Add comment why thiserror is not used

cranelift/codegen/src/data_value.rs

@@ -103,6 +103,8 @@ pub enum DataValueCastFailure {
     FromInteger(i64, Type),
 }
 
+// This is manually implementing Error and Display instead of using thiserror to reduce the amount
+// of dependencies used by Cranelift.
 impl std::error::Error for DataValueCastFailure {}
 
 impl Display for DataValueCastFailure {

cranelift/codegen/src/isa/mod.rs

@@ -186,6 +186,8 @@ pub enum LookupError {
     Unsupported,
 }
 
+// This is manually implementing Error and Display instead of using thiserror to reduce the amount
+// of dependencies used by Cranelift.
 impl std::error::Error for LookupError {}
 
 impl fmt::Display for LookupError {

cranelift/codegen/src/isa/unwind/systemv.rs

@@ -21,6 +21,8 @@ pub enum RegisterMappingError {
     UnsupportedRegisterBank(&'static str),
 }
 
+// This is manually implementing Error and Display instead of using thiserror to reduce the amount
+// of dependencies used by Cranelift.
 impl std::error::Error for RegisterMappingError {}
 
 impl std::fmt::Display for RegisterMappingError {

cranelift/codegen/src/result.rs

@@ -41,6 +41,8 @@ pub enum CodegenError {
 /// A convenient alias for a `Result` that uses `CodegenError` as the error type.
 pub type CodegenResult<T> = Result<T, CodegenError>;
 
+// This is manually implementing Error and Display instead of using thiserror to reduce the amount
+// of dependencies used by Cranelift.
 impl std::error::Error for CodegenError {
     fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
         match self {

cranelift/codegen/src/verifier/mod.rs

@@ -102,6 +102,8 @@ pub struct VerifierError {
     pub message: String,
 }
 
+// This is manually implementing Error and Display instead of using thiserror to reduce the amount
+// of dependencies used by Cranelift.
 impl std::error::Error for VerifierError {}
 
 impl Display for VerifierError {
@@ -179,6 +181,8 @@ pub type VerifierResult<T> = Result<T, VerifierErrors>;
 #[derive(Debug, Default, PartialEq, Eq, Clone)]
 pub struct VerifierErrors(pub Vec<VerifierError>);
 
+// This is manually implementing Error and Display instead of using thiserror to reduce the amount
+// of dependencies used by Cranelift.
 impl std::error::Error for VerifierErrors {}
 
 impl VerifierErrors {

cranelift/module/src/module.rs

@@ -192,6 +192,8 @@ pub enum ModuleError {
     Backend(anyhow::Error),
 }
 
+// This is manually implementing Error and Display instead of using thiserror to reduce the amount
+// of dependencies used by Cranelift.
 impl std::error::Error for ModuleError {
     fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
         match self {