Gleaning rustdocflags from target.cfg(…) is not supported

The bug was `rustflags_from_target` trying to learn rustdocflags from
`target.cfg(…).rustflags`, which is definitely wrong.
As of this writing, either `target.cfg(…).rustdocflags` or
`target.<triple>.rustdocflags` is not supported.

Author: weihanglo

src/cargo/core/compiler/build_context/target_info.rs

@@ -749,9 +749,15 @@ fn rustflags_from_target(
             .target_cfgs()?
             .iter()
             .filter_map(|(key, cfg)| {
-                cfg.rustflags
-                    .as_ref()
-                    .map(|rustflags| (key, &rustflags.val))
+                match flag {
+                    Flags::Rust => cfg
+                        .rustflags
+                        .as_ref()
+                        .map(|rustflags| (key, &rustflags.val)),
+                    // `target.cfg(…).rustdocflags` is currently not supported.
+                    // In fact, neither is `target.<triple>.rustdocflags`.
+                    Flags::Rustdoc => None,
+                }
             })
             .filter(|(key, _rustflags)| CfgExpr::matches_key(key, target_cfg))
             .for_each(|(_key, cfg_rustflags)| {

tests/testsuite/rustdocflags.rs

@@ -148,9 +148,8 @@ fn not_affected_by_target_rustflags() {
         .with_stderr_contains("[RUNNING] `rustc [..] -D missing-docs[..]`")
         .run();
 
-    // This is wrong behaviour. `cargo doc` shouldn't fail.
+    // `cargo doc` shouldn't fail.
     p.cargo("doc -v")
-        .with_status(101)
-        .with_stderr_contains("[RUNNING] `rustdoc [..] -D missing-docs[..]`")
+        .with_stderr_contains("[RUNNING] `rustdoc [..] --cfg foo[..]`")
         .run();
 }