Rollup merge of #122842 - pacak:explicit_name, r=michaelwoerister

Don't emit an error about failing to produce a file with a specific name if user never gave an explicit name

Fixes #122509

You can ask `rustc` to produce some intermediate results with `--emit foo`, this operation comes in two flavors: `--emit asm` and `--emit asm=foo.s`. First one produces one or more `.s` files without any name guarantees, second one renames it into `foo.s`. Second version only works when compiler produces a single file - for asm files this means using a single compilation unit for example.

In case compilation produced more than a single file `rustc` runs following check to emit some warnings:

```rust
            if crate_output.outputs.contains_key(&output_type) {
                // 2) Multiple codegen units, with `--emit foo=some_name`. We have
                //    no good solution for this case, so warn the user.
                sess.dcx().emit_warn(errors::IgnoringEmitPath { extension });
            } else if crate_output.single_output_file.is_some() {
                // 3) Multiple codegen units, with `-o some_name`. We have
                //    no good solution for this case, so warn the user.
                sess.dcx().emit_warn(errors::IgnoringOutput { extension });
            } else {
                // 4) Multiple codegen units, but no explicit name. We
                //    just leave the `foo.0.x` files in place.
                // (We don't have to do any work in this case.)
            }
```

Comment in the final `else` branch implies that if user didn't ask for a specific name - there's no need to emit warnings. However because of the internal representation of `crate_output.outputs` - this doesn't work as expected: if user asked to produce an asm file without giving it an implicit name it will contain `Some(None)`.

To fix the problem new code actually checks if user gave an explicit name. I think this was an original intentional behavior, at least comments imply that.

compiler/rustc_session/src/config.rs

@@ -557,6 +557,11 @@ impl OutputTypes {
         self.0.contains_key(key)
     }
 
+    /// Returns `true` if user specified a name and not just produced type
+    pub fn contains_explicit_name(&self, key: &OutputType) -> bool {
+        self.0.get(key).map_or(false, |f| f.is_some())
+    }
+
     pub fn iter(&self) -> BTreeMapIter<'_, OutputType, Option<OutFileName>> {
         self.0.iter()
     }