Bishonen's Blog - Overflow Yet another software developemnt blog. Zola 2026-04-21T00:00:00+00:00 https://bshn.rs/tags/overflow/atom.xml Not so Trivial Casts 2026-04-21T00:00:00+00:00 2026-04-21T00:00:00+00:00 Nikita Bishonen https://bshn.rs/blog/not-so-trivial-casts/ <h1 id="not-so-trivial-casts"><a class="zola-anchor" href="#not-so-trivial-casts" aria-label="Anchor link for: not-so-trivial-casts"><i class="icon"></i></a> Not so Trivial Casts</h1> <p>There is an interestion lint in Rust called <a rel="external" href="https://doc.rust-lang.org/stable/nightly-rustc/rustc_lint/builtin/static.TRIVIAL_CASTS.html"><code>trivial_casts</code></a>, yet it turns out to be not so trivial.</p> <blockquote> <p>It may be possible that this will become a warning in the future</p> </blockquote> <p>so let’s explore some code examples where I will compare:</p> <ol> <li>Trivial casts</li> <li>Variables types definitions</li> <li>Implicit coercion</li> </ol> <p>I will not add any comments, except compiler outputs, so you will be able to see difference by yourself and decide when and how to use types casting in Rust code. I will only say that <code>trivial_cast_supertype_with_shadowing</code> seems to be a good example on how <strong>trivial_casts</strong> can become not so trivial even in small example (imagine what it can do in a bigger chain of calls and transformations/casts).</p> <pre class="giallo" style="color-scheme: light dark; color: light-dark(#C6D0F5, #565869); background-color: light-dark(#303446, #FAFBFC);"><code data-lang="rust"><span class="giallo-l"><span style="color: light-dark(#E5C890, #ADB1C2);font-style: italic;">#</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;">!</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;">allow</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;">dead_code</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span></span> <span class="giallo-l"><span style="color: light-dark(#E5C890, #ADB1C2);font-style: italic;">#</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;">!</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;">warn</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;">trivial_casts</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);">trait</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;"> Polygon</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span><span style="color: light-dark(#949CBB, #ADB1C2);">}</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);">trait</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;"> TwoSidedPolygon</span><span style="color: light-dark(#81C8BE, #ADB1C2);">:</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> Polygon</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span><span style="color: light-dark(#949CBB, #ADB1C2);">}</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);">struct</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;"> Square</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#CA9EE6, #565869);">u32</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span><span style="color: light-dark(#EF9F76, #FF5C57);"> 8</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);">impl</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> Polygon</span><span style="color: light-dark(#CA9EE6, #F767BB);"> for</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> Square</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span><span style="color: light-dark(#949CBB, #ADB1C2);">}</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);">impl</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> TwoSidedPolygon</span><span style="color: light-dark(#CA9EE6, #F767BB);"> for</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> Square</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span><span style="color: light-dark(#949CBB, #ADB1C2);">}</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);">struct</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;"> Triangle</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#CA9EE6, #565869);">u32</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span><span style="color: light-dark(#EF9F76, #FF5C57);"> 6</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);">impl</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> Polygon</span><span style="color: light-dark(#CA9EE6, #F767BB);"> for</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> Triangle</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span><span style="color: light-dark(#949CBB, #ADB1C2);">}</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span style="color: light-dark(#E5C890, #ADB1C2);font-style: italic;">#</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;">cfg</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;">test</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);">mod</span><span style="color: light-dark(#EF9F76, #565869);"> tests</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> use</span><span style="color: light-dark(#CA9EE6, #F767BB);"> crate</span><span style="color: light-dark(#81C8BE, #ADB1C2);">::</span><span style="color: light-dark(#949CBB, #ADB1C2);">{</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;">Polygon</span><span style="color: light-dark(#949CBB, #ADB1C2);">,</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> Square</span><span style="color: light-dark(#949CBB, #ADB1C2);">,</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> Triangle</span><span style="color: light-dark(#949CBB, #ADB1C2);">,</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> TwoSidedPolygon</span><span style="color: light-dark(#949CBB, #ADB1C2);">}</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span style="color: light-dark(#E5C890, #ADB1C2);font-style: italic;"> #</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;">test</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> fn</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> explicit_definition_supertype</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> Square</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#EF9F76, #FF5C57);">2</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span><span style="color: light-dark(#EF9F76, #FF5C57);"> 8</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);">:</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span style="color: light-dark(#CA9EE6, #F767BB);">dyn</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> Polygon</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> process2(square); the size for values of type `dyn Polygon` cannot be known at compilation time</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> process3(square); the