Add Android support to gimli (#415)

Cargo.toml

@@ -2,6 +2,7 @@
 name = "backtrace"
 version = "0.3.57"
 authors = ["The Rust Project Developers"]
+build = "build.rs"
 license = "MIT/Apache-2.0"
 readme = "README.md"
 repository = "https://github.com/rust-lang/backtrace-rs"
@@ -22,7 +23,7 @@ exclude = ['crates/without_debuginfo', 'crates/macos_frames_test', 'crates/line-
 cfg-if = "1.0"
 rustc-demangle = "0.1.4"
 backtrace-sys = { path = "crates/backtrace-sys", version = "0.1.35", optional = true, default_features = false }
-libc = { version = "0.2.87", default-features = false }
+libc = { version = "0.2.94", default-features = false }
 
 # Optionally enable the ability to serialize a `Backtrace`, controlled through
 # the `serialize-*` features below.
@@ -46,6 +47,11 @@ features = ['read_core', 'elf', 'macho', 'pe', 'unaligned', 'archive']
 [target.'cfg(windows)'.dependencies]
 winapi = { version = "0.3.3", optional = true }
 
+[build-dependencies]
+# Only needed for Android, but cannot be target dependent
+# https://github.com/rust-lang/cargo/issues/4932
+cc = "1.0.67"
+
 [dev-dependencies]
 dylib-dep = { path = "crates/dylib-dep" }
 libloading = "0.6"

build.rs

@@ -0,0 +1,35 @@
+extern crate cc;
+
+use std::env;
+
+fn main() {
+    match env::var("CARGO_CFG_TARGET_OS").unwrap_or_default().as_str() {
+        "android" => build_android(),
+        _ => {}
+    }
+}
+
+fn build_android() {
+    let expansion = cc::Build::new().file("src/android-api.c").expand();
+    let expansion = match std::str::from_utf8(&expansion) {
+        Ok(s) => s,
+        Err(_) => return,
+    };
+    println!("expanded android version detection:\n{}", expansion);
+    let marker = "APIVERSION";
+    let i = match expansion.find(marker) {
+        Some(i) => i,
+        None => return,
+    };
+    let version = match expansion[i + marker.len() + 1..].split_whitespace().next() {
+        Some(s) => s,
+        None => return,
+    };
+    let version = match version.parse::<u32>() {
+        Ok(n) => n,
+        Err(_) => return,
+    };
+    if version >= 21 {
+        println!("cargo:rustc-cfg=feature=\"dl_iterate_phdr\"");
+    }
+}

src/android-api.c

@@ -0,0 +1,4 @@
+// Used from the build script to detect the value of the `__ANDROID_API__`
+// builtin #define
+
+APIVERSION __ANDROID_API__

src/symbolize/gimli.rs

@@ -154,6 +154,7 @@ cfg_if::cfg_if! {
             target_os = "linux",
             target_os = "fuchsia",
             target_os = "freebsd",
+            all(target_os = "android", feature = "dl_iterate_phdr"),
         ),
         not(target_env = "uclibc"),
     ))] {