Add let_underscore_drop lint.

This lint checks for statements similar to `let _ = foo`, where `foo` is
a type that implements `Drop`. These types of let statements cause the
expression in them to be dropped immediately, instead of at the end of
the scope. Such behavior can be surprizing, especially if you are
relying on the value to be dropped at the end of the scope. Instead, the
binding should be an underscore prefixed name (like `_unused`) or the
value should explicitly be passed to `std::mem::drop()` if the value
really should be dropped immediately.

compiler/rustc_lint/src/lib.rs

@@ -54,6 +54,7 @@ mod expect;
 pub mod hidden_unicode_codepoints;
 mod internal;
 mod late;
+mod let_underscore;
 mod levels;
 mod methods;
 mod non_ascii_idents;
@@ -85,6 +86,7 @@ use builtin::*;
 use enum_intrinsics_non_enums::EnumIntrinsicsNonEnums;
 use hidden_unicode_codepoints::*;
 use internal::*;
+use let_underscore::*;
 use methods::*;
 use non_ascii_idents::*;
 use non_fmt_panic::NonPanicFmt;
@@ -199,6 +201,7 @@ macro_rules! late_lint_mod_passes {
                 VariantSizeDifferences: VariantSizeDifferences,
                 BoxPointers: BoxPointers,
                 PathStatements: PathStatements,
+                LetUnderscore: LetUnderscore,
                 // Depends on referenced function signatures in expressions
                 UnusedResults: UnusedResults,
                 NonUpperCaseGlobals: NonUpperCaseGlobals,
@@ -314,6 +317,8 @@ fn register_builtins(store: &mut LintStore, no_interleave_lints: bool) {
         REDUNDANT_SEMICOLONS
     );
 
+    add_lint_group!("let_underscore", LET_UNDERSCORE_DROP);
+
     add_lint_group!(
         "rust_2018_idioms",
         BARE_TRAIT_OBJECTS,