Make http.proxy-cainfo fallback to http.cainfo, like curl.

Author: koxu1996

src/cargo/util/network/http.rs

@@ -61,7 +61,8 @@ pub fn configure_http_handle(gctx: &GlobalContext, handle: &mut Easy) -> CargoRe
         let cainfo = cainfo.resolve_path(gctx);
         handle.cainfo(&cainfo)?;
     }
-    if let Some(proxy_cainfo) = &http.proxy_cainfo {
+    // Use `proxy_cainfo` if explicitly set; otherwise, fall back to `cainfo` as curl does #15376.
+    if let Some(proxy_cainfo) = http.proxy_cainfo.as_ref().or(http.cainfo.as_ref()) {
         let proxy_cainfo = proxy_cainfo.resolve_path(gctx);
         handle.proxy_cainfo(&format!("{}", proxy_cainfo.display()))?;
     }

src/doc/src/reference/config.md

@@ -711,7 +711,7 @@ certificates. If not specified, Cargo attempts to use the system certificates.
 
 #### `http.proxy-cainfo`
 * Type: string (path)
-* Default: none
+* Default: falls back to `http.cainfo` if not set
 * Environment: `CARGO_HTTP_PROXY_CAINFO`
 
 Path to a Certificate Authority (CA) bundle file, used to verify proxy TLS