Auto merge of #9549 - Bryysen:master, r=ehuss

Warn if an "all" target is specified, but we don't match anything

If a combination of --bins, --benches, --examples, --tests flags have
been specified, but we didn't match on anything after resolving the unit-list,
we emit a warning to make it clear that cargo didn't do anything and that the
code is unchecked.

This is my first PR and there are a couple things that I'm unsure about
* The integration test covers only one case (ideally it should cover every combination of the above mentioned flags the user can pass). I figured since the warning function is so simple, it'd best not to clog the testsuite with unnecessary `p.cargo().runs()` and whatnot. If I should make the test more comprehensive I can do that, it's also very easy to write unit tests so i can do that as well if needed.
* I figure we don't actually have to check the `--all-targets`, but i'm doing so for consistency. I also didn't check for the `--lib` flag at all because (I'm assuming) if the user passes `--lib` and there are no libraries, we error.
Edit: I notice (among other things) we sometimes silently skip certain units that have incompatible feature flags (see [here](https://github.com/rust-lang/cargo/blob/ed0c8c6d66e36fafbce4f78907a110838797ae39/src/cargo/ops/cargo_compile.rs#L1140)) so maybe we should be checking the `--lib` flag after all, in the event that a library was silently skipped and we no-opped 🤔

And thanks to `@ehuss` for taking the time to answer my questions and helping me through the contribution process, much appreciated

Closes #9536

src/cargo/ops/cargo_compile.rs

@@ -1140,11 +1140,63 @@ fn generate_targets(
         // else, silently skip target.
     }
     let mut units: Vec<_> = units.into_iter().collect();
+    unmatched_target_filters(&units, &filter, &mut ws.config().shell())?;
+
     // Keep the roots in a consistent order, which helps with checking test output.
     units.sort_unstable();
     Ok(units)
 }
 
+/// Checks if the unit list is empty and the user has passed any combination of
+/// --tests, --examples, --benches or --bins, and we didn't match on any targets.
+/// We want to emit a warning to make sure the user knows that this run is a no-op,
+/// and their code remains unchecked despite cargo not returning any errors
+fn unmatched_target_filters(
+    units: &[Unit],
+    filter: &CompileFilter,
+    shell: &mut Shell,
+) -> CargoResult<()> {
+    if let CompileFilter::Only {
+        all_targets,
+        lib: _,
+        ref bins,
+        ref examples,
+        ref tests,
+        ref benches,
+    } = *filter
+    {
+        if units.is_empty() {
+            let mut filters = String::new();
+            let mut miss_count = 0;
+
+            let mut append = |t: &FilterRule, s| {
+                if let FilterRule::All = *t {
+                    miss_count += 1;
+                    filters.push_str(s);
+                }
+            };
+
+            if all_targets {
+                filters.push_str(" `all-targets`");
+            } else {
+                append(bins, " `bins`,");
+                append(tests, " `tests`,");
+                append(examples, " `examples`,");
+                append(benches, " `benches`,");
+                filters.pop();
+            }
+
+            return shell.warn(format!(
+                "Target {}{} specified, but no targets matched. This is a no-op",
+                if miss_count > 1 { "filters" } else { "filter" },
+                filters,
+            ));
+        }
+    }
+
+    Ok(())
+}
+
 /// Warns if a target's required-features references a feature that doesn't exist.
 ///
 /// This is a warning because historically this was not validated, and it

tests/testsuite/cargo_targets.rs

@@ -2,6 +2,35 @@
 
 use cargo_test_support::project;
 
+#[cargo_test]
+fn warn_unmatched_target_filters() {
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+        [package]
+        name = "foo"
+        version = "0.1.0"
+
+        [lib]
+        test = false
+        bench = false
+        "#,
+        )
+        .file("src/lib.rs", r#"fn main() {}"#)
+        .build();
+
+    p.cargo("check --tests --bins --examples --benches")
+        .with_stderr(
+            "\
+[WARNING] Target filters `bins`, `tests`, `examples`, `benches` specified, \
+but no targets matched. This is a no-op
+[FINISHED][..]
+",
+        )
+        .run();
+}
+
 #[cargo_test]
 fn reserved_windows_target_name() {
     let p = project()