Simplify std_backtrace conditional code

Author: dtolnay

build.rs

@@ -10,10 +10,10 @@ use std::process::{self, Command, Stdio};
 use std::str;
 
 fn main() {
-    let mut error_generic_member_access = false;
     if cfg!(feature = "std") {
         println!("cargo:rerun-if-changed=src/nightly.rs");
 
+        let error_generic_member_access;
         let consider_rustc_bootstrap;
         if compile_probe(false) {
             // This is a nightly or dev compiler, so it supports unstable
@@ -50,7 +50,6 @@ fn main() {
         }
 
         if error_generic_member_access {
-            println!("cargo:rustc-cfg=std_backtrace");
             println!("cargo:rustc-cfg=error_generic_member_access");
         }
 
@@ -69,13 +68,6 @@ fn main() {
         println!("cargo:rustc-check-cfg=cfg(anyhow_no_clippy_format_args)");
         println!("cargo:rustc-check-cfg=cfg(anyhow_no_core_error)");
         println!("cargo:rustc-check-cfg=cfg(error_generic_member_access)");
-        println!("cargo:rustc-check-cfg=cfg(std_backtrace)");
-    }
-
-    if !error_generic_member_access && cfg!(feature = "std") {
-        // std::backtrace::Backtrace
-        // https://blog.rust-lang.org/2022/11/03/Rust-1.65.0.html#stabilized-apis
-        println!("cargo:rustc-cfg=std_backtrace");
     }
 
     if rustc < 81 {

src/backtrace.rs

@@ -1,17 +1,17 @@
-#[cfg(std_backtrace)]
+#[cfg(feature = "std")]
 pub(crate) use std::backtrace::{Backtrace, BacktraceStatus};
 
-#[cfg(not(std_backtrace))]
+#[cfg(not(feature = "std"))]
 pub(crate) enum Backtrace {}
 
-#[cfg(std_backtrace)]
+#[cfg(feature = "std")]
 macro_rules! backtrace {
     () => {
         Some(crate::backtrace::Backtrace::capture())
     };
 }
 
-#[cfg(not(std_backtrace))]
+#[cfg(not(feature = "std"))]
 macro_rules! backtrace {
     () => {
         None
@@ -28,18 +28,14 @@ macro_rules! backtrace_if_absent {
     };
 }
 
-#[cfg(all(
-    any(feature = "std", not(anyhow_no_core_error)),
-    not(error_generic_member_access),
-    std_backtrace
-))]
+#[cfg(all(not(error_generic_member_access), feature = "std"))]
 macro_rules! backtrace_if_absent {
     ($err:expr) => {
         backtrace!()
     };
 }
 
-#[cfg(all(any(feature = "std", not(anyhow_no_core_error)), not(std_backtrace)))]
+#[cfg(all(not(anyhow_no_core_error), not(feature = "std")))]
 macro_rules! backtrace_if_absent {
     ($err:expr) => {
         None

src/error.rs

@@ -155,7 +155,7 @@ impl Error {
             object_reallocate_boxed: object_reallocate_boxed::<E>,
             object_downcast: object_downcast::<E>,
             object_drop_rest: object_drop_front::<E>,
-            #[cfg(all(not(error_generic_member_access), std_backtrace))]
+            #[cfg(all(not(error_generic_member_access), feature = "std"))]
             object_backtrace: no_backtrace,
         };
 
@@ -179,7 +179,7 @@ impl Error {
             object_reallocate_boxed: object_reallocate_boxed::<MessageError<M>>,
             object_downcast: object_downcast::<M>,
             object_drop_rest: object_drop_front::<M>,
-            #[cfg(all(not(error_generic_member_access), std_backtrace))]
+            #[cfg(all(not(error_generic_member_access), feature = "std"))]
             object_backtrace: no_backtrace,
         };
 
@@ -204,7 +204,7 @@ impl Error {
             object_reallocate_boxed: object_reallocate_boxed::<DisplayError<M>>,
             object_downcast: object_downcast::<M>,
             object_drop_rest: object_drop_front::<M>,
-            #[cfg(all(not(error_generic_member_access), std_backtrace))]
+            #[cfg(all(not(error_generic_member_access), feature = "std"))]
             object_backtrace: no_backtrace,
         };
 
