bootstrap: be more explicit on what we collect into. NFC

src/bootstrap/lib.rs

@@ -783,10 +783,10 @@ impl Build {
     fn cflags(&self, target: Interned<String>, which: GitRepo) -> Vec<String> {
         // Filter out -O and /O (the optimization flags) that we picked up from
         // cc-rs because the build scripts will determine that for themselves.
-        let mut base: Vec<String> = self.cc[&target].args().iter()
+        let mut base = self.cc[&target].args().iter()
                            .map(|s| s.to_string_lossy().into_owned())
                            .filter(|s| !s.starts_with("-O") && !s.starts_with("/O"))
-                           .collect::<Vec<_>>();
+                           .collect::<Vec<String>>();
 
         // If we're compiling on macOS then we add a few unconditional flags
         // indicating that we want libc++ (more filled out than libstdc++) and