Don't emit empty DWARF sections

Cargo.toml

@@ -24,6 +24,7 @@ harness = false
 
 [dependencies]
 anyhow = "1.0"
+fallible-iterator = "0.2"
 id-arena = "2.2.1"
 leb128 = "0.2.4"
 log = "0.4.8"

src/module/debug/mod.rs

@@ -4,6 +4,7 @@ mod units;
 
 use crate::emit::{Emit, EmitContext};
 use crate::{CustomSection, Module, RawCustomSection};
+use fallible_iterator::FallibleIterator;
 use gimli::*;
 
 use self::dwarf::{AddressSearchPreference, ConvertContext, DEAD_CODE};
@@ -42,6 +43,16 @@ impl Module {
 
 impl Emit for ModuleDebugData {
     fn emit(&self, cx: &mut EmitContext) {
+        let from_dwarf = cx
+            .module
+            .debug
+            .dwarf
+            .borrow(|sections| EndianSlice::new(sections.as_ref(), LittleEndian));
+
+        if let Ok(true) = from_dwarf.units().all(|unit| Ok(unit.unit_length() == 0)) {
+            return;
+        }
+
         let address_generator = CodeAddressGenerator::new(&cx.module.funcs);
         let address_converter = CodeAddressConverter::new(&cx.code_transform);
 
@@ -55,12 +66,6 @@ impl Emit for ModuleDebugData {
                 .map(write::Address::Constant)
         };
 
-        let from_dwarf = cx
-            .module
-            .debug
-            .dwarf
-            .borrow(|sections| EndianSlice::new(sections.as_ref(), LittleEndian));
-
         let mut dwarf = write::Dwarf::from(&from_dwarf, &|address| {
             if address == 0 || address == DEAD_CODE {
                 Some(write::Address::Constant(address))
@@ -115,10 +120,12 @@ impl Emit for ModuleDebugData {
         sections
             .for_each(
                 |id: SectionId, data: &write::EndianVec<LittleEndian>| -> Result<()> {
-                    cx.wasm_module.section(&wasm_encoder::CustomSection {
-                        name: id.name().into(),
-                        data: data.slice().into(),
-                    });
+                    if !data.slice().is_empty() {
+                        cx.wasm_module.section(&wasm_encoder::CustomSection {
+                            name: id.name().into(),
+                            data: data.slice().into(),
+                        });
+                    }
                     Ok(())
                 },
             )