Run eval_config_entry on all branches so we always emit lints

compiler/rustc_builtin_macros/src/cfg_select.rs

@@ -10,18 +10,21 @@ use crate::errors::{CfgSelectNoMatches, CfgSelectUnreachable};
 
 /// Selects the first arm whose predicate evaluates to true.
 fn select_arm(ecx: &ExtCtxt<'_>, branches: CfgSelectBranches) -> Option<(TokenStream, Span)> {
+    let mut result = None;
     for (cfg, tt, arm_span) in branches.reachable {
         if let EvalConfigResult::True = attr::eval_config_entry(
             &ecx.sess,
             &cfg,
             ecx.current_expansion.lint_node_id,
             ShouldEmit::ErrorsAndLints,
         ) {
-            return Some((tt, arm_span));
+            // FIXME(#149215) Ideally we should short-circuit here, but `eval_config_entry` currently emits lints so we cannot do this yet.
+            result.get_or_insert((tt, arm_span));
         }
     }
 
-    branches.wildcard.map(|(_, tt, span)| (tt, span))
+    let wildcard = branches.wildcard.map(|(_, tt, span)| (tt, span));
+    result.or(wildcard)
 }
 
 pub(super) fn expand_cfg_select<'cx>(

tests/ui/check-cfg/cfg-select.rs

@@ -0,0 +1,18 @@
+//@ check-pass
+
+#![feature(cfg_select)]
+#![crate_type = "lib"]
+
+cfg_select! {
+    true => {}
+    invalid_cfg1 => {}
+    //~^ WARN unexpected `cfg` condition name
+    _ => {}
+}
+
+cfg_select! {
+    invalid_cfg2 => {}
+    //~^ WARN unexpected `cfg` condition name
+    true => {}
+    _ => {}
+}

tests/ui/check-cfg/cfg-select.stderr

@@ -0,0 +1,22 @@
+warning: unexpected `cfg` condition name: `invalid_cfg1`
+  --> $DIR/cfg-select.rs:8:5
+   |
+LL |     invalid_cfg1 => {}
+   |     ^^^^^^^^^^^^
+   |
+   = help: expected names are: `FALSE` and `test` and 31 more
+   = help: to expect this configuration use `--check-cfg=cfg(invalid_cfg1)`
+   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
+   = note: `#[warn(unexpected_cfgs)]` on by default
+
+warning: unexpected `cfg` condition name: `invalid_cfg2`
+  --> $DIR/cfg-select.rs:14:5
+   |
+LL |     invalid_cfg2 => {}
+   |     ^^^^^^^^^^^^
+   |
+   = help: to expect this configuration use `--check-cfg=cfg(invalid_cfg2)`
+   = note: see <https://doc.rust-lang.org/nightly/rustc/check-cfg.html> for more information about checking conditional configuration
+
+warning: 2 warnings emitted
+