #[error("build failed")]
@@ -863,11 +865,11 @@ fn find_rustc_codegen_spirv_in_paths(dylib_paths: Vec<PathBuf>) -> Option<PathBu
 
 fn find_rustc_codegen_spirv() -> Result<PathBuf, SpirvBuilderError> {
     if cfg!(feature = "rustc_codegen_spirv") {
-        if let Some(path) = find_rustc_codegen_spirv_in_paths(dylib_path()) {
-            return Ok(path);
-        }
-        let filename = rustc_codegen_spirv_dylib_name();
-        panic!("Could not find {filename} in library path");
+        find_rustc_codegen_spirv_in_paths(dylib_path()).ok_or_else(|| {
+            SpirvBuilderError::RustcCodegenSpirvDylibNotFound {
+                filename: rustc_codegen_spirv_dylib_name(),
+            }
+        })
     } else {
         Err(SpirvBuilderError::MissingRustcCodegenSpirvDylib)
     }

crates/spirv-builder/src/tests.rs

@@ -20,7 +20,9 @@ mod clap {
 }
 
 mod dylib_lookup {
-    use crate::{find_rustc_codegen_spirv_in_paths, rustc_codegen_spirv_dylib_name};
+    use crate::{
+        SpirvBuilderError, find_rustc_codegen_spirv_in_paths, rustc_codegen_spirv_dylib_name,
+    };
     use std::fs;
     use std::path::{Path, PathBuf};
     use std::time::{Duration, SystemTime, UNIX_EPOCH};
@@ -52,6 +54,43 @@ mod dylib_lookup {
         }
     }
 
+    #[test]
+    fn finds_nothing_in_empty_or_missing_dirs() {
+        let empty = TempDir::new("empty");
+        let missing = empty.path().join("does-not-exist");
+
+        assert_eq!(find_rustc_codegen_spirv_in_paths(vec![]), None);
+        assert_eq!(
+            find_rustc_codegen_spirv_in_paths(vec![empty.path().to_path_buf(), missing]),
+            None
+        );
+    }
+
+    #[test]
+    fn ignores_unrelated_dylibs() {
+        let dir = TempDir::new("unrelated");
+        let unrelated = dir.path().join(format!(
+            "{}some_other_backend{}",
+            std::env::consts::DLL_PREFIX,
+            std::env::consts::DLL_SUFFIX
+        ));
+        fs::File::create(&unrelated).expect("failed to create unrelated dylib file");
+
+        assert_eq!(
+            find_rustc_codegen_spirv_in_paths(vec![dir.path().to_path_buf()]),
+            None
+        );
+    }
+
+    /// A missing backend dylib is reported as an error (it used to `panic!`).
+    #[test]
+    fn not_found_error_names_the_dylib() {
+        let err = SpirvBuilderError::RustcCodegenSpirvDylibNotFound {
+            filename: rustc_codegen_spirv_dylib_name(),
+        };
+        assert!(err.to_string().contains(&rustc_codegen_spirv_dylib_name()));
+    }
+
     #[test]
     fn finds_exact_backend_dylib() {
         let dir = TempDir::new("exact");