@@ -235,7 +235,7 @@ impl Error {
             object_reallocate_boxed: object_reallocate_boxed::<ContextError<C, E>>,
             object_downcast: context_downcast::<C, E>,
             object_drop_rest: context_drop_rest::<C, E>,
-            #[cfg(all(not(error_generic_member_access), std_backtrace))]
+            #[cfg(all(not(error_generic_member_access), feature = "std"))]
             object_backtrace: no_backtrace,
         };
 
@@ -260,7 +260,7 @@ impl Error {
             object_reallocate_boxed: object_reallocate_boxed::<BoxedError>,
             object_downcast: object_downcast::<Box<dyn StdError + Send + Sync>>,
             object_drop_rest: object_drop_front::<Box<dyn StdError + Send + Sync>>,
-            #[cfg(all(not(error_generic_member_access), std_backtrace))]
+            #[cfg(all(not(error_generic_member_access), feature = "std"))]
             object_backtrace: no_backtrace,
         };
 
@@ -372,7 +372,7 @@ impl Error {
             object_reallocate_boxed: object_reallocate_boxed::<ContextError<C, Error>>,
             object_downcast: context_chain_downcast::<C>,
             object_drop_rest: context_chain_drop_rest::<C>,
-            #[cfg(all(not(error_generic_member_access), std_backtrace))]
+            #[cfg(all(not(error_generic_member_access), feature = "std"))]
             object_backtrace: context_backtrace::<C>,
         };
 
@@ -410,7 +410,7 @@ impl Error {
     /// [dependencies]
     /// anyhow = { version = "1.0", features = ["backtrace"] }
     /// ```
-    #[cfg(std_backtrace)]
+    #[cfg(feature = "std")]
     pub fn backtrace(&self) -> &std::backtrace::Backtrace {
         unsafe { ErrorImpl::backtrace(self.inner.by_ref()) }
     }
@@ -728,7 +728,7 @@ struct ErrorVTable {
     object_reallocate_boxed: unsafe fn(Own<ErrorImpl>) -> Box<dyn StdError + Send + Sync + 'static>,
     object_downcast: unsafe fn(Ref<ErrorImpl>, TypeId) -> Option<Ref<()>>,
     object_drop_rest: unsafe fn(Own<ErrorImpl>, TypeId),
-    #[cfg(all(not(error_generic_member_access), std_backtrace))]
+    #[cfg(all(not(error_generic_member_access), feature = "std"))]
     object_backtrace: unsafe fn(Ref<ErrorImpl>) -> Option<&Backtrace>,
 }
 
@@ -804,7 +804,7 @@ where
     }
 }
 
-#[cfg(all(not(error_generic_member_access), std_backtrace))]
+#[cfg(all(not(error_generic_member_access), feature = "std"))]
 fn no_backtrace(e: Ref<ErrorImpl>) -> Option<&Backtrace> {
     let _ = e;
     None
@@ -888,7 +888,7 @@ where
 }
 
 // Safety: requires layout of *e to match ErrorImpl<ContextError<C, Error>>.
-#[cfg(all(not(error_generic_member_access), std_backtrace))]
+#[cfg(all(not(error_generic_member_access), feature = "std"))]
 #[allow(clippy::unnecessary_wraps)]
 unsafe fn context_backtrace<C>(e: Ref<ErrorImpl>) -> Option<&Backtrace>
 where
@@ -954,7 +954,7 @@ impl ErrorImpl {
         }
     }
 
-    #[cfg(std_backtrace)]
+    #[cfg(feature = "std")]
     pub(crate) unsafe fn backtrace(this: Ref<Self>) -> &Backtrace {
         // This unwrap can only panic if the underlying error's backtrace method
         // is nondeterministic, which would only happen in maliciously

src/fmt.rs

@@ -40,7 +40,7 @@ impl ErrorImpl {
             }
         }
 
-        #[cfg(std_backtrace)]
+        #[cfg(feature = "std")]
         {
             use crate::backtrace::BacktraceStatus;
             use alloc::string::ToString;