Deny useing tool paths

compiler/rustc_resolve/messages.ftl

@@ -207,5 +207,9 @@ resolve_expected_found =
 resolve_indeterminate =
     cannot determine resolution for the visibility
 
+resolve_tool_module_imported =
+    cannot use a tool module through an import
+    .note = the tool module imported here
+
 resolve_module_only =
     visibility must resolve to a module

compiler/rustc_resolve/src/errors.rs

@@ -469,6 +469,15 @@ pub(crate) struct ExpectedFound {
 #[diag(resolve_indeterminate, code = "E0578")]
 pub(crate) struct Indeterminate(#[primary_span] pub(crate) Span);
 
+#[derive(Diagnostic)]
+#[diag(resolve_tool_module_imported)]
+pub(crate) struct ToolModuleImported {
+    #[primary_span]
+    pub(crate) span: Span,
+    #[note]
+    pub(crate) import: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(resolve_module_only)]
 pub(crate) struct ModuleOnly(#[primary_span] pub(crate) Span);

compiler/rustc_resolve/src/ident.rs

@@ -17,7 +17,7 @@ use crate::late::{
     ConstantHasGenerics, ConstantItemKind, HasGenericParams, PathSource, Rib, RibKind,
 };
 use crate::macros::{sub_namespace_match, MacroRulesScope};
-use crate::{AmbiguityError, AmbiguityErrorMisc, AmbiguityKind, Determinacy, Finalize};
+use crate::{errors, AmbiguityError, AmbiguityErrorMisc, AmbiguityKind, Determinacy, Finalize};
 use crate::{Import, ImportKind, LexicalScopeBinding, Module, ModuleKind, ModuleOrUniformRoot};
 use crate::{NameBinding, NameBindingKind, ParentScope, PathResult, PrivacyError, Res};
 use crate::{ResolutionError, Resolver, Scope, ScopeSet, Segment, ToNameBinding, Weak};
@@ -1357,7 +1357,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                 }
             };
 
-            let is_last = i == path.len() - 1;
+            let is_last = i + 1 == path.len();
             let ns = if is_last { opt_ns.unwrap_or(TypeNS) } else { TypeNS };
             let name = ident.name;
 
@@ -1494,16 +1494,12 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
                     if let Some(next_module) = binding.module() {
                         module = Some(ModuleOrUniformRoot::Module(next_module));
                         record_segment_res(self, res);
-                    } else if res == Res::ToolMod && i + 1 != path.len() {
+                    } else if res == Res::ToolMod && !is_last && opt_ns.is_some() {
                         if binding.is_import() {
-                            self.tcx
-                                .sess
-                                .struct_span_err(
-                                    ident.span,
-                                    "cannot use a tool module through an import",
-                                )
-                                .span_note(binding.span, "the tool module imported here")
-                                .emit();
+                            self.tcx.sess.emit_err(errors::ToolModuleImported {
+                                span: ident.span,
+                                import: binding.span,
+                            });
                         }
                         let res = Res::NonMacroAttr(NonMacroAttrKind::Tool);
                         return PathResult::NonModule(PartialRes::new(res));

tests/ui/resolve/tool-import.rs

@@ -0,0 +1,8 @@
+// edition: 2018
+
+use clippy::time::Instant;
+//~^ `clippy` is a tool module
+
+fn main() {
+    Instant::now();
+}

tests/ui/resolve/tool-import.stderr

@@ -0,0 +1,9 @@
+error[E0433]: failed to resolve: `clippy` is a tool module, not a module
+  --> $DIR/tool-import.rs:3:5
+   |
+LL | use clippy::time::Instant;
+   |     ^^^^^^ `clippy` is a tool module, not a module
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0433`.