A proof-of-concept for defmt *without* interning strings.

This is just an experiment for now.

If the 'no-interning' feature is set, instead of

#[export_name = "{JSON...}"]
static S: u8 = 0;
&S as *const _ as usize

You get:

static S: &'static str = "{JSON...}";
&S as *const _ as usize

Note that I had to change the size of the interned ID from u16 to usize, because with no-interning it's now a pointer to actual data.

Author: jonathanpallant

book/src/setup.md

@@ -59,4 +59,10 @@ To learn how to enable other logging levels and filters logs per modules read th
 
 ### Memory use
 
-When in a tight memory situation and logging over RTT, the buffer size (default: 1024 bytes) can be configured with the `DEFMT_RTT_BUFFER_SIZE` environment variable. Use a power of 2 for best performance.
+When in a tight memory situation and logging over RTT, the buffer size (default: 1024 bytes) can be configured with the `DEFMT_RTT_BUFFER_SIZE` environment variable. Use a power of 2 for best performance.
+
+### Use on Windows/macOS/Linux
+
+You can use `defmt` without string-interning by enabling the `no-interning` feature. This will mean that format strings are included as literal strings and not hidden in the symbol name.
+
+You can optionally send the output to the `log` crate by enabling the `use-log-crate` feature.

defmt/Cargo.toml

@@ -21,6 +21,8 @@ version = "0.3.8"
 alloc = []
 avoid-default-panic = []
 ip_in_core = []
+no-interning = [ "defmt-macros/no-interning" ]
+use-log-crate = []
 
 # Encoding feature flags. These should only be set by end-user crates, not by library crates.
 #

defmt/src/export/mod.rs

@@ -99,7 +99,7 @@ pub fn timestamp(fmt: crate::Formatter<'_>) {
 }
 
 /// Returns the interned string at `address`.
-pub fn make_istr(address: u16) -> Str {
+pub fn make_istr(address: usize) -> Str {
     Str { address }
 }
 

defmt/src/formatter.rs

@@ -11,6 +11,6 @@ pub struct Formatter<'a> {
 /// [`intern!`]: macro.intern.html
 #[derive(Clone, Copy)]
 pub struct Str {
-    /// 16-bit address
-    pub(crate) address: u16,
+    /// The address of the interned string
+    pub(crate) address: usize,
 }

macros/Cargo.toml

@@ -15,6 +15,8 @@ proc-macro = true
 [features]
 # WARNING: for internal use only, not covered by semver guarantees
 unstable-test = []
+# Turns off string interning
+no-interning = []
 
 [dependencies]
 defmt-parser = { version = "=0.3.4", path = "../parser" }

macros/src/construct.rs

@@ -47,7 +47,7 @@ pub(crate) fn interned_string(
         let var_item = static_variable(&var_name, string, tag, prefix);
         quote!({
             #var_item
-            &#var_name as *const u8 as u16
+            &#var_name as *const _ as usize
         })
     };
 
@@ -61,17 +61,23 @@ pub(crate) fn interned_string(
 ///   under macos: ".defmt," + 16 character hex digest of symbol's hash
 ///   otherwise:   ".defmt." + prefix + symbol
 pub(crate) fn linker_section(for_macos: bool, prefix: Option<&str>, symbol: &str) -> String {
-    let mut sub_section = if let Some(prefix) = prefix {
-        format!(".{prefix}.{symbol}")
+    let symbol_hash = hash(symbol);
+    let suffix = if for_macos {
+        // The Apple linker doesn't like `.` in input section names
+        if let Some(prefix) = prefix {
+            format!(",{prefix}_{symbol_hash:x}")
+        } else {
+            format!(",{symbol_hash:x}")
+        }
     } else {
-        format!(".{symbol}")
+        if let Some(prefix) = prefix {
+            format!(".{prefix}.{symbol_hash:x}")
+        } else {
+            format!(".{symbol_hash:x}")
+        }
     };
 
-    if for_macos {
-        sub_section = format!(",{:x}", hash(&sub_section));
-    }
-
-    format!(".defmt{sub_section}")
+    format!(".defmt{suffix}")
 }
 
 pub(crate) fn static_variable(
@@ -83,13 +89,20 @@ pub(crate) fn static_variable(
     let sym_name = mangled_symbol_name(tag, data);
     let section = linker_section(false, prefix, &sym_name);
     let section_for_macos = linker_section(true, prefix, &sym_name);
-
-    quote!(
-        #[cfg_attr(target_os = "macos", link_section = #section_for_macos)]
-        #[cfg_attr(not(target_os = "macos"), link_section = #section)]
-        #[export_name = #sym_name]
-        static #name: u8 = 0;
-    )
+    if cfg!(feature = "no-interning") {
+        quote!(
+            #[cfg_attr(target_os = "macos", link_section = #section_for_macos)]
+            #[cfg_attr(not(target_os = "macos"), link_section = #section)]
+            static #name: &'static str = #sym_name;
+        )
+    } else {
+        quote!(
+            #[cfg_attr(target_os = "macos", link_section = #section_for_macos)]
+            #[cfg_attr(not(target_os = "macos"), link_section = #section)]
+            #[export_name = #sym_name]
+            static #name: u8 = 0;
+        )
+    }
 }
 
 pub(crate) fn string_literal(content: &str) -> LitStr {

macros/src/function_like/internp.rs

@@ -13,14 +13,21 @@ pub(crate) fn expand(args: TokenStream) -> TokenStream {
     let section_for_macos = construct::linker_section(true, prefix, &sym_name);
 
     let var_addr = if cfg!(feature = "unstable-test") {
-        quote!({ defmt::export::fetch_add_string_index() as u16 })
+        quote!({ defmt::export::fetch_add_string_index() as usize })
+    } else if cfg!(feature = "no-interning") {
+        quote!({
+            #[cfg_attr(target_os = "macos", link_section = #section_for_macos)]
+            #[cfg_attr(not(target_os = "macos"), link_section = #section)]
+            static S: &'static str = #sym_name;
+            &S as *const _ as usize
+        })
     } else {
         quote!({
             #[cfg_attr(target_os = "macos", link_section = #section_for_macos)]
             #[cfg_attr(not(target_os = "macos"), link_section = #section)]
             #[export_name = #sym_name]
             static S: u8 = 0;
-            &S as *const u8 as u16
+            &S as *const _ as usize
         })
     };