x/tools/gopls/internal/analyzer/modernize: bug in slices.Contains transformation

[adonovan]

[split out of https://github.com//issues/70815#issuecomment-2598663385]

@findleyr says:
I encountered a bug in the slices.Contains modernizer today.

Modernizing using slices.Contains here results in an error:
https://cs.opensource.google/go/x/tools/+/master:gopls/internal/golang/highlight.go;l=348;drc=344e48255740736de8c8277e9a286cf3231c7e13

		case *ast.CallExpr:
			// If cursor is an arg in a callExpr, we don't want control flow highlighting.
			if i > 0 {
				for _, arg := range n.Args {
					if arg == path[i-1] {
						return
					}
				}
			}

Error is: S (type []ast.Expr) does not satisfy ~[]E.
For now, I think we need to check that the types in the match condition are identical.

[adonovan]

Related: we need a means of testing the welltypedness of the transformed code. I'll keep that in mind as I rework the apply-fix logic (which is currently forked between RunWithSuggestedFixes and analysis/checker, but I think deserves a common implementation).

[adonovan]

There's a similar problem in the maps.Copy modernizer if the two maps don't have the same type:

func (fs FolderSettings) set(opts *runConfig) {
	// Re-use the Settings type, for symmetry, but translate back into maps for
	// the editor config.
	folders := make(map[string]map[string]any)
	maps.Copy(folders, fs)  /// oops

	opts.editor.FolderSettings = folders
}