Add a feature to stop linking the default panic

When end-user crates provide a panic handler, this symbol is not needed
... but the linker isn't smart enough to optimize it away, even though
the PROVIDE line isn't used (the symbol is provided by the rust function
tagged with #[panic_handler]).  Those crates can enable this feature to
remove the PROVIDE line from the linker script, to stop linking this
default panic handler.

CHANGELOG.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+- [#812]: `defmt`: Add a feature to stop linking a default panic handler
+
+[#812]: https://github.com/knurling-rs/defmt/pull/812
+
 ## [v0.3.6] - 2024-02-05
 
 - [#804]: `CI`: Remove mdbook strategy

defmt/Cargo.toml

@@ -20,6 +20,7 @@ version = "0.3.6"
 [features]
 alloc = []
 ip_in_core = []
+avoid-default-panic = []
 
 # Encoding feature flags. These should only be set by end-user crates, not by library crates.
 #

defmt/build.rs

@@ -1,10 +1,21 @@
 use std::{env, error::Error, fs, path::PathBuf};
 
+fn sub(linker_script: String) -> String {
+    #[cfg(not(feature = "avoid-default-panic"))]
+    {
+        linker_script
+    }
+    #[cfg(feature = "avoid-default-panic")]
+    {
+        linker_script.replacen("PROVIDE(_defmt_panic = __defmt_default_panic);", "", 1)
+    }
+}
+
 fn main() -> Result<(), Box<dyn Error>> {
     // Put the linker script somewhere the linker can find it
     let out = &PathBuf::from(env::var("OUT_DIR")?);
     let linker_script = fs::read_to_string("defmt.x.in")?;
-    fs::write(out.join("defmt.x"), linker_script)?;
+    fs::write(out.join("defmt.x"), sub(linker_script))?;
     println!("cargo:rustc-link-search={}", out.display());
     let target = env::var("TARGET")?;