Remove 2 frames of noise from 'context' backtraces

dtolnay · dtolnay · commit 131249b11c0f · 2022-10-20T14:25:48.000-07:00
Repro:

    use anyhow::Context;

    fn main() -&gt; anyhow::Result&lt;()&gt; {
        let result = Err(std::fmt::Error);
        result.context("...")
    }

Before:

    0: &lt;E as anyhow::context::ext::StdError&gt;::ext_context
              at /git/anyhow/src/context.rs:27:29
    1: anyhow::context::&lt;impl anyhow::Context&lt;T,E&gt; for core::result::Result&lt;T,E&gt;&gt;::context::{{closure}}
              at /git/anyhow/src/context.rs:50:30
    2: core::result::Result&lt;T,E&gt;::map_err
              at /rustc/4b8f4319954ff2642690b9e5cbe4af352d095bf6/library/core/src/result.rs:861:27
    3: anyhow::context::&lt;impl anyhow::Context&lt;T,E&gt; for core::result::Result&lt;T,E&gt;&gt;::context
              at /git/anyhow/src/context.rs:50:9
    4: testing::main
              at ./src/main.rs:5:5

After:

    0: &lt;E as anyhow::context::ext::StdError&gt;::ext_context
              at /git/anyhow/src/context.rs:27:29
    1: anyhow::context::&lt;impl anyhow::Context&lt;T,E&gt; for core::result::Result&lt;T,E&gt;&gt;::context
              at /git/anyhow/src/context.rs:52:31
    2: testing::main
              at ./src/main.rs:5:5
diff --git a/src/context.rs b/src/context.rs
@@ -47,15 +47,23 @@ where
     where
         C: Display + Send + Sync + 'static,
     {
-        self.map_err(|error| error.ext_context(context))
+        // Not using map_err to save 2 useless frames off the captured backtrace
+        // in ext_context.
+        match self {
+            Ok(ok) => Ok(ok),
+            Err(error) => Err(error.ext_context(context)),
+        }
     }
 
     fn with_context<C, F>(self, context: F) -> Result<T, Error>
     where
         C: Display + Send + Sync + 'static,
         F: FnOnce() -> C,
     {
-        self.map_err(|error| error.ext_context(context()))
+        match self {
+            Ok(ok) => Ok(ok),
+            Err(error) => Err(error.ext_context(context())),
+        }
     }
 }
 

Original file line number	Diff line number	Diff line change
`@@ -47,15 +47,23 @@ where`
`47`	`47`	`where`
`48`	`48`	`C: Display + Send + Sync + 'static,`
`49`	`49`	`{`
`50`		`- self.map_err(\|error\| error.ext_context(context))`
	`50`	`+ // Not using map_err to save 2 useless frames off the captured backtrace`
	`51`	`+ // in ext_context.`
	`52`	`+ match self {`
	`53`	`+ Ok(ok) => Ok(ok),`
	`54`	`+ Err(error) => Err(error.ext_context(context)),`
	`55`	`+ }`
`51`	`56`	`}`
`52`	`57`
`53`	`58`	`fn with_context<C, F>(self, context: F) -> Result<T, Error>`
`54`	`59`	`where`
`55`	`60`	`C: Display + Send + Sync + 'static,`
`56`	`61`	`F: FnOnce() -> C,`
`57`	`62`	`{`
`58`		`- self.map_err(\|error\| error.ext_context(context()))`
	`63`	`+ match self {`
	`64`	`+ Ok(ok) => Ok(ok),`
	`65`	`+ Err(error) => Err(error.ext_context(context())),`
	`66`	`+ }`
`59`	`67`	`}`
`60`	`68`	`}`
`61`	`69`