Migrate from failure to thiserror

The failure crate invents its own traits that don't use
std::error::Error (because failure predates certain features added to
Error); this prevents using ? on an error from failure in a function
using Error. The thiserror crate integrates with the standard Error
trait instead.

cranelift/codegen/src/result.rs

@@ -1,19 +1,19 @@
 //! Result and error types representing the outcome of compiling a function.
 
 use crate::verifier::VerifierErrors;
-use failure_derive::Fail;
+use thiserror::Error;
 
 /// A compilation error.
 ///
 /// When Cranelift fails to compile a function, it will return one of these error codes.
-#[derive(Fail, Debug, PartialEq, Eq)]
+#[derive(Error, Debug, PartialEq, Eq)]
 pub enum CodegenError {
     /// A list of IR verifier errors.
     ///
     /// This always represents a bug, either in the code that generated IR for Cranelift, or a bug
     /// in Cranelift itself.
-    #[fail(display = "Verifier errors:\n{}", _0)]
-    Verifier(#[cause] VerifierErrors),
+    #[error("Verifier errors")]
+    Verifier(#[from] VerifierErrors),
 
     /// An implementation limit was exceeded.
     ///
@@ -21,22 +21,16 @@ pub enum CodegenError {
     /// limits][limits] that cause compilation to fail when they are exceeded.
     ///
     /// [limits]: https://cranelift.readthedocs.io/en/latest/ir.html#implementation-limits
-    #[fail(display = "Implementation limit exceeded")]
+    #[error("Implementation limit exceeded")]
     ImplLimitExceeded,
 
     /// The code size for the function is too large.
     ///
     /// Different target ISAs may impose a limit on the size of a compiled function. If that limit
     /// is exceeded, compilation fails.
-    #[fail(display = "Code for function is too large")]
+    #[error("Code for function is too large")]
     CodeTooLarge,
 }
 
 /// A convenient alias for a `Result` that uses `CodegenError` as the error type.
 pub type CodegenResult<T> = Result<T, CodegenError>;
-
-impl From<VerifierErrors> for CodegenError {
-    fn from(e: VerifierErrors) -> Self {
-        Self::Verifier(e)
-    }
-}