[ty] Improve debug messages when imports fail (#21555)

Author: AlexWaygood

crates/ty_python_semantic/resources/mdtest/snapshots/unresolved_import.md_-_Unresolved_import_di…_-_Using_`from`_with_to…_(4b8ba6ee48180cdd).snap

@@ -40,6 +40,8 @@ error[unresolved-import]: Cannot resolve imported module `....foo`
 2 |
 3 | stat = add(10, 15)
   |
+help: The module can be resolved if the number of leading dots is reduced
+help: Did you mean `...foo`?
 info: Searched in the following paths during module resolution:
 info:   1. /src (first-party code)
 info:   2. vendored://stdlib (stdlib typeshed stubs vendored by ty)

crates/ty_python_semantic/src/types/infer/builder.rs

@@ -5799,9 +5799,8 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             return;
         };
         let mut diagnostic = builder.into_diagnostic(format_args!(
-            "Cannot resolve imported module `{}{}`",
-            ".".repeat(level as usize),
-            module.unwrap_or_default()
+            "Cannot resolve imported module `{}`",
+            format_import_from_module(level, module)
         ));
 
         if level == 0 {
@@ -5832,6 +5831,30 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
                     }
                 }
             }
+        } else {
+            if let Some(better_level) = (0..level).rev().find(|reduced_level| {
+                let Ok(module_name) = ModuleName::from_identifier_parts(
+                    self.db(),
+                    self.file(),
+                    module,
+                    *reduced_level,
+                ) else {
+                    return false;
+                };
+                resolve_module(self.db(), &module_name).is_some()
+            }) {
+                diagnostic
+                    .help("The module can be resolved if the number of leading dots is reduced");
+                diagnostic.help(format_args!(
+                    "Did you mean `{}`?",
+                    format_import_from_module(better_level, module)
+                ));
+                diagnostic.set_concise_message(format_args!(
+                    "Cannot resolve imported module `{}` - did you mean `{}`?",
+                    format_import_from_module(level, module),
+                    format_import_from_module(better_level, module)
+                ));
+            }
         }
 
         // Add search paths information to the diagnostic
@@ -6039,7 +6062,8 @@ impl<'db, 'ast> TypeInferenceBuilder<'db, 'ast> {
             }
             Err(ModuleNameResolutionError::UnknownCurrentModule) => {
                 tracing::debug!(
-                    "Relative module resolution `{}` failed; could not resolve file `{}` to a module",
+                    "Relative module resolution `{}` failed: could not resolve file `{}` to a module \
+                    (try adjusting configured search paths?)",
                     format_import_from_module(*level, module),
                     self.file().path(self.db())
                 );