trait bound `dyn Polygon: TwoSidedPolygon` is not satisfied</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> process_square(square); expected &amp;Square, found &amp;(dyn Polygon + &#39;static)</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);"> }</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span style="color: light-dark(#E5C890, #ADB1C2);font-style: italic;"> #</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;">test</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> fn</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> explicit_definition_subtype</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> Square</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#EF9F76, #FF5C57);">2</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span><span style="color: light-dark(#EF9F76, #FF5C57);"> 8</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);">:</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span style="color: light-dark(#CA9EE6, #F767BB);">dyn</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> TwoSidedPolygon</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> process2(square); the size for values of type `dyn Polygon` cannot be known at compilation time</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process3</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> process_square(square); expected &amp;Square, found &amp;(dyn TwoSidedPolygon + &#39;static)</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);"> }</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span style="color: light-dark(#E5C890, #ADB1C2);font-style: italic;"> #</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;">test</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> fn</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> trivial_cast_supertype</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> Square</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#EF9F76, #FF5C57);">2</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span><span style="color: light-dark(#EF9F76, #FF5C57);"> 8</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span>square</span><span style="color: light-dark(#CA9EE6, #F767BB);"> as</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span style="color: light-dark(#CA9EE6, #F767BB);">dyn</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> Polygon</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> trivial cast: `&amp;Square` as `&amp;dyn Polygon`</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> process2(square); the size for values of type `dyn Polygon` cannot be known at compilation time</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> process3(square); the trait bound `dyn Polygon: TwoSidedPolygon` is not satisfied</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> process_square(square); expected &amp;Square, found &amp;dyn Polygon</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);"> }</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span style="color: light-dark(#E5C890, #ADB1C2);font-style: italic;"> #</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;">test</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> fn</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> trivial_cast_subtype</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> Square</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#EF9F76, #FF5C57);">2</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span><span style="color: light-dark(#EF9F76, #FF5C57);"> 8</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span>square</span><span style="color: light-dark(#CA9EE6, #F767BB);"> as</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span style="color: light-dark(#CA9EE6, #F767BB);">dyn</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> TwoSidedPolygon</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> trivial cast: `&amp;Square` as `&amp;dyn TwoSidedPolygon` </span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> process2(square); the size for values of type `dyn Polygon` cannot be known at compilation time</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process3</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> process_square(square); expected &amp;Square, found &amp;(dyn TwoSidedPolygon + &#39;static)</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);"> }</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span style="color: light-dark(#E5C890, #ADB1C2);font-style: italic;"> #</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;">test</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> fn</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> trivial_cast_struct_type</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> Square</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#EF9F76, #FF5C57);">2</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span><span style="color: light-dark(#EF9F76, #FF5C57);"> 8</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span>square</span><span style="color: light-dark(#CA9EE6, #F767BB);"> as</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;">Square</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> trivial cast: `&amp;Square` as `&amp;Square`</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process2</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process3</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process_square</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);"> }</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span style="color: light-dark(#E5C890, #ADB1C2);font-style: italic;"> #</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;">test</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> fn</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> auto_coercion</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> Square</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#EF9F76, #FF5C57);">2</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span><span style="color: light-dark(#EF9F76, #FF5C57);"> 8</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> polygon</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>polygon</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process2</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>polygon</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process3</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>polygon</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process_square</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>polygon</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);"> }</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span