Ignore imported items in min_ident_chars

Suppress the `min_ident_chars` warning for items whose name we cannot
control. Do not warn for `use a::b`, but warn for `use a::b as c`, since
`c` is a local identifier.

Fixes rust-lang#8573

Author: Ethiraric

clippy_lints/src/min_ident_chars.rs

@@ -3,7 +3,7 @@ use clippy_utils::is_from_proc_macro;
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir::def::{DefKind, Res};
 use rustc_hir::intravisit::{walk_item, Visitor};
-use rustc_hir::{GenericParamKind, HirId, Item, ItemKind, ItemLocalId, Node, Pat, PatKind};
+use rustc_hir::{GenericParamKind, HirId, Item, ItemKind, ItemLocalId, Node, Pat, PatKind, UsePath};
 use rustc_lint::{LateContext, LateLintPass, LintContext};
 use rustc_middle::lint::in_external_macro;
 use rustc_session::impl_lint_pass;
@@ -105,11 +105,26 @@ impl Visitor<'_> for IdentVisitor<'_, '_> {
 
         let str = ident.as_str();
         if conf.is_ident_too_short(cx, str, ident.span) {
-            if let Node::Item(item) = node
-                && let ItemKind::Use(..) = item.kind
+            // Check whether the node is part of a `use statement. We don't want to emit a warning if the user
+            // has no control over the type.
+            let usenode = opt_as_use_node(node).or_else(|| {
+                cx.tcx
+                    .hir()
+                    .parent_iter(hir_id)
+                    .find_map(|(_, node)| opt_as_use_node(node))
+            });
+
+            // If the name of the identifier is the same from the one of the imported item, this means that we
+            // found a `use foo::bar`. We can early-return to not emit the warning.
+            // If however the identifier is different, this means it is an alias (`use foo::bar as baz`). In
+            // this case, we need to emit the warning for `baz`.
+            if let Some(imported_item_path) = usenode
+                && let Some(Res::Def(_, imported_item_defid)) = imported_item_path.res.first()
+                && cx.tcx.item_name(*imported_item_defid).as_str() == str
             {
                 return;
             }
+
             // `struct Awa<T>(T)`
             //                ^
             if let Node::PathSegment(path) = node {
@@ -160,3 +175,16 @@ fn emit_min_ident_chars(conf: &MinIdentChars, cx: &impl LintContext, ident: &str
     };
     span_lint(cx, MIN_IDENT_CHARS, span, &help);
 }
+
+/// Attempt to convert the node to an [`ItemKind::Use`] node.
+///
+/// If it is, return the [`UsePath`] contained within.
+fn opt_as_use_node(node: Node<'_>) -> Option<&'_ UsePath<'_>> {
+    if let Node::Item(item) = node
+        && let ItemKind::Use(path, _) = item.kind
+    {
+        Some(path)
+    } else {
+        None
+    }
+}

tests/ui-toml/min_ident_chars/auxiliary/extern_types.rs

@@ -1,3 +1,6 @@
 #![allow(nonstandard_style, unused)]
 
 pub struct Aaa;
+pub struct Bbb;
+
+pub const N: u32 = 3;

tests/ui-toml/min_ident_chars/min_ident_chars.rs

@@ -3,7 +3,7 @@
 #![warn(clippy::min_ident_chars)]
 
 extern crate extern_types;
-use extern_types::Aaa;
+use extern_types::{Aaa, N as W};
 
 struct Owo {
     Uwu: u128,

tests/ui-toml/min_ident_chars/min_ident_chars.stderr

@@ -1,8 +1,8 @@
-error: this ident is too short (3 <= 3)
-  --> $DIR/min_ident_chars.rs:6:19
+error: this ident is too short (1 <= 3)
+  --> $DIR/min_ident_chars.rs:6:30
    |
-LL | use extern_types::Aaa;
-   |                   ^^^
+LL | use extern_types::{Aaa, N as W};
+   |                              ^
    |
    = note: `-D clippy::min-ident-chars` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(clippy::min_ident_chars)]`