Support target.'cfg(..)'.rustdocflags analogously to rustflags

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

@@ -870,15 +870,15 @@ fn rustflags_from_target(
     if let Some(target_cfg) = target_cfg {
         gctx.target_cfgs()?
             .iter()
-            .filter_map(|(key, cfg)| {
-                match flag {
-                    Flags::Rust => cfg
-                        .rustflags
-                        .as_ref()
-                        .map(|rustflags| (key, &rustflags.val)),
-                    // `target.cfg(…).rustdocflags` is currently not supported.
-                    Flags::Rustdoc => None,
-                }
+            .filter_map(|(key, cfg)| match flag {
+                Flags::Rust => cfg
+                    .rustflags
+                    .as_ref()
+                    .map(|rustflags| (key, &rustflags.val)),
+                Flags::Rustdoc => cfg
+                    .rustdocflags
+                    .as_ref()
+                    .map(|rustdocflags| (key, &rustdocflags.val)),
             })
             .filter(|(key, _rustflags)| CfgExpr::matches_key(key, target_cfg))
             .for_each(|(_key, cfg_rustflags)| {

src/cargo/util/context/target.rs

@@ -13,6 +13,7 @@ use std::rc::Rc;
 pub struct TargetCfgConfig {
     pub runner: OptValue<PathAndArgs>,
     pub rustflags: OptValue<StringList>,
+    pub rustdocflags: OptValue<StringList>,
     pub linker: OptValue<ConfigRelativePath>,
     // This is here just to ignore fields from normal `TargetConfig` because
     // all `[target]` tables are getting deserialized, whether they start with

src/doc/src/reference/config.md

@@ -605,7 +605,8 @@ order, with the first one being used:
 
 1. `CARGO_ENCODED_RUSTDOCFLAGS` environment variable.
 2. `RUSTDOCFLAGS` environment variable.
-3. All matching `target.<triple>.rustdocflags` config entries joined together.
+3. All matching `target.<triple>.rustdocflags` and `target.<cfg>.rustdocflags`
+  config entries joined together.
 4. `build.rustdocflags` config value.
 
 Additional flags may also be passed with the [`cargo rustdoc`] command.
@@ -1394,6 +1395,12 @@ The value may be an array of strings or a space-separated string.
 See [`build.rustdocflags`](#buildrustdocflags) for more details on the different
 ways to specific extra flags.
 
+#### `target.<cfg>.rustdocflags`
+
+This is similar to the [target rustdocflags](#targettriplerustdocflags), but
+using a [`cfg()` expression]. If several `<cfg>` and [`<triple>`] entries
+match the current target, the flags are joined together.
+
 #### `target.<triple>.<links>`
 
 The links sub-table provides a way to [override a build script]. When

tests/testsuite/rustdocflags.rs

@@ -193,6 +193,72 @@ fn not_affected_by_target_rustflags() {
         .run();
 }
 
+#[cargo_test]
+fn target_cfg_rustdocflags_works() {
+    let p = project()
+        .file("src/lib.rs", "")
+        .file(
+            ".cargo/config.toml",
+            r#"
+                    [target.'cfg(true)']
+                    rustdocflags = ["--cfg", "from_target_cfg"]
+
+                    [build]
+                    rustdocflags = ["--cfg", "from_build"]
+                "#,
+        )
+        .build();
+
+    p.cargo("doc -v")
+        .with_stderr_data(str![[r#"
+...
+[RUNNING] `rustdoc [..] --cfg from_target_cfg[..]`
+...
+"#]])
+        .run();
+}
+
+#[cargo_test]
+fn target_cfg_rustdocflags_works_through_cargo_test() {
+    let p = project()
+        .file(
+            "src/lib.rs",
+            r#"
+                //! ```
+                //! assert!(cfg!(from_target_cfg));
+                //! ```
+            "#,
+        )
+        .file(
+            ".cargo/config.toml",
+            r#"
+                    [target.'cfg(true)']
+                    rustdocflags = ["--cfg", "from_target_cfg"]
+
+                    [build]
+                    rustdocflags = ["--cfg", "from_build"]
+                "#,
+        )
+        .build();
+
+    p.cargo("test --doc -v")
+        .with_stderr_data(str![[r#"
+...
+[RUNNING] `rustdoc[..]--test[..]--cfg[..]from_target_cfg[..]`
+
+"#]])
+        .with_stdout_data(str![[r#"
+
+running 1 test
+test src/lib.rs - (line 2) ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in [ELAPSED]s
+
+
+"#]])
+        .run();
+}
+
 #[cargo_test]
 fn target_triple_rustdocflags_works() {
     let host = rustc_host();