style="color: light-dark(#E5C890, #ADB1C2);font-style: italic;"> #</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;">test</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> fn</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> shadowed_auto_coercion</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> Square</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#EF9F76, #FF5C57);">2</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span><span style="color: light-dark(#EF9F76, #FF5C57);"> 8</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> Triangle</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#EF9F76, #FF5C57);">3</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span><span style="color: light-dark(#EF9F76, #FF5C57);"> 6</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process2</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> process3(square); the trait bound `Triangle: TwoSidedPolygon` is not satisfied</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> process_square(square); expected &amp;Square, found &amp;Triangle</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process_triangle</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);"> }</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span style="color: light-dark(#E5C890, #ADB1C2);font-style: italic;"> #</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#E5C890, #565869);font-style: italic;">test</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> fn</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> trivial_cast_supertype_with_shadowing</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> Square</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#EF9F76, #FF5C57);">2</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span><span style="color: light-dark(#EF9F76, #FF5C57);"> 8</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> Triangle</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#949CBB, #ADB1C2);">[</span><span style="color: light-dark(#EF9F76, #FF5C57);">3</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span><span style="color: light-dark(#EF9F76, #FF5C57);"> 6</span><span style="color: light-dark(#949CBB, #ADB1C2);">]</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> let</span><span> square</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> =</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span>square</span><span style="color: light-dark(#CA9EE6, #F767BB);"> as</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span style="color: light-dark(#CA9EE6, #F767BB);">dyn</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> Polygon</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> trivial cast: `&amp;Square` as `&amp;Square`</span></span> <span class="giallo-l"><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span>square</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">;</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> process2(square); the size for values of type `dyn Polygon` cannot be known at compilation time</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> process3(square); the trait bound `dyn Polygon: TwoSidedPolygon` is not satisfied</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> //</span><span style="color: light-dark(#949CBB, #ADB1C2);font-style: italic;"> process_square(square); expected &amp;Square, found &amp;dyn Polygon</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);"> }</span></span> <span class="giallo-l"></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> fn</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process_square</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#EA999C, #565869);">_square</span><span style="color: light-dark(#81C8BE, #ADB1C2);">:</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;">Square</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span><span style="color: light-dark(#949CBB, #ADB1C2);">}</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> fn</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process_triangle</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#EA999C, #565869);">_triangle</span><span style="color: light-dark(#81C8BE, #ADB1C2);">:</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;">Triangle</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span><span style="color: light-dark(#949CBB, #ADB1C2);">}</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> fn</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#EA999C, #565869);">_polygon</span><span style="color: light-dark(#81C8BE, #ADB1C2);">:</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#CA9EE6, #F767BB);">impl</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> Polygon</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> +</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> ?</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;">Sized</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span><span style="color: light-dark(#949CBB, #ADB1C2);">}</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> fn</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process2</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#EA999C, #565869);">_polygon</span><span style="color: light-dark(#81C8BE, #ADB1C2);">:</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span style="color: light-dark(#CA9EE6, #F767BB);">impl</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> Polygon</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span><span style="color: light-dark(#949CBB, #ADB1C2);">}</span></span> <span class="giallo-l"><span style="color: light-dark(#CA9EE6, #F767BB);"> fn</span><span style="color: light-dark(#8CAAEE, #09A1ED);font-style: italic;"> process3</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#EA999C, #565869);">_polygon</span><span style="color: light-dark(#81C8BE, #ADB1C2);">:</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> &amp;</span><span style="color: light-dark(#949CBB, #ADB1C2);">(</span><span style="color: light-dark(#CA9EE6, #F767BB);">impl</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;"> TwoSidedPolygon</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> +</span><span style="color: light-dark(#81C8BE, #ADB1C2);"> ?</span><span style="color: light-dark(#E5C890, #13BBB7);font-style: italic;">Sized</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);">)</span><span style="color: light-dark(#949CBB, #ADB1C2);"> {</span><span style="color: light-dark(#949CBB, #ADB1C2);">}</span></span> <span class="giallo-l"><span style="color: light-dark(#949CBB, #ADB1C2);">}</span></span></code></pre>