internal: start new diagnostics API

At the moment, this moves only a single diagnostic, but the idea is
reafactor the rest to use the same pattern. We are going to have a
single file per diagnostic. This file will define diagnostics code,
rendering range and fixes, if any. It'll also have all of the tests.
This is similar to how we deal with assists.

After we refactor all diagnostics to follow this pattern, we'll probably
move them to a new `ide_diagnostics` crate.

Not that we intentionally want to test all diagnostics on this layer,
despite the fact that they are generally emitted in the guts on the
compiler. Diagnostics care to much about the end presentation
details/fixes to be worth-while "unit" testing. So, we'll unit-test only
the primary output of compilation process (types and name res tables),
and will use integrated UI tests for diagnostics.

crates/ide/src/diagnostics.rs

@@ -4,6 +4,8 @@
 //! macro-expanded files, but we need to present them to the users in terms of
 //! original files. So we need to map the ranges.
 
+mod unresolved_module;
+
 mod fixes;
 mod field_shorthand;
 mod unlinked_file;
@@ -12,7 +14,7 @@ use std::cell::RefCell;
 
 use hir::{
     db::AstDatabase,
-    diagnostics::{Diagnostic as _, DiagnosticCode, DiagnosticSinkBuilder},
+    diagnostics::{AnyDiagnostic, Diagnostic as _, DiagnosticCode, DiagnosticSinkBuilder},
     InFile, Semantics,
 };
 use ide_assists::AssistResolveStrategy;
@@ -42,6 +44,12 @@ pub struct Diagnostic {
 }
 
 impl Diagnostic {
+    fn new(code: &'static str, message: impl Into<String>, range: TextRange) -> Diagnostic {
+        let message = message.into();
+        let code = Some(DiagnosticCode(code));
+        Self { message, range, severity: Severity::Error, fixes: None, unused: false, code }
+    }
+
     fn error(range: TextRange, message: String) -> Self {
         Self { message, range, severity: Severity::Error, fixes: None, unused: false, code: None }
     }
@@ -82,6 +90,13 @@ pub struct DiagnosticsConfig {
     pub disabled: FxHashSet<String>,
 }
 
+struct DiagnosticsContext<'a> {
+    config: &'a DiagnosticsConfig,
+    sema: Semantics<'a, RootDatabase>,
+    #[allow(unused)]
+    resolve: &'a AssistResolveStrategy,
+}
+
 pub(crate) fn diagnostics(
     db: &RootDatabase,
     config: &DiagnosticsConfig,
@@ -108,9 +123,6 @@ pub(crate) fn diagnostics(
     }
     let res = RefCell::new(res);
     let sink_builder = DiagnosticSinkBuilder::new()
-        .on::<hir::diagnostics::UnresolvedModule, _>(|d| {
-            res.borrow_mut().push(diagnostic_with_fix(d, &sema, resolve));
-        })
         .on::<hir::diagnostics::MissingFields, _>(|d| {
             res.borrow_mut().push(diagnostic_with_fix(d, &sema, resolve));
         })
@@ -204,16 +216,33 @@ pub(crate) fn diagnostics(
             );
         });
 
+    let mut diags = Vec::new();
     let internal_diagnostics = cfg!(test);
     match sema.to_module_def(file_id) {
-        Some(m) => m.diagnostics(db, &mut sink, internal_diagnostics),
+        Some(m) => diags = m.diagnostics(db, &mut sink, internal_diagnostics),
         None => {
             sink.push(UnlinkedFile { file_id, node: SyntaxNodePtr::new(parse.tree().syntax()) });
         }
     }
 
     drop(sink);
-    res.into_inner()
+
+    let mut res = res.into_inner();
+
+    let ctx = DiagnosticsContext { config, sema, resolve };
+    for diag in diags {
+        let d = match diag {
+            AnyDiagnostic::UnresolvedModule(d) => unresolved_module::render(&ctx, &d),
+        };
+        if let Some(code) = d.code {
+            if ctx.config.disabled.contains(code.as_str()) {
+                continue;
+            }
+        }
+        res.push(d)
+    }
+
+    res
 }
 
 fn diagnostic_with_fix<D: DiagnosticWithFixes>(