don't enable warnings by default if CFLAGS/CXXFLAGS defined in environment

If CFLAGS/CXXFLAGS is defined in the environment, then presumably the consumer has already determined the set of warnings flags to pass to the compiler, so we shouldn't enable warnings by default in that case.

src/lib.rs

@@ -116,8 +116,8 @@ pub struct Build {
     shared_flag: Option<bool>,
     static_flag: Option<bool>,
     warnings_into_errors: bool,
-    warnings: bool,
-    extra_warnings: bool,
+    warnings: Option<bool>,
+    extra_warnings: Option<bool>,
 }
 
 /// Represents the types of errors that may occur while using cc-rs.
@@ -319,8 +319,8 @@ impl Build {
             cargo_metadata: true,
             pic: None,
             static_crt: None,
-            warnings: true,
-            extra_warnings: true,
+            warnings: None,
+            extra_warnings: None,
             warnings_into_errors: false,
         }
     }
@@ -597,8 +597,8 @@ impl Build {
     ///     .compile("libfoo.a");
     /// ```
     pub fn warnings(&mut self, warnings: bool) -> &mut Build {
-        self.warnings = warnings;
-        self.extra_warnings = warnings;
+        self.warnings = Some(warnings);
+        self.extra_warnings = Some(warnings);
         self
     }
 
@@ -620,7 +620,7 @@ impl Build {
     ///     .compile("libfoo.a");
     /// ```
     pub fn extra_warnings(&mut self, warnings: bool) -> &mut Build {
-        self.extra_warnings = warnings;
+        self.extra_warnings = Some(warnings);
         self
     }
 
@@ -1324,12 +1324,17 @@ impl Build {
             cmd.args.push(directory.into());
         }
 
-        if self.warnings {
+        // If warnings and/or extra_warnings haven't been explicitly set,
+        // then we set them only if the environment doesn't already have
+        // CFLAGS/CXXFLAGS, since those variables presumably already contain
+        // the desired set of warnings flags.
+
+        if self.warnings.unwrap_or(if self.has_flags() { false } else { true }) {
             let wflags = cmd.family.warnings_flags().into();
             cmd.push_cc_arg(wflags);
         }
 
-        if self.extra_warnings {
+        if self.extra_warnings.unwrap_or(if self.has_flags() { false } else { true }) {
             if let Some(wflags) = cmd.family.extra_warnings_flags() {
                 cmd.push_cc_arg(wflags.into());
             }
@@ -1366,6 +1371,12 @@ impl Build {
         Ok(cmd)
     }
 
+    fn has_flags(&self) -> bool {
+        let flags_env_var_name = if self.cpp { "CXXFLAGS" } else { "CFLAGS" };
+        let flags_env_var_value = self.get_var(flags_env_var_name);
+        if let Ok(_) = flags_env_var_value { true } else { false }
+    }
+
     fn msvc_macro_assembler(&self) -> Result<(Command, String), Error> {
         let target = self.get_target()?;
         let tool = if target.contains("x86_64") {

tests/test.rs

@@ -1,6 +1,7 @@
 extern crate cc;
 extern crate tempdir;
 
+use std::env;
 use support::Test;
 
 mod support;
@@ -110,6 +111,30 @@ fn gnu_warnings_overridable() {
         .must_have_in_order("-Wall", "-Wno-missing-field-initializers");
 }
 
+#[test]
+fn gnu_no_warnings_if_cflags() {
+    env::set_var("CFLAGS", "-Wflag-does-not-exist");
+    let test = Test::gnu();
+    test.gcc()
+        .file("foo.c")
+        .compile("foo");
+
+    test.cmd(0).must_not_have("-Wall").must_not_have("-Wextra");
+    env::set_var("CFLAGS", "");
+}
+
+#[test]
+fn gnu_no_warnings_if_cxxflags() {
+    env::set_var("CXXFLAGS", "-Wflag-does-not-exist");
+    let test = Test::gnu();
+    test.gcc()
+        .file("foo.c")
+        .compile("foo");
+
+    test.cmd(0).must_not_have("-Wall").must_not_have("-Wextra");
+    env::set_var("CXXFLAGS", "");
+}
+
 #[test]
 fn gnu_x86_64() {
     for vendor in &["unknown-linux-gnu", "apple-darwin"] {